Running head: AFFINAL CONFLICT 1

Mother-in-law daughter-in-law conflict: An evolutionary perspective, ethnographic review, and

report of empirical data from the United States

Jessica D. Ayers1, Jaimie Arona Krems2, Nicole Hess3, & Athena Aktipis1,4

1 Department of Psychology, Arizona State University

2 Oklahoma Center for Evolutionary Analysis (OCEAN), Department of Psychology, Oklahoma

State University

3 Department of Anthropology, Washington State University

4Center for Evolution and Medicine, Arizona State University

Corresponding author: Jessica D. Ayers, [email protected], Department of Psychology, Arizona

State University, Tempe, AZ 85287-1104, United States.

Acknowledgments: We would like to thank Lee Cronk for his helpful comments on this manuscript. AFFINAL CONFLICT 2

Abstract

Relationships with genetic relatives have been extensively studied in the evolutionary social sciences, but affinal relationships have received much less attention, and little work has examined both cooperation and conflict among affines from an evolutionary perspective. Yet humans have extensive interactions with the kin of their mates, i.e., in-laws or affines, as humans form long-term pair bonds with mates, with both sexes investing heavily in resulting offspring, thus leading to many opportunities for interacting with extended kinship networks. To contribute to the scholarship on affinal bonds, and particularly on perceptions of affinal conflict, we conducted an ethnographic review and collected empirical data on cooperation and conflict among affines. Here we present (1) a sample 37 of ethnographies showing cross-cultural evidence of conflict in affinal relationships. We also report (2) empirical evidence of self-reported cooperative and conflictual aspects in affinal relationships in a Western sample. U.S. men and women both reported more conflict with mothers-in-law than with mothers, and mothers reported more conflict with their daughters-in-law than with their daughters. We also found that there was high conflict between mother- and daughter-in-laws in domains of family dynamics/ status and social role expectations in both our ethnographic sample and among the Western participants. We discuss the implications of this work and directions for future research.

Keywords: affines, kin, in-laws, cooperation, conflict, pair-bonds AFFINAL CONFLICT 3

Mother-in-law daughter-in-law conflict: An evolutionary perspective, ethnographic review, and

report of empirical data from the United States

1. Introduction

Unlike most other social animals, humans form enduring relationships with the kin of their long-term mates. These relationships are referred to with terms like “in-laws” or “affines”, which at once signal kinship through marriage while also differentiating these relationships from those with genetic kin. As a result of long-term pair-bonding, marriage patterns, and extensive investment in offspring by parents as well as other kin, humans often live with and have ongoing interactions and relationships with their affines1 (D. E. Brown, 1991). Despite the strong emphases on both kinship bonds (Ackerman et al., 2007; Eberhard, 1975; Hamilton, 1964; Ko et al., 2020; Koster et al., 2019; Lieberman et al., 2007) and mating relationships (David M.

Buss, 1985, 1989; David M. Buss & Barnes, 1986; D. M. Buss, 2017; D. M. Buss & Schmitt,

1993; Gangestad & Simpson, 2000; Symons, 1979) in evolutionary social science, there has been surprisingly little quantitative examination of the psychology bearing on relationships at the intersection of these foci; that is, on relationships among affines.

Evolutionary-minded work on affinal relationships has been primarily descriptive and has primarily investigated the positive side of in-law relationships, specifically the ways in which affinal relationships are cooperative and beneficial for both parties (Dyble et al., 2018; Hrdy,

2007; Macfarlan et al., 2018; Willführ et al., 2018). However, conflict is also a central feature of

1 Technically speaking, some other species also have affines. For example, species that live in larger kin-based groups are more likely to have interactions with their affinal kin. Known dispersal patterns across non-human primate species are helpful in determining the kin relationships a focal individual has with others in their group, e.g., genetic vs. affinal, matrilineal vs. patrilineal. Among human populations, however, studying these relationships is potentially more complicated given that humans do not have universal dispersal patterns: sometimes females leave their natal groups (Ember, 1978; Marlowe, 2004; Murdock, 1967; Rodseth et al., 1991; Seielstad et al., 1998), other times males leave their natal groups (Ember, 1975; Murdock, 1949; Turnbull, 1976), and sometimes both sexes leave their natal groups (Koenig & Borries, 2012; Kramer et al., 2017; Marlowe, 2003). Additionally, affinal relationships can take different shapes across cultures due to differences in socioecological variables and other cultural factors, such as traditional marriage practices, inheritance rules, and descent tracing. AFFINAL CONFLICT 4 affinal relationships. For example, work in evolutionary anthropology suggests that child survivorship and mortality are influenced by female cooperation and conflict over resources and child care (Kaptijn et al., 2010; Sheppard & Sear, 2016; Voland & Beise, 2002, 2005)(Voland &

Beise, 2002, 2005)owever, even in these papers, the focus of the papers is on the calculus behind female cooperation and conflict within families and its influence on long-term reproductive behavior (Ruth Mace, 2013; Strassmann, 2011) as opposed to the psychological experience of female cooperation and conflict within families. In this paper, we build off of this foundational work to focus on the experience and perceptions of cooperation and conflict, as these proximate factors should influence the ultimate reproductive outcomes, childhood survival, and relationships within the family unit.

Outside of the evolutionary social science, other disciplines have made headway towards understanding the mother- and daughter-in-law dynamic. For example, across cultures, mothers-in-law are perceived to be “distant” and perceived less positively than mothers in relationships with their daughters-in-law (Adler et al., 1989), mothers-in-law experience more strained relationships with their daughters-in-law following the birth of a grandchild (Fischer,

1983), and mothers-in-law also often are faced with an unequal relationship power dynamic that causes strife in the relationship (Linn & Breslerman, 1996). In addition, mothers- and daughters- in-law generally have differences in goals, values, and communication styles (which we would argue also stem from differences in fitness-linked outcomes). When mother- and daughter-in- law relationships are successful, it is generally attributed to daughters-in-law being compliant to their mothers-in-law’s demands (Jackson & Berg-Cross, 1988). However, what is not addressed in these articles is why the mother- and daughter-in-law relationship has these negative and conflictual features. This is why an evolutionary perspective is necessary for understanding the ultimate reasons for cooperation and conflict in the mother- and daughter-in-law relationship.

In this paper, we further develop the evolutionary logic behind in-law cooperation and conflict, arguing for the presence of conflict in affinal relationships. To do this, we present AFFINAL CONFLICT 5 evidence from ethnographic literature suggesting that conflict among affines is likely to be a human universal, and we also report findings from empirical data collected in a modern western population showing that both cooperation and conflict are components of relationships with in- laws. In sum, we find some support for our proposition that affinal conflict--and here, specifically--mother- and daughter-in-law conflict--is common across societies, and we suggest that this is a rich and understudied area for research on familial relationships in evolutionary perspective.

1.1 Genetic conflict might explain differences in cooperation and conflict.

How do affinal relationships differ from relationships with biological kin? Hamilton’s

(1964) rule is often used to explain cooperative behavior with biological kin because such cooperation can increase one’s inclusive fitness and also cause kin to feel stake in another’s welfare and survival (Roberts, 2005). We see examples of this in the non-human animal literature, for example, where older females signal that a predator is near, revealing their location and sometimes sacrificing their lives, in order to warn the group that danger is near

(Sherman, 1977). Ultimately this stake in another’s welfare is caused by genes shared between the individual and their kin (i.e., inclusive fitness).

But what about when two individuals have no genes in common? For example, while mothers and fathers are not closely related2, they still cooperate and share a stake in the survival of any shared offspring (Burton-Chellew & Dunbar, 2011; Dow, 1984; Hughes, 1988), creating interdependence between family members (Aktipis et al., 2018; Barclay, 2020; Sznycer et al., 2020). Such cooperation was almost certainly prevalent in the human ancestral past as well. At the same time, the lack of shared genes -- along with other fitness-linked asymmetries

(e.g., age and sex differences in potential future reproductive output and preferred mating

2 In this paper, we are exclusively talking about relationships between mothers and fathers who are not closely related for simplicity. There is robust evidence that in many cultures across the world cross-cousin marriages are preferred (see (Chagnon et al., 2017; Goody & Goody, 1966; Gray & Gulliver, 2013; Leach, 1951; Murphy & Kasdan, 1959) for ethnographic data on this). We acknowledge this in the discussion of the paper and discuss how cross-cousin marriages may influence the nature of in-law conflict. AFFINAL CONFLICT 6 strategies) -- between parents can lead to conflict. This conflict can manifest explicitly, as when there is disagreement about what is in the best interest for these shared descendants

(Borgerhoff Mulder, 2007). Genetic conflict can also manifest implicitly through physiological processes, like when genes from mothers and fathers vie for primary control over offspring phenotypes through growth and resource consumption rates (Werren, 2011) both in utero

(Fowden & Moore, 2012; Haig, 1993) and after birth (Cui & Donnellan, 2009; T. Shackelford &

Goetz, 2009).

