Running head: ATTACHMENT SYSTEMS 1

Manuscript in press at Social and Personality Psychology Compass, accepted September 19, 2020

Please cite as: Barbaro, N. (in press). The Nature of Attachment Systems. Social and Personality Psychology

Compass.

The Nature of Attachment Systems

Nicole Barbaro

Oakland University

Main text: 5,111 words

Correspondence: Nicole Barbaro, Oakland University, Department of Psychology, 654 Pioneer Drive, Rochester,

MI 48309. Email: [email protected] ATTACHMENT SYSTEMS 2

Abstract

Attachment theory is an enduring and generative framework for understanding infant and romantic relationships.

As a lifespan phenomenon, attachment research has focused largely on developmental processes and how to link the infant attachments with later close relationships, but there has been comparably less scholarly attention targeted at the evolutionary origins and functionality of romantic attachments, in particular. Here, I advance a two-system approach to attachment, proposing that infant attachments and romantic attachments constitute etiologically distinct systems that evolved in response to different selection pressures, serve different evolutionary functions, and are fundamentally different in nature with regard to operation and necessity toward their respective evolutionary goals. This two-system approach has downstream implications for future attachment research, most notably with regard to how the romantic attachment system develops over the lifespan, and the extent to which romantic attachments guide the formation of relationships in adulthood.

Keywords: ; evolution; mate retention; development; close relationships; romantic attachment. ATTACHMENT SYSTEMS 3

The Nature of Attachment Systems

Attachment theory (Bowlby, 1969, 1982) has remained an enduring and popular framework for understanding close interpersonal relationships across the lifespan, namely with parents and romantic partners

(Fraley, 2019). The two primary foci of attachment theory – infant-caregiver relationships and close romantic relationships – have received enormous empirical and theoretical attention in recent decades. There is little scholarly debate about the nature and development of infant attachment bonds. There is, however, active debate about the nature and development of adult attachments with romantic partners (see Barbaro et al., 2017; Fearon &

Roisman, 2017; Fraley & Roisman, 2019; Verhage et al., 2016). The primacy of these debates center on how adult attachments develop, and how to understand individual variation in adult attachments (see Fraley, 2019 for an excellent review of the primary issues). Less attention has been paid on the evolution of adult attachments with romantic partners, however. Although rich conversation existed at the formal inception of adult attachment theory

(Hazan & Diamond, 2000; Hazan & Shaver, 1987; Kirkpatrick, 1997), targeted attention to these issues has declined in the last decade (cf. Del Giudice, 2009), despite many interesting issues remaining unresolved.

The purpose of this article is to focus on unresolved issues with regard to romantic attachments, in particular, and their relationship with infant-caregiver attachments. Here, I will argue that ‘attachment’ is comprised of two etiologically distinct attachment systems: the infant attachment system and the romantic attachment system. From this perspective I specifically focus on the following issues:

(1) The evolutionary foundations of adult romantic attachments. What was the primary selection pressure

that prompted the evolution of the romantic attachment system?

(2) The current functionality of the romantic attachment system. How does the romantic attachment

system operate, and how does it manifest in relationships?

(3) The fundamental differences between infant attachment bonds and romantic attachment bonds. How

are the two systems distinct, given the above two premises?

An attachment systems perspective, advanced here, proposes that the infant attachment system and the romantic attachment system are distinct suites of psychological adaptations arising over deep evolutionary history. The etiology of each system must be viewed as conceptually independent in that each system has evolved ATTACHMENT SYSTEMS 4 in response to different selection pressures; therefore, the infant attachment system and the romantic attachment system each solve unique adaptive problems. The etiology of the infant attachment system within this framework accords with consensus view in the field: infant attachment to a primary caregiver (evolutionarily, the mother) ultimately solves the problem of the infant’s own survival given altricial features of human infants. The etiology of the romantic attachment system, however, has been subject to great debate as to what prompted romantic attachment to evolve, and what evolutionary purpose it serves.