Affinal relationships are expected to have similar elements of genetic conflict as relationships between to unrelated parents. Affines generally have genes in common with only one parent3 and so their fitness interests are aligned with only one parent. This may lead them to behave in a way that favors the best interest of their genetic kin, even at a cost to their affinal kin. Affinal conflict can be thought of as extended manifestation of the genetic conflict between mothers and fathers, and we expect this affinal conflict to manifest via psychology and behavior as well.

1.2 Affinal relationships can be cooperative

Most of the work that has been conducted on affinal relationships has focused on cooperation in these relationships, and on female-female affinal relationships. Humans are cooperative breeders who form long-term pair bonds and have shorter interbirth intervals relative to extant non-human apes (Emery Thompson, 2013; Waynforth, 2018). Because the time between births in hunter-gatherer populations is two to three years on average (Jones,

1986; Marlowe, 2005), and significantly shorter than that of non-human apes like chimpanzees

(Emery Thompson, 2013), human females face the sex-specific challenge of caring for multiple dependent offspring at one time, and can reap fitness benefits from positive affinal relationships

(Hrdy, 2017). 3 With the exception of cousin marriages, where men and women have a small proportion of genes in common via a shared family member (resulting in at most 1/8th, or 12.5%, shared genes). We discuss how this genetic relatedness may influence in-law conflict in the discussion but do not discuss it in the introduction for simplicity’s sake. AFFINAL CONFLICT 7

Ancestral females are thought to have relied on their female kin to assist with child- caring responsibilities (Coxworth et al., 2015; Kramer, 2005; Rodseth et al., 1991), but this was not always possible because genetic relatives may not always have lived close enough to provide consistent help. This would have created an opportunity to foster positive and supportive female affinal relationships in order to care for children (Fletcher et al., 2015;

Jankowiak & Fischer, 1992; Power & Ready, 2019). Affines have a vested interest in caring for shared descendants, and this stake can evolutionarily incentivize affines to provide the same level and quality of care to children as care provided by women’s genetic kin (Dyble et al., 2018;

Willführ et al., 2018). This evolutionary alignment of interests suggests that cooperation should be an important feature of mother- and daughter-in-law relationships.

There is comparatively less work on male-male affinal bonds. Men’s social networks are typically coalitionally-based and can be strengthened via positive relationships with genetic and affinal kin. In Yanomamö warfare coalitions co-unokais, or men who kill enemies together during war, come from different familial lineages but engage in marriage alliances, become affines, and live in the same village as one another later in life (Macfarlan et al., 2014). Thus, one of the benefits to being in co-unokais includes an avenue for mate acquisition that strengthens existing social bonds. Similarly, Waorani warfare coalitions contain males who are potential affines, actual affines, or kin, and use these connections to gain new potential allies or affinal kin from raiding parties (Macfarlan et al., 2018). It is clear that men’s social relationships as a whole are strengthened by strong, positive affinal relationships. Therefore, men may perceive benefits to affinal relationships if these relationships provide coalitional benefits.

Maintaining affinal bonds is associated with cooperative benefits, with female affinal cooperation centered around 1) kin care domains and 2) maintaining social bonds, whereas male affinal cooperation seems to be centered around 1) gaining and maintaining social relationships and 2) mate acquisition. However, an evolutionary perspective also predicts that there should be cooperation in domains such as self- and other-protection as well as the AFFINAL CONFLICT 8 investment of resources in children, as these domains also promote cooperative relationships between kin.

1.3 Affinal relationships can also be conflictual

Much existing work on affinal relationships has focused on cooperation rather than potential conflict. But conflict is present and important: because of underlying genetic conflict, affines are likely to disagree about what is in the best interest of their respective genes and thus their shared descendants (although this conflict need not be consciously about genetic fitness)

(Leonetti et al., 2007; Mulder & Rauch, 2009). At least some affinal conflict may be due to non- alignment in fitness interests. Indeed, mothers- and daughters-in-law typically do not have fully- aligned fitness interests (Dow, 1984; R. Mace & Alvergne, 2012). Different fitness optima and subsequent reproductive conflict can lead to later conflict over resources. For example, a mother-in-law’s son/ daughter-in-law’s partner has finite resources; if those were to go to the daughter-in-law and their children, that would benefit the daughter-in-law and, to a lesser extent, the mother-in-law; but if those were to go directly to his mother or perhaps to her daughters, that would more greatly benefit the mother-in-law at the expense of the daughter-in-law. Thus conflict over reproductive outcomes can manifest as conflict over resources, and this pay be particularly salient with respect to the allocation of the son/husband’s material resources. As such, daughters-in-law should favor her husband allocating all material resources to herself and her own children and family, while her mother-in-law may favor her son allocating some resources to the children of a different daughters-in-law if the son has co-wives or previous wives. Competition over resources can lead to negative outcomes such as child mortality

(Lahdenperä et al., 2012; Sheppard & Sear, 2016). However, conflict over resources generally resolves in the daughter-in-law’s favor (Cant & Johnstone, 2008; Koster et al., 2019). This makes evolutionary sense for mothers-in-law to concede the resources they think daughters-in- law need to raise children, though there should be disagreement over the amount of resources needed to rear offspring. AFFINAL CONFLICT 9

Daughters- and mothers-in-law can also be in conflict over how their husband/son allocates investment to parties other than co-wives. The daughter-in-law should favor her husband allocating all of his investment to her own children, while the mother-in-law should favor him dividing his investment among the daughter-in-law’s children (Daly & Perry, 2017;

Danielsbacka et al., 2011) his parents, his siblings, his nieces/nephews, or to securing additional mates. Again, these “preferences” need not be conscious, and may often be completely unconscious manifestations of underlying evolutionary interests. Because grandparents may be biased to invest more in their “needier” descendants (Snopkowski & Sear,

2015), daughters-in-law may be able to “win” disagreements over investment by feigning that their children are in need (Busch et al., 2018).

Of the limited literature investigating conflict in affinal relationships, it seems that the domains where affines experience conflict are the very same in which affines experience cooperation. That is, there is some evidence that affines experience conflict over 1) hands-on care of children and other needy kin, 2) giving material resources to kin, and 3) spending time with kids. Conflict in these domains may also influence the quantity, quality, and need for additional social relationships, which in turn may become another source of conflict in these relationships. For example, conflict over childcare may influence the quality of the mother- and daughter-in-law relationship, causing the two to spend less time together, forcing the daughter- in-law to seek out additional social relationships to supplement childcare -- which may in turn worsen her relationship with her mother-in-law. We can also expect that domains such as (new) mate acquisition, mate retention, and self-protection will also be a source of conflict in affinal relationships, as these behaviors can directly interfere with both mother- and daughter-in-law’s fitness interests.

1.4 Current studies

Given the importance that evolutionary social sciences have placed on understanding both mating relationships and kinship, it is surprising that so little research exists at the AFFINAL CONFLICT 10 intersection of these literatures (i.e., on affinal relationships). In order to address the gap in the literature, we first conducted a search for ethnographies published over the past 100 years that mention affinal conflict. Building on the ethnographic literature, we also conducted a study of modern affinal relationships. Specifically, we assessed both cooperation and also conflict among in-laws via an online study with participants from the United States. Because there was more information about female affinal relationships in the literature, and also because prevailing folk knowledge suggests that the mother- and daughter-in-law relationship may be especially fraught with conflict, we focused our literature search and empirical study on relationships between female affines. We hypothesized that women would report more conflict with their mothers-in-law than with their mothers in the U.S. sample. We also hypothesized that mothers- in-law would report more conflict with their daughters-in-law compared to their daughters in the

U.S. sample.

2. Cross-cultural ethnographic data shows that there is conflict among affines

Before we conducted our empirical study of female affinal relationships, we reviewed the ethnographic literature on conflict in affinal relationships. Most of the ethnographic literature focuses on cooperative interactions between female affines, so we conducted this review as a proof of concept that female affinal relationships are also conflictual. More specifically, we conducted this review to show that conflict is reported in the ethnographic record even though there is a bias in reporting to focus on the cooperative and positive aspects of these relationships. We conducted a literature search in Google Scholar for ethnographies published over the last 100 years that discussed conflict between female affines. Undergraduate research assistants who were partially blinded to the hypotheses conducted the literature search under the third author’s supervision. These undergraduates were instructed to look for descriptions of in-law relationships and use search terms such as “mother-in-law” and “conflict” to find the sample of ethnographic literature. This search strategy resulted in a sample of 60 ethnographies. AFFINAL CONFLICT 11

Next, a separate team of undergraduate research assistants who were blind to the hypotheses were instructed to read and summarize each of the ethnographies under the first author’s supervision. Because the topics of the ethnographies were fairly general, the undergraduates were instructed to note instances when the cooperation or conflict was discussed for any affinal relationship. The summaries were completed independently and returned to the first author. After compiling the summaries, we determined that 37 ethnographies contained information relevant to conflict in female affinal relationships. Here we present a summary of the content of these ethnographies (see Table 1).