Etiology of the Infant Attachment System

Bowlby (1969, 1982) postulated that infants had an innate motivation to form attachment bonds with their primary caregiver (most often, evolutionarily the mother). Infant attachments function as a means to increase the infant’s proximity to the mother, thereby increasing protection and conferring a reliable survival advantage for the offspring. Natural selection therefore favored the evolution of parental attachment bonds in humans, consequently resulting in a biological predisposition for human infants to form an attachment to their mother. The general consensus being, that, attachment bonds between infants and primary caregivers (or, at least, the motivation to form an attachment) is an innate and normative characteristic of human development.

It is not only human infants that form maternal attachments after birth, however. Infant attachments are ubiquitous across species for which offspring survival is dependent upon some degree of care from (typically) the mother (Insel & Young, 2001). In other ape species (our closest living genetically related relatives) infants form strong attachments to their mothers after birth. Infant attachments in apes and other mammalian species have a deep evolutionary history. Species for which maternal care is necessary for survival via, namely, feeding and protection, natural selection has favored motivational systems that facilitate proximity maintenance—a fundamental feature of attachment bonds (Cassidy, 2016). Infant-parent attachment bonds therefore did not evolve over human evolution specifically, but rather are a consequence of mammalian and primate evolution more generally.

Pioneering research demonstrated that infant attachment bonds not only function to motivate fundamental attachment behaviors, such as feeding and protection, but that attachment bonds are imperative for normative social and emotional development as well (Ainsworth, 1964; Ainsworth et al., 1978; Harlow & Zimmermann, ATTACHMENT SYSTEMS 5

1959). Socioemotional development in infants and young children is certainly influenced by the quality of the attachment bond with the primary caregiver, with the most well-investigated qualitative feature being maternal sensitivity (Verhage et al., 2016). Yet, socioemotional development is unlikely to be the primary selection pressure prompting the evolution of the infant attachment system. Feeding and protection needs of infants as a direct result of infant attachment bonds are both necessary and sufficient for infant survival, and thus, the evolutionary origins of the infant attachment bond; whereas the socioemotional benefits are arguably necessary now, but are not sufficient. The etiology of the infant attachment system in humans and other primates is therefore most likely to be the result of direct survival benefits conferred to offspring over deep evolutionary history, maintained and refined via socioemotional benefits across (human) evolutionary history.

Etiology of the Romantic Attachment System

Interest in the study of adult attachment was based on Bowlby’s proposition that early attachments have enduring influence “from the cradle to the grave.” Adult attachments were originally assumed to be a natural and self-evident developmental consequence of infant attachment relationships, and a normative aspect of human development. Working under this assumption, characteristics of infant attachments were extrapolated to adult relationships. Adult attachment theory (Hazan & Shaver, 1987) initially held the central propositions that: infant attachments and romantic attachments are governed by the same system (the attachment-behavioral system as proposed by Bowlby); variation, or categories, of romantic attachments mirrored those of infant attachments; and, internal working models of attachment developed via infant-caregiver interactions are guiding, endogenous influences on adult attachment relationships. Whereas the foundational working assumption of contemporary attachment theory is that a single attachment system operates throughout development (Bowbly, 1969, 1982;

Hazan & Shaver, 1987), there remain debates on how focal attachment figures shift and change across development (e.g., Carli et al., 2019; Rowe & Carnelley, 2005; Umemura et al., 2017). Theorizing on adult attachment largely sidestepped investigations of unique evolutionary etiology in favor of focusing on parallels between infant and adult attachments across the lifespan, and on identifying the developmental mechanisms by which infant attachments influenced later adult attachments. ATTACHMENT SYSTEMS 6

Comprehensive theoretical accounts of adult attachment etiology, however, require evolutionary explanations for the existence of romantic attachment bonds. Put differently, the evolutionary benefits of romantic attachments are not self-evident in the same ways for which the evolutionary benefits of an infant’s attachment to a parent are clear. Rather than assuming romantic attachments are an enduring consequence of infant attachments, etiological accounts for romantic attachments must explain: (1) the existence of romantic attachment bonds – what selection pressure could have prompted the evolution of the romantic attachment system or, alternatively, whether adult attachment is better conceptualized as a byproduct of infant attachment; (2) the functionality of romantic attachments, or how the romantic attachment system in human relationships operates and manifests; and

(3) the observation that romantic attachments are not necessary or sufficient for successful reproductive relationships.

Hypothesized Evolutionary Origins. Several hypotheses have been proposed for why romantic attachments evolved. The neoteny or developmental immaturity hypothesis proposes that attachments persist into adolescence and adulthood in humans because humans have an unusually prolonged juvenile period (see

Brumbaugh & Fraley, 2006; Fraley & Shaver, 2000). From this perspective, the attachment system fails to “turn off” as it does in other mammalian species for which adult pair-bonds are not formed. Because the attachment system does not turn off, it remains reactive to similar cues as it did in infancy, but the attachment figure shifts throughout development (see Del Giudice, 2009; Umemura et al., 2017). This view accords with the assumption of a single attachment system, and furthers the perspective of necessary and inherent similarities between infant and romantic attachments (Fraley & Shaver, 2000; Hazan & Diamond, 2000). If the long-term activation of the attachment system in humans is a result of the characteristic prolonged juvenile period in humans, however, then adult attachment could more parsimoniously be viewed as largely a developmental byproduct of infant attachments.

The byproduct perspective of adult attachment etiology, however, appears incompatible with a large and growing body of work suggestive of independent and specialized function of adult attachment (e.g., Barbaro et al.,

2016a, 2019; Kim et al., 2018; Kruger et al., 2013; Overall et al., 2014). Adult attachment theory (Hazan &

Diamond, 2000; Fraley & Shaver, 2000; Shaver et al., 1988; Zeifman & Hazan, 2016) has since culminated on the ATTACHMENT SYSTEMS 7 view that romantic attachments ultimately evolved to cement or maintain pair-bonds between romantic partners. It is argued that romantic attachments within pair-bonds contribute to fitness via provisioning of offspring, thereby increasing the resources invested in offspring and, for men, increasing assurance of resource provisioning to genetically related offspring—referred to as the paternal care or male provisioning hypothesis (Fraley & Shaver,

2000; see also Lancaster & Lancaster, 1987; Lovejoy, 1981). Brumbaugh and Fraley (2006) provided phylogenetic support for the paternal care hypothesis by showing that the evolution of paternal care covaries with the evolution of pair-bonding across the primate order; and, that because paternal care had earlier evolutionary origins than the other traits investigated – such as neoteny – the results therefore suggested that the evolution of paternal care played an important role in the evolution of adult attachments.

Consensus view within psychology supposes that romantic attachments co-evolved with increases of male provisioning to ensure offspring survival to reproductive age (Brumbaugh & Fraley, 2006; Finkel & Eastwick,

2015; Fraley & Shaver, 2000; Hazan & Diamond, 2000; Kirkpatrick, 1998; Simpson & Belsky, 2016). The male provisioning hypothesis for romantic attachment is circular, however. The hypothesis supposes that male provisioning made pair-bonding more beneficial due to increased resources (e.g., food) provisioned to offspring.

For males, they would be more certain of their resource investment into an offspring if bonded with the mother, and for females, they would have a reliable source of male investment into their offspring if bonded. The next question is, then, why did males begin bonding with females. The answer to which is that males could then more judiciously allocate resources to genetically related offspring if they were pair-bonded. Therefore, the evolutionary reasons for each – male-provisioning and pair-bonding – both rely on each other as explanations.

The male-provisioning hypothesis, then, still leaves unanswered the question of what selection pressure prompted the evolution of pair-bonds, thus also neglecting the question of evolutionary etiology of romantic attachment.