Table 1. Evidence for mother- and daughter-in-law conflict across societies. Society/ People (Continent) Domain of Conflict Summary of Affine Conflict

Ibibio (Africa) Marriage negotiations Mothers-in-law have conflict with Social role expectations daughters-in-law and her family due to different expectations for the marriage (Charles, 2005)

Nuer (Africa) Food and eating Daughter-in-law cannot eat with her mother-in-law; if mother-in- law dies, then the husband’s oldest sister assumes this role (Evans-Pritchard, 1948)

Zande (Africa) Social role expectations Mother-in-law is seen as the person who resolves the conflict between husband and wife (Evans-Pritchard, 1970)

Sidāma (Africa) Verbal abuse Mother-in-law verbally abuses daughter-in-law and insults her original clan/ family (Goody, 1959)

Gusii (Africa) Marriage negotiations If there are problems in the Resources and economics marriage, wife and her family can prevent divorce by refusing to return bridewealth (Hakansson, 1994)

Ibibio (Africa) Social role expectations Mother-in-law is the first one to know about conflict between son and daughter-in-law (Offiong, 1997)

Tuareg Techawait (Africa) Living arrangements Mother-in-law determines where AFFINAL CONFLICT 12

son and daughter-in-law live after marriage (Rasmussen, 2000)

Najd, Hijaz, and Wahabist Kin care Mother-in-law will nurse infants if peoples in Saudi Arabia (Asia) daughter-in-law leaves the family due to conflict (Altorki, 1980)

Dravidian and Indo-Aryan Family dynamics and status Mothers-in-law always have a peoples of India (Asia) higher status than daughters-in- law (Alvi, 2007)

Malaysian peoples of the Sik Social role expectations Daughter-in-law can never meet region (Asia) “standards of perfection” set by mother-in-law (Banks, 1972)

Burmese (Asia) Family dynamics and status Daughter-in-law serves mother- Social role expectations in-law; mother-in-law controls access to son (Brant & Khaing, 1951)

Pakistani Punjabis (Asia) Family dynamics and status Daughter-in-law is subordinate Social role expectations to mother-in-law and must change to meet her expectations (Charsley, 2005)

Hindu in Rural West Bengal Mother- and daughter-in-law are (Asia) Family dynamics and status in direct competition for the Social role expectations attention/ affection of the son; daughter-in-law is expected to enter the household without “disrupting” the husband’s relationships with his family members (Davis, 1976)

Taiwanese (Asia) Physical abuse Mother-in-law “trains” and Social role expectations “socializes” new daughter-in-law Verbal abuse by treating her “harshly” physically and verbally; older mothers-in-law upset that daughters-in-law are less subservient than in the past (Gallin, 1994)

South Korea, urban middle-class Kin care Mother-in-law and daughter-in- families (Asia) Social role expectations law fight over daughter-in-law working outside of the household and raising the children (Kim, 1996)

Japanese (Asia) Family dynamics and status Mother-in-law controls access to Resources and economics son; mother-in-law controls the Social role expectations household and allows daughter- in-law very little freedom (Lebra, AFFINAL CONFLICT 13

1978)

Khasi (Asia) Family dynamics and status Mothers- and daughters-in-law Social role expectations treat each other poorly; mother- in-law “tests” daughter-in-law to “win” the groom (Leonetti et al., 2007)

North Malayan Teochiu Chinese Family dynamics and status Mother-in-law disagrees with (Asia) Kin care daughter-in-law over her child- rearing practices; mother- and daughter-in-law speak poorly about each other (Newell, 1957)

Contemporary Japan (Asia) Family dynamics and status Disagreements between mothers- and daughters-in-law can be so severe that the daughter-in-law will move away (Nosaka, 2010)

Indian people in the Himalayan Family dynamics and status Daughters-in-law wear veils to village of Ghanyari (Asia) Social role expectations show subordination to their mother-in-law (Sharma, 1978)

Urban North India (Asia) Family dynamics and status Daughter-in-law is expected to Social role expectations be subservient to older women in the household; mother- and daughter-in-law often have conflict over son’s intentions in marriage (Vatuk, 1971)

Indian peoples from rural Tamil Family dynamics and status Mother- and daughter-in-law Nadu (Asia) Physical abuse have conflict over household Social role expectations finances and chores; mother-in- Verbal abuse law controls access to son; mother-in-law ridicules and physically harms daughter-in-law to get her way (Vera-Sanso, 1999)

Chinese (Asia) Family status and dynamics Daughter-in-law must submit to Social role expectations foot binding done by her mother- in-law; mother- and daughter-in- law bicker over housework; mother-in-law is resentful of the daughter-in-law’s relationship with son (Wolf, 1968)

Mehinacu (South America) Social role expectations Daughter-in-law is subordinate Family dynamics and status to mother-in-law at the beginning of their relationship, but this may change as time goes on (Gregor, 1974)

Rural Yucatán (South America) Food and eating Mother-in-law’s acceptance of AFFINAL CONFLICT 14

Marriage negotiations daughter-in-law is dependant on Social role expectations the presentation of food and gifts at ritual ceremonies (Woodrick, 1995)

Eastern Canadian Eskimo Living arrangements Mother- and daughter-in-law (North America) fight if son and daughter-in-law decide to live in a different community (Dunning, 1966)

Teton Dakota tribe (North Avoidance Mother-in-law is in control of the America) Kin care household; daughter-in-law Resources and economics relies on mother-in-law for childcare but also wants to avoid mother-in-law (Hassrick, R. B., 1944)

American middle-class families Kin care Mothers- and daughters-in-law (North America) fight over mother-in-law’s access to children after divorce (Johnson, 1989)

Northwest coast peoples (North Family dynamics and status Mother-in-law will reject potential America) Marriage negotiations daughters-in-law if she has Social role expectations “lazy” brothers; mothers-in-law look down on daughters-in-law if they use contraceptives (Walter, 2006)

Papua New Guinea and Marriage negotiations Mothers-in-law have a lot of traditional Australian peoples influence in marriage (Australia) negotiations (Dickerson-Putman, 1996)

Australian Aboriginals (Australia) Avoidance Mothers-in-law are avoided and viewed negatively by daughters- in-law (Hiatt, 1984)

Bosnian Muslim village (Europe) Family dynamics and status Maidens insult potential Social role expectations mothers-in-law before marriage; Verbal abuse mother-in-law has power over daughter-in-law in the social structure; songs sung by mother-in-law designed to reinforce daughter-in-law’s fears about mother-in-law and test authority; daughter-in-law is expected to accept mother-in- law as a new mother (Lockwood, 1995)

Korowai (Oceania) Avoidance Mother-in-law and daughter-in- Verbal abuse law harshly joke about and avoid one another (Stasch, 2002) AFFINAL CONFLICT 15

Cross-cultural comparisons Resources and economics Daughter-in-law performs Social role expectations domestic labor for mother-in-law; mother-in-law has control over the economic activities of her daughter-in-law (Bossen, 1988)

Cross-cultural comparisons Family dynamics and status Daughter-in-law is subservient to Social role expectations mother-in-law; mother-in-law has power over the daughter-in-law (J. K. Brown et al., 1982)

Cross-cultural comparisons Family dynamics and status Daughter-in-law is subordinate Resources and economics to the mother-in-law; mother-in- Social role expectations law has economic power over daughter-in-law; daughter-in-law “strain” the relationship between mother-in-law and son (Michaelson & Goldschmidt, 1971)

Cross-cultural comparisons Avoidance Mother-in-law relationship is Family dynamics and status more formal in matrilineal Social role expectations societies than in patrilineal societies; mothers-in-law avoided by both sexes in cross- cousin marriages (Goody & Buckley, 1974)

The major pattern that emerged from the ethnographic record is that mothers-in-law often have a lot of control over the social roles and family dynamics of the daughters-in-law.