Theoretical work on the evolutionary origins of pair-bonding and consequent romantic attachment bonds have thus far been parochial by neglecting comparative analyses across the primate order (cf. Brumbaugh &

Fraley, 2006), viewing the benefits of paternal care from an anthropocentric perspective (e.g., greater paternal care is associated with lower depression levels in adolescents thereby rendering paternal care beneficial), and ATTACHMENT SYSTEMS 8

WEIRD perspective by not taking into account that intensive male provisioning and its impact are highly variable across cultures (Fernandez-Duque et al., 2009; Sear & Mace, 2008).

The evolution of romantic attachments is dependent upon the evolution of pair-bonding. Any theory concerning evolution of romantic attachment must then subsume two broad explanations: (1) an explanation for why the trait originated over evolutionary history (i.e., the selection pressures prompted the evolution of the trait), and (2) an explanation for the how the trait has been maintained over time (i.e., the operation of the trait; how it manifests). Although previous attachment scholars have discussed potential evolutionary hypotheses for the evolution of romantic attachments (see Brumbaugh & Fraley, 2006 for an excellent review of psychological perspectives), distinction between selection pressures and maintenance mechanisms have not been formalized for romantic attachments. Here, I take a broader comparative, evolutionary perspective for the etiology of the romantic attachment system, proposing two hypotheses: (1) the infanticide risk hypothesis for the selection pressure that prompted the evolution of romantic attachments; and (2) the mate guarding hypothesis as the primary functional operation and manifestation of romantic attachments.

Infanticide Risk Hypothesis. Approximately 15%-20% of primate species exhibit pair-bonding

(Brumbaugh & Fraley, 2006; Kleiman, 1977; Munshi-South, 2007; Opie et al., 2013), as compared to approximately only approximately 3%-5% of mammalian species. Comparative perspectives beyond the traditional psychological sciences, therefore, offer important insights to the selection pressures that prompted pair- bonding, because phylogenetic analyses of primate species suggest that pair-bonding has evolved independently seven to ten times across all major primate radiations (Fuentes, 2000; Kappeler & van Schaik, 2002).

The paternal care, or male provisioning hypothesis for human romantic-partner attachments (Brumbaugh

& Fraley, 2006; Finkel & Eastwick, 2015), which is largely favored in the psychological sciences, is discordant with research and theorizing on the infanticide risk hypothesis for pair-bonding in non-human primates (Hawkes,

2004; Opie, 2013; Palombit, 1999, 2015; Smuts, 1992; van Schaik & Dunbar, 1990; cf. Dixon, 2013). One reason for this neglect of the infanticide risk hypothesis within the psychological attachment literature is that male provisioning and male protection are often conflated in human research (see Hawkes, 2004), such that protection from male infanticide is considered and measured as a component of paternal care (see Brumbaugh & Fraley, ATTACHMENT SYSTEMS 9

2006; Symons, 1979). Provisioning and protection, however, are distinct components of male behavior (Hawkes,

2004; see also Opie et al., 2013). Male provisioning entails direct investment of resources to offspring, which is an aspect of parenting effort, or intersexual competition. Male protection, on the other hand, is direct competition with another male, and therefore is an aspect of intrasexual competition, or mating effort. It may certainly be possible for males to optimize both mating and parenting effort simultaneously, such that, for example, male-male competition behaviors (e.g., hunting) may also result in greater resources indirectly provisioned to offspring (see

Hawkes, 2004 for extended discussion). Such optimization of mating and parenting effort, however, is a-typical given trade-off assumptions of reproductive effort (e.g., Del Giudice et al., 2015; Hawkes, 2004; Richardson et al., 2017; Roff, 2002). From this perspective, Hawkes (2004) argues that infanticide protection (intrasexual competition) and male provisioning (intersexual competition) are two competing hypotheses for the evolutionary origins of pair-bonds, rather than components of the same hypothesis.

Favoring of the male provisioning hypothesis in the psychological literature may also result from the perceived ubiquity of pair-bonding across human societies (Hawkes, 2004). From a cross-cultural perspective, however, paternal provisioning is not a ubiquitous feature of pair-bond relationships with large variation observed across societies, although paternal is quite common in humans when pair-bonding does occur (Fernandez-Duque et al., 2009; French et al., 2018; Quinlan, 2007, 2008; Quinlan & Quinlan, 2007). The direct adaptive benefits of paternal provisioning are also inconsistent across offspring outcomes (see Sear, 2016; Sear & Mace, 2008;

Sheppard et al., 2014). Despite the ubiquity of romantic attachment between partners in pair-bonded relationships, from cross-cultural perspective, pair-bonds serve a broad array of functions, aside from copulation and , for which a more diverse array of potential evolutionarily-relevant benefits of pair-bonds for both men and women must also be considered.

The infanticide risk hypothesis (infanticide, here, defined as an unrelated male killing a female’s offspring) has received support as a primary selection pressure prompting the evolution of pair-bonding in primates (Hawkes, 2004; Opie et al., 2013; Palombit, 1999, 2015; Smuts, 1992; van Schaik & Janson, 2000; van

Schaik & Dunbar, 1990). Risk of infanticide by males among primate species is one of the highest among mammalian species (Hausfater & Hrdy, 1984, Yamagiwa, 2015), thus coinciding with primates’ proportionally ATTACHMENT SYSTEMS 10 high rates of pair-bonding. Protection against infanticide by year-round association via pair-bonding is one adaptive response against the risk of infanticide (Hawkes, 2004; Opie et al., 2013; Palombit, 1999, 2015). The crucial factor for infanticide avoidance as a primary selection pressure for pair-bonding in primates appears to be the slow life histories characteristic of primates (Yamagiwa, 2015). Infanticide is more likely to occur in species with slow life histories because of longer nursing times and, in particular, among species characterized by long juvenile periods and long duration of lactation relative to gestation, such as gibbons and humans, for example

(Kappeler & van Schaik, 2000).

Evolutionary reconstructions support that infanticide risk likely prompted shifts to pair-bonding, rather than merely co-evolving with pair-bonding as is the case for male-provisioning. Opie et al. (2013) used phylogenetic analyses to differentiate factors prompting pair-bonding from responses following pair-bonding.

Across 230 primate species, three selection pressures were examined for their correlated evolution with pair- bonding: infanticide risk, paternal care, and mate guarding. Although all three traits were shown to covary with pair-bonding (similar to results of male-provisioning documented by Brumbaugh & Fraley, 2006, described above), infanticide risk shows the most compelling explanation for the initial shift to pair-bonding among primates. Only the presence of infanticide risk reliably increased the probability of shifts to pair-bonding across the primate order, and was the only factor investigated to reliably precede pair-bonding (Opie et al., 2013; see also, Shultz et al., 2014), which is one reliable counterstrategy to infanticide (Palombit, 1999, 2015). Therefore, it appears more plausible that infanticide risk prompted the shift to pair-bonding, after which male-provisioning became increasingly common within pair-bonds. Opie et al. (2013) show also that once pair-bonding evolves as an effective counterstrategy to infanticide, back-transitions nearly never occur, suggesting that pair-bonding with consequent low infanticide is a relatively stable evolutionary strategy in primates (Shultz et al., 2014).

Mate Guarding Hypothesis. Once pair-bonding and consequent attachments are established, mate guarding ensures the optimization of resource investment for both males and females (Opie et al., 2013; see also

Brumbaugh & Fraley, 2006). Here, I propose that mate guarding is the primary manifestation of the romantic attachment system, whereby romantic attachment bonds are a primary motivational source underlying mate guarding behaviors. Mate guarding behavior occurs in response to threats to the pair-bonded relationship (Buss, ATTACHMENT SYSTEMS 11

1988; Buss et al., 2008). Threats can be exogenous in origin (intrasexual competitors), or endogenous in origin

(emotional detachment or disinterest of mate). Threats may be acute in nature (a present, intrasexual competitor), ongoing (chronic jealousy), or perceived (suspicion of partner’s activities). Threats to a pair-bonded relationship can thus be operationalized as endogenous or exogenous threats to the stability of the pair-bond that negatively affect optimal investment of resources into the mateship, for either partner. The romantic attachment system, then, is a suite of psychological mechanisms that primarily operate to prevent, correct, and address threats to the pair- bond most often by regulating mate guarding behavior, broadly construed (Barbaro et al., 2016a,b, 2019).