Mothers-in-law often have control over their daughters-in-law’s households, economic decisions, and access to the daughter-in-law’s husband, for example. When problems in the marriage arise, mothers-in-law are normally the first to know and are the ones responsible for

“fixing” the problems. Mothers-in-law often are verbally and physically abusive towards their daughters-in-law in order to mold their daughters-in-law’s behavior to the mother-in-law’s expectations of her as a wife to her son. When afforded the luxury, daughters-in-law often avoid being in the presence of their mothers-in-law. These patterns of conflict are quite similar to domains reported in the introduction including cooperation and conflict over kin care, resources, investment in children, and mating. Figure 1 represents the number of times each of these domains was mentioned in our ethnographic sample. AFFINAL CONFLICT 16

Figure 1. The number of times different kinds of conflict were mentioned in our sample of ethnographies. Our results show that conflict over social roles, and family dynamics/status were the most common domains of conflict and food/eating, living arrangements, and physical abuse were the least common domains of conflict.

These patterns of conflict between mothers- and daughters-in-law are reported even with the typical emphasis on cooperative and communal relationships between female kin in the anthropological literature. The documented conflict in these societies is similar to the folk knowledge in the United States that mother- and daughter-in-law relationships can be especially tense. But social roles and living patterns have changed drastically over the last century; men in

Western societies have more contact with their affinal kin and women have more contact with their genetic kin compared to men and women in traditional societies (Bielby & Bielby, 1992;

Brandén & Haandrikman, 2019; Donato et al., 2006; Komarovsky & Philips, 1967; Leichter et AFFINAL CONFLICT 17 al., 1967; Reiss, 1962)4. Even so, mothers-in-law still have a large influence on modern women's lives (Apter, 2010; Bryant et al., 2001; Carey, 2018; Hill, 2008).

There are additional factors that can influence conflict between mothers- and daughters- in-law that we were not able to address in the current ethnographic review. The ethnographies that we included from our search span the better part of the last century and, as a result, not all of the ethnographies included relevant demographic information about each culture. Information on subsistence patterns, marital and residence types, inheritance rules, and integration into market systems was not reported on in every ethnography. The omission of this information does not detract from our ability to provide a proof of concept that conflict between female affines in the ethnographic record, the omission does hinder our ability to detect nuances in these findings. Specifically, the omission of these factors across the ethnographies prevents us from being able to draw firm conclusions about how these factors influence conflict between female kin and affines at the times that the ethnographies were conducted. Since all of these factors have the potential to change over time, we also cannot deduce what this information would have been from ethnographies that were published later since we cannot be sure that cultural practices have not changed between the time the original ethnography was written to the time that the new ethnography was written. Future research should look at investigating how the domains of conflict we report here interact with factors such as subsistence patterns, marital and residence types, inheritance rules, and integration into market systems and the resulting kinds of conflict that are reported.

3. Study 1 Method

4 This is an ongoing topic of debate within the anthropological literature. There is variability in post- marriage residential practices. We do not seek to engage in that debate here, though it should be noted that spouses in matrilineal societies have less spousal cooperation (Lowes, 2017). In addition, to the extent that humans have an evolutionary history of engaging in patrilocal residence patterns, it would have been evolutionarily advantageous for men and women to have different psychological mechanisms for interacting with biological and affinal kin depending on residency patterns. It is also possible that men and women both have less contact with their biological kin in the modern world due to relocating for their, or their partner’s, jobs. But there is evidence to suggest that women are more likely to relocate for the relationship than men (Brandén & Haandrikman, 2019). AFFINAL CONFLICT 18

3.1 Participants. Participants (N = 308, 153 males, Mage = 36.24, SDage = 12.18, 75.2%

White/ Caucasian, 67.4% with some college education) were recruited online using two

5 participant recruitment platforms (MTurk: N = 181, 110 males, Mage = 33.73, SDage = 11.34,

71.2% White/ Caucasian, 70.6% with some college education; Prolific: N = 121, 52 males, Mage

= 40.51, SDage = 12.20, 81.8% White/ Caucasian, 64.4% with some college education).

Approximately 85% of participants were currently in a romantic relationship, and approximately

64% were currently married.

All participants answered questions about their relationships with different genetic and affinal family members. We had strict inclusion guidelines for these analyses to ensure that participants had the correct comparison groups to assess perceptions of cooperation and conflict in their relationships. This means that our overall sample includes participants who may not have relationships with genetic family members, in-laws, or both, because not having these relationships did not exclude participants from completing the sections for the relationships they do currently have. We only report on the results for participants who had current relationships with their genetic and affinal mothers and daughters. Participants who indicated that they currently had relationships with both their living mothers and mothers-in-law (N = 87) were included in the analyses about genetic vs. affinal mothers. Participants who indicated that they currently had relationships with their both living daughters and daughters-in-law (N = 16) were included in the analyses about genetic vs. affinal daughters. There were not enough participants who reported having adopted or step mothers or daughters (step-mother N = 3, adoptive mother N = 4; step-daughter N = 5, adoptive daughter N = 5) to meaningfully compare these groups to genetically related mothers and daughters.

We conducted sensitivity analyses to determine the minimum effect size we would be able to detect with our sample sizes. All sensitivity analyses were conducted in G*Power 5 We used two online recruitment platforms because, during the course of data collection, a massive infiltration of bots was discovered (Dreyfuss, 2018). Due to the bot issue, we switched data collection sites to avoid collecting data from bots. We checked our already collected data from MTurk and determined that the data we had already collected did not contain any bots. AFFINAL CONFLICT 19

(Erdfelder et al., 1996) with 0.8 power and ⍺ = 0.05 for a 2 (participant’s sex; between- subjects) x 2 (genetic vs affinal relationship; within-subjects) design. We investigated the sensitivity of our main effects and interactions and report the sensitivity analyses for the interactions here. For our analyses on overall conflict with genetic and affinal mothers, the correlation between repeated measures was

0.74, which suggests that our sample size was large enough to detect effects larger than partial η2 = 0.01. For our analyses on overall conflict with genetic and affinal daughters, the correlation between repeated measures was 0.60, which suggests that our sample size was large enough to detect effects larger than partial η2 = 0.10.

3.2 Measures and Procedures. Participants first answered standard demographic questions and provided names (or initials) of their affinal and genetic kin that were currently living and in contact with the participant. In later questions about these individuals, names or initials were propagated into the question forms (e.g., “How much conflict do you have with your mother-in-law [X] in the following areas of life: ...”). Participants answered questions for the following targets: mother, father, sister, brother, son, and daughter for both genetic kin and in- laws/ affines. Participants were not shown questions about relationships that they indicated they did not currently have. For the purposes of this study, we only report the results of the genetic and affinal mothers and daughters.

Next, participants answered basic questions about these relationships (e.g., how long they have had this relationship), questions about the overall amount of cooperation, domains of cooperation (e.g., cooperation over material resources, kin care, social relationships), the overall amount of conflict, and domains of conflict (e.g., conflict over material resources, kin care, social relationships). Questions are provided in the supplemental materials. Additionally, participants were given open-ended questions for elaboration on the circumstances surrounding cooperation and conflict with each reported relationship partner. The order of the relationships, as well as the cooperation and conflict questions, were completely randomized within-subject. AFFINAL CONFLICT 20

In the cooperation section, participants answered “How much cooperation do you have with X?” to assess the overall amount of cooperation in the relationship using a 1 No cooperation to 7 Very high cooperation likert-type scale. Then, participants indicated the amount of cooperation in their relationships across different fundamental social domains (Kenrick et al.,

2010; Neel et al., 2015). For the purposes of this study, we adapted questions for the domains to include resources (i.e., material resources, daily finances, long-term finances), social relationships (i.e., gaining and maintaining social relationships and social partners), physical safety, social status, romantic partners, and kin care (i.e., caring for kin who are not children/ grandchildren, caring for children/ grandchildren that are not your own, and caring for your own children/ grandchildren). These domains were also selected based on the results of our ethnographic investigation into conflict in affinal relationships.

In the conflict section of the survey, the same sets of questions were asked but framed to be about the overall conflict in the relationship and the amount of conflict in each domain. All questions were answered on a 1 No conflict to 7 Very high/ much conflict likert-type scales.

Participants were then thanked for their time and debriefed regarding the purpose of the study.

Ratio of conflict in interactions. To assess the participant’s perception of conflict and cooperation in their relationships, we calculated the ratio of conflict to the total of both cooperation and conflict (conflict/ (conflict + cooperation)). The ratio allows us to assess if the interactions among the participant and target were characterized primarily by conflict (indicated by scores closer to 1) or cooperation (indicated by scores closer to 0). It was important for us to create ratio scores before comparing the cooperation and conflict between affines and kin, as there is potentially more conflict in close relationships (Fincham et al., 1990), so using the ratio scores allows us to control for potential differences in closeness between genetic and affinal relationships. We created similar ratio scores for the conflict in each distinct domain.