Men and women both perform mate guarding behavior (Buss, 1988; Buss & Shackelford, 1997).

Humans live in multi-male, multi-female groups meaning that men and women may both surreptitiously divert resources (sexual, emotional, economical) outside the pair-bond, rendering the deceived partner vulnerable to exploitation (Buss et al., 2017). Threats to a pair-bonded relationship may predictably differ for men and women given differential obligatory parental investment and asymmetry of reproductive variance (Bateman, 1948;

Trivers, 1972). Moreover, because women’s initial investment in offspring is greater relative to men (Trivers,

1972), women, on average, may be more responsive to relationship threats, especially during early stages of relationship formation relative to men, on average. That women’s attachment mechanisms may be more responsive than men’s, especially early on during attachment bond formation, corresponds with work showing small, but reliable sex-differences in anxious (higher in women) and avoidance (higher in men) attachment responses in men (Del Giudice, 2011) and with work showing women are more likely than men to interpret ambiguous behaviors as relationship threats (Kruger et al., 2013). The benefits of pair-bonding for men and women also may differ in some ways given fundamental asymmetries of biological reproduction. The mate guarding hypothesis of romantic attachments must therefore explain the benefits of mate guarding (especially for males) and the threats to pair-bond stability for which men and women may be sensitive to.

Benefits of pair-bonding for women are relatively straightforward. Because internal fertilization and gestation occur in women, women’s reproductive output is constrained relative to men’s, and women cannot increase their reproductive output the same as men by simply copulating with more partners. Women also invest substantial time and energy to gestating offspring and several years of nursing, during which time ovulation ATTACHMENT SYSTEMS 12 typically ceases. Pair-bonding with an investing male partner retained via effective mate guarding affords many benefits for women. It allows women to shorten lactation duration (see Quinlan & Quinlan, 2008) resulting in relatively shorter inter-birth ratios (Shultz et al., 2014; Yamagiwa, 2015), thereby potentially increasing women’s reproductive rates, and decreasing the benefits of infanticide by other males given that offspring are weaned relatively quickly (Opie et al., 2013). Women also benefit both directly and indirectly from male protection.

Women are at lesser risk for male violence when pair-bonded, referred to as the bodyguard hypothesis (Wilson &

Mesnick, 1997). Importantly, women’s offspring are less likely to be abused or killed by an outside male if women remain mated to their offspring’s father (Archer, 2013; Daly & Wilson, 1988).

Pair-bonding in males, however, results in a dramatic shift in reproductive strategy, rendering benefits of pair-bonding for men comparatively less straightforward than for women. Ejaculate production is relatively cheap, with men needing to contribute only an ejaculate to the production an offspring, creating a drastic asymmetry of minimum obligatory parental investment between the sexes (Trivers, 1972). Men can substantially increase their reproductive output by having more copulations with a variety of different (fertile) women

(Bateman, 1948). Pair-bonding can, however, be advantageous for men on the condition that the benefits of mate guarding a single female can result in greater fitness (i.e., surviving, reproducing offspring) for that male (Hawkes et al., 1995). Mate guarding can increase probability of offspring survival (by, in part, reducing infanticide risk) and allow for greater reproductive output of the female partner by shortening her inter-birth duration. Pair- bonding only via effective mate guarding can therefore be an advantageous reproductive strategy for men.