4. Study 1a Results

4.1 Preliminary analysis of raw scores AFFINAL CONFLICT 21

Before conducting our focal analyses, we investigate the base levels of cooperation and conflict in the participant’s relationships with genetic and affinal mothers and daughters using mixed model AONVAs with participant sex as the between subjects factor and cooperation and conflict with genetic and affinal mothers and daughters as the within subject factors. First, we investigated the raw amount of cooperation in the participants’ relationships with their genetic and affinal mothers. We found that both men and women reported more cooperation with their mothers (MMen = 5.33, SDMen = 1.76; MWomen = 4.86, SDWomen = 2.00) compared to their mothers- in-law (MMen = 4.52, SDMen = 1.81; MWomen = 4.16, SDWomen = 2.16), F(1, 84) = 8.77, p = 0.004, partial η2 = 0.004. There was no interaction between participant sex and cooperation with genetic vs. affinal mothers (p > 0.83). It is interesting to note that with both genetic and affinal mothers, men reported more cooperation in their relationships with genetic and affinal mothers compared to the amount of the cooperation women reported in their relationships.

We then investigated the raw amount of conflict in the participant’s relationships with their genetic and affinal mothers. We found that there were no significant differences in the amount of conflict that men and women reported in their relationships with their mothers (MMen =

3.05, SDMen = 1.96; MWomen = 2.76, SDWomen = 1.77) and their mothers-in-law (MMen = 3.02, SDMen

2 = 2.08; MWomen = 2.82, SDWomen = 1.99), F(1, 85) = 0.007, p = 0.93, partial η = 0.00. Similarly, there was no interaction between participant sex and conflict with genetic vs. affinal mothers (p

> 0.86). Again, we found that men reported more conflict with both genetic and affinal mothers compared to the amount of conflict that women reported.

Next, we investigated the raw amount of cooperation in the participants’ relationships with their genetic and affinal daughters. We found that there were not significant differences in the amount of cooperation that mothers and fathers reported in their relationships with their daughters (MMen = 4.43, SDMen = 1.90; MWomen = 5.67, SDWomen = 2.00) and their daughters-in-law

2 (MMen = 4.29, SDMen = 1.80; MWomen = 4.44, SDWomen = 2.46), F(1, 14) = 3.17, p = 0.10, partial η =

0.19. Similarly, there was no interaction between participant sex and cooperation with genetic AFFINAL CONFLICT 22 vs. affinal daughters (p > 0.18). It is interesting to note that with both genetic and affinal daughters, mothers reported more cooperation in their relationships with genetic and affinal daughters compared to the amount of cooperation that fathers reported.

Finally, we investigated the raw amount of conflict in the participants’ relationships with their genetic and affinal daughters. We found that there were not significant differences in the amount of conflict that mothers and fathers reported in their relationships with their daughters

(MMen = 4.86, SDMen = 1.57; MWomen = 2.00, SDWomen = 1.66) and their daughters-in-law (MMen =

2 4.00, SDMen = 2.31; MWomen = 3.11, SDWomen = 2.32), F(1, 14) = 0.04, p = 0.84, partial η = 0.003.

Similarly, there was no interaction between participant sex and conflict with genetic vs. affinal daughters (p > 0.12). It is interesting to note that with both genetic and affinal daughters, fathers reported more conflict in their relationships with genetic and affinal daughters compared to the amount of conflict that mothers reported.

Given the bias found in these analyses, such that men reported more conflict with genetic and affinal mothers and daughters compared to women and women reported more cooperation with genetic and affinal daughters compared to men, we determined that the use of the ratio scores would be necessary in order to control for the biases in reporting perceptions of cooperation and conflict in their relationships. By using the ratio scores, we can control for potential biases that participants have when reporting the cooperation and conflict in their relationships.

4.2 Relationships with mothers vs. mothers-in-law

First, we performed a mixed model ANOVA looking at men’s and women’s (between subjects) perceptions of the ratio of conflict in their relationship with their genetic vs. affinal mothers (within subjects). Men and women reported having significantly more conflict with their mothers-in-law (MMen = 0.44, SDMen = 0.11; MWomen = 0.44, SDWomen = 0.18) than their mothers

2 (MMen = 0.39, SDMen = 0.14; MWomen = 0.40, SDWomen = 0.17), F(1, 84) = 4.43, p = 0.038, partial η = AFFINAL CONFLICT 23

0.05. There was not a significant interaction between participant sex and genetic vs. affinal mothers, F(1, 85) = 0.001, p = 0.977, partial η2 = 0.00.

Figure 2. Both men and women (N = 87) reported that they perceived more conflict in their relationships with their mothers-in-law than in their relationships with their mothers, F(1, 85) =

4.43, p = 0.038, partial η2 = 0.05.

4.3 Relationships with daughters vs. daughters-in-law

Next, we performed a mixed model ANOVA looking at mothers’ and fathers’ (between subjects) perceptions of the ratio of conflict in their relationship with genetic vs. affinal daughters

(within subjects). There was not a statistically significant main effect of genetic vs. affinal daughters, F(1, 14) = 0.74, p 0.405, partial η2 = 0.05. However, there was a marginally significant interaction between participant sex and genetic vs. affinal daughters, F(1, 14) = 3.92, p = 0.068, partial η2 = 0.219. Mothers reported less conflict with their daughters (M = 0.32, SD =

0.17) compared to their daughters-in-law (M = 0.41, SD = 0.18), whereas fathers reported AFFINAL CONFLICT 24 similar levels of conflict with their daughters (M = 0.49, SD = 0.35) and daughters-in-law (M =

0.46, SD = 0.11).

Figure 3. Fathers reported similar levels of conflict in their relationships with their daughters-in- law and daughters while mothers reported more conflict in their relationship with their daughters-in-law compared to their relationship with their daughters (N = 16), F(1, 14) = 3.92, p

= 0.068, partial η2 = 0.219.

5. Study 1b Results

5.1 Overall patterns. Study 1b was an exploratory study aimed at assessing whether the pattern of conflict from study 1a replicated across all fundamental social domains. To do this, we performed mixed model ANOVAs comparing each relationship type (genetic vs. affinal mothers and daughters) in each domain. Across all domains, we replicate the same pattern that we observed in study 1a: Both men and women report more conflict in their relationships with their mothers-in-law compared to their relationships with their genetic mothers; mothers reported more conflict in their relationships with their daughters-in-law compared to their genetic AFFINAL CONFLICT 25 daughters. Figures 3 and 4 show the percentage of conflict in each domain reported by male and female participants (“Ego”) when they are thinking about their relationships with their mother, mother-in-law, daughter, and daughter-in-law. AFFINAL CONFLICT 26 AFFINAL CONFLICT 27 AFFINAL CONFLICT 28

Figure 4. Panel A shows female participants’ responses regarding the percentage of conflict in their relationships with their mothers and mothers-in-law across different domains. We see that, across these domains, women reported a greater percentage of conflict in their interactions with their mothers-in-law compared to their interactions with their mothers. Panel B shows male participants’ responses regarding the percentage of conflict in their relationships with their mothers and mothers-in-law across different domains. Across these domains, men reported a greater percentage of conflict in their interactions with their mothers-in-law compared to their interactions with their mothers. This replicates the results of study 1a. Error bars represent standard errors. N = 87. AFFINAL CONFLICT 29 AFFINAL CONFLICT 30 AFFINAL CONFLICT 31 AFFINAL CONFLICT 32

Figure 5. Panel A shows female participants’ responses regarding the percentage of conflict in their relationships with their daughters and daughters-in-law across different domains. We see that, across these domains, women reported a greater percentage of conflict in their interactions with their daughters-in-law compared to their interactions with their daughters. Panel B shows male participants’ responses regarding the percentage of conflict in their relationships with their daughters and daughters-in-law across different domains. Across domains, men reported a greater percentage of conflict in their interactions with their daughters compared to their interactions with their daughters-in-law. This replicates the results from study 1a. Error bars represent standard errors. N = 16.

5.2 Specific domains of conflict.

5.2.1 Material resources. First, we investigated the ratio of conflict over material resources in these relationships. There was a statistically significant main effect of genetic vs. AFFINAL CONFLICT 33 affinal mothers, F(1, 83) = 4.97, p = 0.028, partial η2 = 0.057. Men and women both reported more conflict with their mothers-in-law (MMen = 0.45, SDMen = 0.16; MWomen = 0.45, SDWomen = 0.19) over gaining and/or maintaining material resources than with their mothers (MMen = 0.39, SDMen =

0.18; MWomen = 0.40, SDWomen = 0.22). There was not a statistically significant interaction between participant sex and genetic vs. affinal mothers, F(1, 83) = 0.08, p = 0.784, partial η2 = 0.001.

There was not a significant main effect of genetic vs. affinal daughters, F(1, 14) = 0.20, p

= 0.666, partial η2 = 0.014, but there was a significant interaction of participant sex and genetic vs. affinal daughters, F(1, 14) = 4.99, p = 0.042, partial η2 = 0.263. Fathers reported more conflict over material resources with their daughters (M = 0.52, SD = 0.05) compared to their daughters-in-law (M = 0.44, SD = 0.13). Mothers reported more conflict over material resources with their daughters-in-law (M = 0.51, SD = 0.24) compared to their daughters (M = 0.38, SD =

0.24). See Figure 5 and 6 for means and standard errors. AFFINAL CONFLICT 34 AFFINAL CONFLICT 35 AFFINAL CONFLICT 36 AFFINAL CONFLICT 37

Figure 6. Panel A shows women’s responses to questions about conflict over finances with their mothers-in-law and mothers and panel B shows men’s responses to questions about conflict over finances with their mothers-in-law and mothers. There was a significant main effect of conflict with biological vs. affinal mothers over material resources, F(1, 83) = 4.97, p = 0.028, partial η2 = 0.057, such that both men and women reported more conflict with their mothers-in- law compared to their mothers. The graph shows that this pattern is observable in other financial domains that did not reach statistical significance. Error bars represent standard errors. N = 87. AFFINAL CONFLICT 38 AFFINAL CONFLICT 39 AFFINAL CONFLICT 40 AFFINAL CONFLICT 41

Figure 7. Panel A shows women’s responses to questions about conflict over finances with their daughters-in-law and daughters and panel B shows men’s responses to questions about conflict over finances with their daughters-in-law and daughters. There was a significant interaction of participant sex and conflict with genetic vs. affinal daughters over material resources, F(1, 14) =

4.99, p = 0.042, partial η2 = 0.263, such that fathers reported more conflict with their daughters and mothers reported more conflict with their daughters-in-law. The graph shows that this pattern is observable in other financial domains that did not reach statistical significance. Error bars represent standard errors. N = 16. AFFINAL CONFLICT 42

5.2.2 Social partners. Next, we investigated the ratio of conflict over gaining/ maintaining valued social partners in these relationships. There was not a significant main effect of genetic vs. affinal mothers, F(1, 84) = 2.53, p = 0.115, partial η2 = 0.029, nor a significant interaction of participant sex and genetic vs. affinal mothers, F(1, 84) = 1.12 p = 0.293, partial η2

= 0.013. This suggests that both men and women perceive similar amounts of conflict with their genetic and affinal mothers over gaining and/ or maintaining valued social partners

There was not a significant main effect of genetic vs. affinal daughters, F(1, 14) = 1.98, p

= 0.181, partial η2 = 0.124. However, there was a significant interaction between participant sex and genetic vs. affinal daughters, F(1, 14) = 7.09, p = 0.019, partial η2 = 0.336. Fathers reported more conflict over gaining and/ or maintaining valued social partners with their daughters (M =

0.48, SD = 0.47) compared to their daughters-in-law (M = 0.44, SD = 0.12). Mothers reported more conflict over gaining and/ or maintaining valued social partners with their daughters-in-law

(M = 0.43, SD = 0.24) compared to their daughters (M = 0.27, SD = 0.17).

5.2.3 Caring for other kin members (not children). We then investigated the ratio of conflict over caring for kin members that are not children. There was a significant main effect of genetic vs. affinal mothers, F(1, 83) = 4.43, p = 0.038, partial η2 = 0.051. Men and women reported more conflict over caring for other family members with their mothers-in-law (MMen =

0.44, SDMen = 0.13; MWomen = 0.45, SDWomen = 0.19) compared to with their mothers (MMen = 0.41,

SDMen = 0.17; MWomen = 0.38, SDWomen = 0.19). There was not a significant interaction between participant sex and genetic vs. affinal mothers, F(1, 83) = 0.45, p = 0.505, partial η2 = 0.005.

There was not a significant main effect of genetic vs. affinal daughters, F(1, 14) = 0.09, p

= 0.766, η2 = 0.007. However, there was a significant interaction of participant sex and genetic vs. affinal daughters, F(1, 14) = 8.33, p = 0.012, partial η2 = 0.373. Fathers reported more conflict over caring for other family members with their daughters (M = 0.47, SD = 0.09) compared to their daughters-in-law (M = 0.40, SD = 0.12). Mothers reported more conflict over AFFINAL CONFLICT 43 caring for other family members with their daughters-in-law (M = 0.46, SD = 0.21) compared to their daughters (M = 0.37, SD = 0.16). See Figures 7 and 8 for means and standard errors.

5.2.4 Caring for biological children/grandchildren. We then investigated the ratio of conflict over caring for one’s own genetic children/ grandchildren. There was a significant main effect of genetic vs. affinal mothers, F(1, 84) = 6.83, p = 0.011, partial η2 = 0.075. Men and women reported more conflict with their mothers-in-law (MMen = 0.43, SDMen = 0.15; MWomen =

0.47, SDWomen = 0.21) over caring for their own children and grandchildren compared to their genetic mothers (MMen = 0.38, SDMen = 0.16; MWomen = 0.40, SDWomen = 0.20). There was not a significant interaction of participant sex and genetic vs. affinal mothers, F(1, 84) = 0.11, p =

0.736, partial η2 = 0.001. There was not a significant main effect or interaction of conflict over caring for their own children and grandchildren for genetic vs. affinal daughter analysis. See

Figures 7 and 8 for means and standard errors. AFFINAL CONFLICT 44 AFFINAL CONFLICT 45 AFFINAL CONFLICT 46 AFFINAL CONFLICT 47

Figure 8. Panel A shows women’s responses to questions about conflict over kin care with their mothers-in-law and mothers and panel B shows men’s responses to questions about conflict over kin care with their mothers-in-law and mothers. There was a significant main effect of genetic vs. affinal mothers over other kin care (not including children), F(1, 83) = 4.43, p =

0.038, partial η2 = 0.051, and over caring for biological children/ grandchildren, F(1, 84) = 6.83, p = 0.011, partial η2 = 0.075, such that both men and women reported more conflict with their mothers-in-law than their mothers over caring for biological children/ grandchildren and other forms of kin care not involving children. Error bars represent standard errors. N = 87. AFFINAL CONFLICT 48 AFFINAL CONFLICT 49 AFFINAL CONFLICT 50 AFFINAL CONFLICT 51

Figure 9. Panel A shows women’s responses to questions about conflict over kin care with their daughters-in-law and daughters and panel B shows men’s responses to questions about conflict over kin care with their daughters-in-law and daughters. There was a significant interaction of participant sex and genetic vs. affinal daughters over kin care that did not involve children, F(1,

14) = 8.33, p = 0.012, partial η2 = 0.373, such that mothers reported more conflict with their daughters-in-law than their daughters over kin care that did not involve children and fathers reported more conflict with their daughters than their daughters-in-law over kin care that did not involve children. The graph shows that the pattern of men having more conflict with their genetic daughters in these kin care domains is present even if the analyses did not reach statistical significance. Error bars represent standard errors. N = 16. AFFINAL CONFLICT 52

5.2.5 Other domains. We also investigated the ratio of conflict over long-term finances, gaining and maintaining social relationships, physical safety, gaining social status, gaining new romantic partners, maintaining current romantic partners, and caring for children and grandchildren that are not yours in each of these relationships. We did not find significant main effects or interactions in any of these domains. However, even though these comparisons did not reach statistical significance, the trends were in the predicted direction. Both men and women reported more conflict with their mothers-in-law compared to their mothers in these domains, and mothers reported more conflict with their daughters-in-law compared to their daughters in these domains.

6. Study 1c results

Next, conducted an exploratory investigation of the instances of conflict that participants reported in the open-ended questions. Specifically, we aimed to explore whether the themes that we identified in our ethnographic survey (e.g., social role expectations, family dynamics and status, resources and economics, living arrangements) were also reflected in the conflict that

US participants reported in their affinal relationships.

To assess this, we had undergraduate research assistants (all blind to hypotheses) code the open-ended responses using the domains of conflict identified in our ethnographic survey.

Research assistants were instructed to code each response with the most representative category. Responses to each question were rated by three research assistants. Research assistants first coded the responses independently and then, if there were any disagreements between codings, discussed the coding with the other raters until all raters agreed on the final coding. We present results from the subset of participants who did respond to the open-ended questions (N = 65)6.