The (Different) Nature of Attachment Systems

Evolutionary perspectives on attachment have proposed that, in humans, adaptations for romantic attachments in adulthood were ‘co-opted’ from infant attachments via natural selection over human evolutionary history (Hazan & Diamond, 2000). Evolution via natural selection operates on existing genetic variation in a population and therefore designs adaptations from material (available cognitive adaptations; genetic variants) present in the population. It is therefore reasonable to suppose that natural selection co-opted features (and their underlying genetic variants) of infant attachment adaptations for the new purpose of forming attachments between romantic partners (Hazan & Diamond, 2000; Hazan & Shaver, 1987), thereby maintaining that infant and ATTACHMENT SYSTEMS 13 romantic attachments are behavioral homologies serving a similar overarching goal (Fraley & Shaver, 2000). An adaptation co-opted from existing adaptations, however, does not necessarily sequitur to inherent phenotypic similarity as is largely assumed by adult attachment theory (Fraley & Shaver, 2000; Hazan & Diamond, 2000;

Hazan & Shaver, 1987). As Kirkpatrick (1998) argues, only important features of a system of cognitive adaptations need to be maintained over development for such co-opted processes to occur; and that natural selection would have discarded unnecessary components and added more specified components to the romantic attachment system over time. This process could result in a fundamentally reorganized and functionally different system. From this view, the infant attachment system needed to supply only particular features necessary for the evolution of the romantic attachment system (Kirkpatrick, 1998). The romantic attachment system should not necessarily be considered a behavioral homologue to the infant attachment system.

The distinct evolutionary etiologies of the infant attachment system and romantic attachment system proposed here suggest differences with regard to the core functional nature of each attachment system. First, the romantic attachment system was likely, in part, co-opted from the infant attachment system, as proposed by others. I propose, specifically, that natural selection co-opted the structural features of infant attachments (see

Bowbly, 1969, 1982; Hazan & Ziefman, 1999) for the romantic attachment system. Attachment features include proximity maintenance, separation distress, safe haven, and secure base (Bowbly, 1969, 1982; Hazan & Zeifman,

1999), and are the defining features of true attachment bonds between two individuals. From this view, the romantic attachment system and infant attachment system use the same criteria for assessing what constitutes a reliable attachment figure, and both take time to develop a true attachment bond. If true, we would expect to observe these features that define true attachment figures in only parents or romantic partners, but not other individuals such as friends and acquaintances. Evidence supports the prediction that romantic partners and primary caregivers are the only individuals for which all four features of attachments are reliably observed in the same individual (Fraley & Davis, 1997; Hazan & Zeifman, 1999), and therefore, attachments in the strict sense are between only infants and parents, and between romantic partners (Hazan & Diamond, 2000).

Second, each attachment system is facultative toward different evolutionary-relevant goals (see also

Kirkpatrick, 1998; see also Schaller et al., 2017 who also argue that parenting motives and mate retention motives ATTACHMENT SYSTEMS 14 are distinct fundamental motivational systems for humans). The infant attachment system very clearly functions to facilitate the infant’s survival by means of soliciting protection from the primary caregiver(s) and by adequate provisions of sustenance, especially from the mother. And, the formation of an attachment bond, from the infant’s perspective, is absolutely necessary for (ancestral) infant survival. Attachments to a romantic partner, on the other hand, function to promote replicative success of each individual partner by (ultimately) reducing risk of infanticide for their offspring which is one means by which the probability of the parents’ genes being propagated into subsequent generations is increased. This differs subtly from functional accounts proposed by standard attachment perspectives (Finkel & Eastwick, 2015; Fraley & Shaver, 2000; Kirkpatrick, 1998) in that such proposals center around provisioning and care to promote survival offspring as the ultimate function of romantic partner attachments. Provisioning, however, is a secondary function of romantic partner attachments in that provisioning co-evolved with, or as a consequence of, pair-bonded attachment (Opie et al., 2013). The functional importance of the romantic attachment system should be focused (selfishly) on the replicative benefits accrued to the individual romantic partner, not focused on the offspring per se.

Third, the operational nature of the attachment bond within each system is qualitatively different. Infant- parent attachments are largely unidirectional in nature, meaning that the flow of resources is directly almost entirely only from parent to infant. The parent gains no direct survival advantage by virtue of the infant’s bond to her, and if anything, may be more vulnerable to injury and incur an increased risk of death (e.g., during childbirth; depleting caloric resources; attracting conspecifics or predators). The operational nature of romantic attachments, in contrast, is bidirectional in nature, meaning that replicative benefits are conferred between partners. Given that each parent shares 50% of their genetic material with their offspring, each parent can benefit from an attachment relationship. Proponents of a single attachment system approach acknowledge the different operational nature of infant and romantic attachments, but reconcile the ‘problem’ by proposing that the infant attachments develop into a more mutual attachment partnership (i.e., a “corrected partnership” (Ainsworth, 1985)), and that this relatively more mutual attachment relationship between infant (child) and parent is what actually serves as the prototype for future romantic relationships. The different operational nature of each attachment system should, I ATTACHMENT SYSTEMS 15 argue, be viewed as unique defining features of each attachment system, rather than a problem to be reconciled to accord with assumptions of inherent similarity and continuity.

Finally, theory must account for the observation that formation of infant attachments (to varying degrees) are necessary for infant survival, whereas, formation of romantic attachments are not at all necessary for successful replicative success of adults (see also, Kirkpatrick, 1998). Traditional perspectives on attachment propose that trait-like attachment styles or working models are initially formed early in development, and then are refined and developed into cognitive prototypes or internal working models of attachment relationships

(Bretherton & Munholland, 2016; Fraley, 2002). These trait-like attachment styles are proposed to guide an increasing array of interpersonal relationships during childhood and adolescence, and throughout adulthood, including both short- and long-term romantic partners (Belsky & Simpson, 2016; Del Giudice, 2009). The view that adult attachment styles are largely relationship-general, and precede or guide the formation of attachment relationships (reflected by attachment’s trait-like conceptualization and measurement) can be criticized on multiple accounts given that, in adults: (1) attachment bonds are not fundamentally necessary for successful reproductive relationships (Kirkpatrick, 1998); (2) attachment bonds can take up to two years to fully develop

(Hazan & Zeifman, 1999); (3) attachment bonds are reliably only observed between committed romantic partners

(and between infants and primary caregivers) (Hazan & Diamond, 2000); and, (4) partner-specific attachments have greater explanatory power of partner interactions than do partner-general attachments (Barry et al., 2007;

Sibley et al., 2005).

Bowbly (1969, 1982), in fact, cautioned against conflating true attachments with general socialization systems that guide other interpersonal interactions over the lifespan. Instead, the romantic attachment system should be viewed as a context specific cognitive system that is activated in only the context of pair-bonded relationships; attachments in the strict sense are not necessarily activated or fully developed in many adult romantic relationships. The cognitive machinery for the formation of romantic attachments is likely a human universal given that pair-bonding has evolved as a in Homo sapiens (Buss et al., 2017; Opie et al.,

2013). It is a non-sequitur to assume, however, that the romantic attachment system is therefore influential for all types of mateships (e.g., Shaver et al., 2019; Simpson et al., 2011), or that the romantic attachment styles precede ATTACHMENT SYSTEMS 16 or guide the formation of mateships (e.g., Del Giudice, 2009). True romantic attachment bonds regulate threats to pair-bonded relationships in relatively long-term mating contexts (Barbaro et al., 2019). Other cognitive systems of specified mating adaptations (e.g., sexual strategies; Buss & Schmitt, 1993) are sufficient for navigating and explaining other mating contexts.

Conclusions

Attachment theory has generated numerous insights the understanding of close relationships across the lifespan, and has been remarkably influential across sub-disciplines of psychology. Although much has been learned about the development of attachment across the lifespan, much less scholarly attention has been targeted at the evolution of adult romantic attachments in recent years. I propose that infant attachments and romantic attachments have distinct evolutionary etiologies, and that the nature of each attachment relationship is fundamentally different. These two premises have important implications for the continued study of attachment as a lifespan phenomenon, particularly with regard to developmental processes and mechanisms. ATTACHMENT SYSTEMS 17

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