6 Before completing the quantitative assessments of cooperation and conflict, participants were presented with the following prompt with the name of the relationship target piped in from the participants’ earlier responses: “We are interested in the conflict that you have with your AFFINAL CONFLICT 53

First, we report on the domains of conflict involving mothers- and daughters-in-law. To conceptually replicate the style of the ethnographies, reports of conflict with both affine categories are included here in a single graph (see Figure 10). The most common source of conflict between mothers- and daughters-in-law was in regards to family dynamics and status, and the least common sources of conflict were marriage expectations and verbal abuse. These results are similar to the ethnographic results (but not identical); this suggests that the domains of conflict we identified from the ethnographies are also representative of the kinds of conflict that US participants report, thereby implying some cross-cultural similarity in affinal conflict domains.

Figure 10. Among mothers and daughter-in-laws in our Western sample, conflict in the domain of family dynamics/status was mentioned most often in qualitative responses, followed by conflict over kin care. mother-in-law [daughter-in-law, mother, daughter] X. Is there anything you'd like to share about the circumstances where you have conflict with your mother-in-law [daughter-in-law, mother, daughter] X and what it's like?” As responses to these qualitative questions were not required, many participants chose not to answer these questions. Here we present the results from those who did answer these questions as a method to document that the domains of conflict from the ethnographic record are the same domains of conflict that our Western sample of participants experience. AFFINAL CONFLICT 54

Next, we asked our participants to report on the domains of conflict they experience with their biological mothers and daughters, using the same format as above. By including conflict between mothers and daughters we can determine if our domains of conflict extend to biological relationships as well. Again, the most common source of conflict in relationships between mothers and daughters involves family dynamics and status, and the least common sources of conflict are about avoidance, food and eating, and marriage negotiations. The fact that the domains of conflict in mother and daughter relationships is similar to the domains of conflict in mother- and daughter-in-law relationships, something that we did not predict but isn’t unexpected, further suggests that the domains of conflict we identified in the ethnographies are representative of the kinds of conflict our participants experience in their daily lives.

Figure 11. Among mothers and daughters in our Western sample, conflict in the domain of family dynamics/ status was mentioned most often in qualitative responses from our Western participants, followed by conflict over social role expectations and verbal abuse. AFFINAL CONFLICT 55

We also note one additional and unpredicted, interpretation of these qualitative results. As discussed in the method section, we decided to create a ratio score to assess the amount of conflict that participants reported in their relationships. We made this decision to control for the fact that there may be more conflict in close relationships (Fincham et al., 1990) that biases participants to report more conflict with biological kin. The results of our investigation into the raw scores as well as our qualitative analyses presented in this section suggest that this is indeed the case. Participants, in general, were more likely to talk about conflict with their biological mothers and daughters compared to their conflict with their mothers- and daughters- in-law. Taking these two factors into consideration, it becomes clear that researchers studying conflict and cooperation in kin relationships need to control for this bias in reporting in order to fully understand the nuances in genetic and affinal relationships.

7. Discussion

We explored a relatively understudied topic in evolutionary social science--affinal bonds, and particularly conflict in mother- and daughter-in-law relationships. We provide an evolutionary-minded rationale behind the conflict between mothers- and daughters-in-law, review the cross-cultural ethnographic literature of mother- and daughter-in-law conflict, and provide new data from a modern, Western sample on perceived conflict (and cooperation) in both affinal and biological relationships.

Results from those novel data suggest that there are important differences in perceptions of conflict with affines as compared to genetic relatives: Both men and women reported having more conflict with their mothers-in-law than their mothers, and mothers indicated having more conflict with their daughters-in-law than their daughters. These novel findings are consistent with both (a) the conflict reported in qualitative, cross-cultural ethnographic data (Table 1) and (b) also past research indicating that mother- and daughter-in- law conflict may be a category of conflict with important fitness implications (Sear, 2008; Sear &

Mace, 2008). AFFINAL CONFLICT 56

Additionally, we found evidence that fathers indicated having more conflict with their daughters than their daughters-in-law. This finding was not expected, but could be attributable to paternity uncertainty. The more uncertain a man is about his paternity, the less likely he might be to invest in his children (Alvergne et al., 2009) and grandchildren (Michalski & Shackelford,

2005), and this non-investment could facilitate conflict with children (daughters or sons). If a father is uncertain about his putative daughter’s paternity, he may experience more conflict with her than he does with his daughter-in-law since he does not question her paternity and has not invested his resources in her for her entire life. While one may think that a father should have less conflict with his daughter even if he is uncertain about her paternity due to the sunken costs of previous investment, it is also possible that this uncertainty would lead him to stop investing once his daughter can care for herself to avoid investing additional resources into a child and potential grandchildren that are not genetically related to him. This prediction also implies that fathers should report more conflict with their sons compared to their sons-in-law. While we do not report the relationship here, our preliminary data does suggest that men report more conflict with their sons compared to their sons-in-law. It is also possible that fathers have more conflict with their daughters compared to their daughters-in-law because fathers are engaging in daughter guarding (Flinn, 1988; Perilloux et al., 2008; Sundin & Shackelford, 2019), behavior whereby fathers protect their daughters’ sexual reputation and mate value by influencing their daughters’ mating decisions (e.g., who is and is not an appropriate long-term partner). The reported conflicts (kin care and resources) we see in our data may represent domains where fathers attempt to change their daughters’ behavior to be more “favorable” for the fathers’ interests.

These differences in cooperation and conflict between genetic and affinal kin relationships may be driven by genetic conflict: affinal relationships manifest elements of genetic conflict because affines have genes in common with one parent and may unconsciously act in the interest of their genetic kin. The lack of shared genes between affines and their non-genetic AFFINAL CONFLICT 57 kin may lead to conflict when the actions of affines come at a detriment to their non-genetic kin.

Mothers- and daughters-in-law may disagree about the distribution of the son’s/ husband’s resources and investment in children, just as we see mothers and fathers disagreeing in these domains. But affines should also disagree about other domains as well, such as gaining new social relationships, maintaining current social relationships, gaining new romantic partners, and maintaining current romantic relationships as relationships in these domains can directly influence both the mother- and daughter-in-law’s fitness outcomes. Our results are consistent with the hypothesis that genetic conflict may underlie some of the negative social interactions that can occur in affinal relationships.

There could be more proximal reasons for conflict between affines, which may themselves be a product of ultimate explanations (e.g., genetic conflict). As with our genetic kin, our relationships with affines are generally relationships that we cannot pick -- affinal relationships are an unintended consequence of mating pair-bonds. But unlike relationships with our genetic kin, affinal bonds may possess more underlying fitness conflicts than shared fitness interests, suggesting that affinal relationships represent a separate category of social relationships with the potential for both extreme conflict and cooperation without the element of choice. This may be particularly true for the mother- and daughter-in-law relationship. For example, mothers- and daughters-in-law do not choose to have relationships with one another and instead have relationships with one another as a byproduct of their unique relationships with the mother-in-law’s son/ daughter-in-law’s husband. Due to this, mothers- and daughters- in-law do not form relationships based on their shared interests in multiple domains and have relationships that are more conflictual. In contrast to this, mother and daughter relationships are other kin relationships that are not “chosen” in the traditional sense but build off of years of living and working together that allows for both parties to figure out how to realize the benefits of cooperation and avoid or manage conflict. AFFINAL CONFLICT 58

Some interesting patterns emerge when we look at the ethnographic data and qualitative responses from Western participants. The most striking patterns is that family dynamics/status are reported to be a high source of conflict between mother- and daughter-in-laws, both in our sample from the ethnographic literature (Morr Serewicz & Canary, 2008; Prentice, 2008; Rew et al., 2013; Silverstein, 1992) and in the responses from Western participants (Morr Serewicz &

Canary, 2008; Prentice, 2008; Rew et al., 2013; Silverstein, 1992). It is not surprising that family dynamics/status was a common area of conflict in both the ethnographic literature and Western participants, as conflict about family dynamics and status have been reported in many societies

(Morr Serewicz & Canary, 2008; Prentice, 2008; Rew et al., 2013; Silverstein, 1992)

(Morr Serewicz & Canary, 2008; Prentice, 2008; Rew et al., 2013; Silverstein,

1992)Social roles are generally related to expectations about family dynamics (Kiecolt & Acock,

1988; Marks et al., 2009; Oláh et al., 2018), and so it is possible that there is less conflict in our

Western sample because in our Western participants are less likely to live in extended family households (Georgas, 2011; Georgas et al., 2001)more economic freedom and leads to lower levels of financial interdependence (Reyes, 2018), which might lead to less conflict over social roles.

However, the remaining categories in the top five most common sources of conflict are different between the ethnographic societies and Wesntern participants. The ethnographic data indicated that marriage negotiations, verbal abuse, and resources and economics were the next most common sources of conflict. On the other hand, western participants indicated that kin care, avoiding interacting, and conflict over living arrangements were the next most common sources of conflict between mothers- and daughters-in-law.

7.1 Limitations

We bring together the evolutionary logic of in-law conflict with qualitative ethnographic literature and new empirical data to explore the intersection of mating relationships and kinship. AFFINAL CONFLICT 59

The study design has some limitations, however. To be included in analyses, participants had to have current relationships with both living genetic and affinal kin. This resulted in many participants being excluded. The exclusion of these participants might cause under- representation of high conflict relationships. For example, one reason participants may no longer have these relationships could be due to extreme conflict. Excluding these participants may systematically bias our results to show more cooperation (via lower conflict ratios) across all relationships. Future work could address this potential confound by asking about relationships with genetic relatives and in-laws who are currently living, even if the participant is no longer in contact with them.

Another limitation to the current design was that questions were not designed to address the myriad nuances in cooperation and conflict with genetic and affinal kin. For example, although we asked how much cooperation and conflict was in the relationship (i.e., frequency), we are not able to assess how intense this cooperation and conflict was (i.e., intensity).

Therefore we have no way of differentiating between participants who have frequent but mild conflict in their social relationships from participants who have rare but intense conflict. This limitation would also apply to the intensity and frequency of cooperation in these relationships.

Future research should tease apart the intensity and frequency of cooperation and conflict in these relationships.

We investigated cooperation and conflict across some of the fundamental domains of social life (Kenrick et al., 2010). However, the questions did not assess all factors within each domain. We did not, ask, for example, about reproductive decision-making, which is a domain in which conflict between affines may be high. Women’s affines may often prefer a woman to have more offspring than is in her individual fitness evolutionary interests, leading to familial conflict and negative health outcomes for women (Penn & Smith, 2007). Thus, although we did ask participants how much cooperation and conflict they experience over caring for genetic children/ grandchildren, non-genetic children/ grandchildren, and kin who are not children (aunts, uncles, AFFINAL CONFLICT 60 siblings, etc.), we did not ask participants about whether there was conflict with affines over the questions of if and when to have children.

7.2 Future directions

There are still unanswered questions regarding differences in cooperation and conflict in genetic and affinal kin relationships. For example, we dichotomized relatedness here into biologically related kin or presumably not biologically related affines. Yet when affines are more strongly genetically related (as in cross-cousin marriages), this might lead to lesser conflict between affines. This prediction is derived from Hamilton’s (Hamilton, 1964) rule, where increasing genetic relatedness should increase cooperativeness between individuals. However, the incidence of genetic diseases goes up when close relatives reproduce together, leading to other fitness costs (Antfolk et al., 2012).

Additionally, we would expect that conflict between mothers- and daughters-in-law would be moderated by the mother-in-law’s certainty that her son is the father of the daughter-in-law’s children. We see evidence for this with grandparents, where 1) maternal grandparents invest more in their daughter’s children than their son’s children because there may be uncertainty regarding the children’s paternity (Daly & Perry, 2017; Danielsbacka et al., 2011) and 2) grandmother’s invest more in their grandchildren than grandfathers because grandmothers can be sure of their relatedness to their own children (M. S. Smith, 2017). Conflict should lessen as the daughter-in-law and son have more offspring, as this increases perceptions of fitness interdependence between mother- and daughter-in-law.

Additionally, differences in cooperation and conflict between genetic and affinal siblings should be examined in future studies. In our review of the ethnographic literature, we discovered several mentions of cooperation and conflict with sisters- or brothers-in-law beyond mention of cooperative relationships between male affines (Macfarlan et al., 2018). However, genetic and affinal sibling relationships should still embody aspects of both cooperation and conflict as genetic siblings may compete for investment from parents (Dixon & Stein, 2006) and cooperate AFFINAL CONFLICT 61 with one another to increase inclusive fitness (Pollet, 2007), while affinal siblings may help protect mateships (Pham et al., 2014) and limit investment in new affines until there is shared interest in common descendants.

Finally, one thing that we were not able to assess in this study was how multiple demographic factors can further exacerbate cooperation and conflict. Ideally, future work will investigate additional factors that could influence conflict with affines. For example, it is possible that women with higher socioeconomic status experience less conflict over monetary resources with their mothers- and daughters-in-law because it is less likely that these individuals will experience trade-offs between their own monetary needs and those of their mother- or daughter-in-law. Other demographic factors, such as age, education, distance between their homes, and presence of other adults in the home could also increase or decrease cooperation and conflict. However, given that our sample of ethnographies did not report all of this information and our fairly homogenous sample, we were not able to investigate the effects of relevant demographic factors (e.g., residence patterns, socioeconomic status). Future studies could employ computer-assisted coding when coding the ethnographies to gather information missing from any single ethnography and address how these demographic factors influence cooperation and conflict with affines. Similarly, future studies should focus on collecting a more heterogeneous sample so the influence of factors such as socioeconomic status can be investigated.

7.3 Conclusions

The goal of the current work was to investigate the understudied intersection of kin and mating relationships -- affinal relationships. We aimed to provide an evolutionary rationale behind the conflict between mothers- and daughters-in-law, review the ethnographic literature of mother- and daughter-in-law conflict, and provide new data from a modern, Western sample.

Results indicated that conflict between mothers- and daughters-in law replicated: women reported that they have more conflict with their mothers-in-law than with their mothers, and AFFINAL CONFLICT 62 mothers reported that they have more conflict with their daughters-in-law compared to their daughters. Interestingly, we also found evidence that men reported that they have more conflict with their mothers-in-law, and fathers report that they have more conflict with their daughters than their daughters-in-law. Findings suggest that more nuanced investigations into genetic and affinal kin relationships are needed to understand the full complexity of cooperation and conflict in these relationships. AFFINAL CONFLICT 63

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Supplemental materials Questionnaire for mother-in-law target. Participants responded to the same set of questions for each relationship they reported. Participants were asked to report the target’s name so it could be propagated into later questions. Italicized words represent wording changes between targets.

What is your mother-in-law’s name? Please provide her first name.If you have more than one, please report your current or most recent mother-in-law. ______

How long have you had this relationship (in years)? ______

How much conflict do you have with your mother-in-law [mother-in-law’s name]? No conflict Very high conflict

1 2 3 4 5 6 7

How much conflict do you have with your mother-in-law [mother-in-law’s name] in the following areas of life:

To gain and/or maintain maternal resources, such as money No conflict Very high conflict

1 2 3 4 5 6 7

To help with daily financial needs No conflict Very high conflict

1 2 3 4 5 6 7

To help with long-term financial needs No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain social relationships, such as friendships No conflict Very high conflict

1 2 3 4 5 6 7 AFFINAL CONFLICT 78

To ensure physical safety No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain valued social partners No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain social status No conflict Very high conflict

1 2 3 4 5 6 7

To attract NEW desirable sexual or romantic partners No conflict Very high conflict

1 2 3 4 5 6 7

To maintain a CURRENT long-term romantic relationship No conflict Very high conflict

1 2 3 4 5 6 7

To help care for family members (but NOT children or grandchildren) No conflict Very high conflict

1 2 3 4 5 6 7

To help care for children (but NOT your own children or grandchildren) No conflict Very high conflict

1 2 3 4 5 6 7 AFFINAL CONFLICT 79

To help care for YOUR OWN children or grandchildren No conflict Very high conflict

1 2 3 4 5 6 7

How much do you cooperate with your mother-in-law [mother-in-law’s name]? No conflict Very high conflict

1 2 3 4 5 6 7

How much do you cooperate with your mother-in-law [mother-in-law’s name] in the following areas of life:

To gain and/or maintain maternal resources, such as money No conflict Very high conflict

1 2 3 4 5 6 7

To help with daily financial needs No conflict Very high conflict

1 2 3 4 5 6 7

To help with long-term financial needs No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain social relationships, such as friendships No conflict Very high conflict

1 2 3 4 5 6 7 AFFINAL CONFLICT 80

To ensure physical safety No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain valued social partners No conflict Very high conflict

1 2 3 4 5 6 7

To gain and/or maintain social status No conflict Very high conflict

1 2 3 4 5 6 7

To attract NEW desirable sexual or romantic partners No conflict Very high conflict

1 2 3 4 5 6 7

To maintain a CURRENT long-term romantic relationship No conflict Very high conflict

1 2 3 4 5 6 7

To help care for family members (but NOT children or grandchildren) No conflict Very high conflict

1 2 3 4 5 6 7

To help care for children (but NOT your own children or grandchildren) No conflict Very high conflict

1 2 3 4 5 6 7 AFFINAL CONFLICT 81

To help care for YOUR OWN children or grandchildren No conflict Very high conflict

1 2 3 4 5 6 7