Running head: BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 1

Behavior Genetics Research on Personality:

Moving Beyond Traits to Examine Characteristic Adaptations

Phuong Linh L. Nguyen*, Moin Syed, Matt McGue

University of Minnesota – Twin Cities, Department of Psychology

*Corresponding Author:

Phuong Linh L. Nguyen

75 East River Parkway

Minneapolis, MN 55455 [email protected]

This manuscript is the submitted version as of May 26, 2021 and not the final accepted version.

BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 2

Abstract

This paper argues that behavior genetics research on personality should expand beyond universal traits to include characteristic adaptations. Trait research examines broad, decontextualized, and universal domains such as the Five Factor Model. Characteristic adaptations are more contextualized than traits, such as goals and life strategies as responses to specific life demands. The paper is organized into three sections: (1) a review of the abundance of behavior genetics research on personality traits, which has reached a convergent point at which few further findings are reported beyond the classic distribution of high genetic and non- shared environmental influences with little to no shared environmental effect; (2) a review of existing behavior genetics research on characteristic adaptations that, although limited in volume, has demonstrated patterns far less consistent than traits; and (3) a discussion on future directions and important limitations to consider in conducting and interpreting behavior genetics research on non-trait personality. The connection between characteristic adaptations and contextualized life outcomes, the preponderance of homogenous findings on traits, and the sparse yet promising findings of characteristic adaptations, all support the need for behavior genetics research on personality to expand beyond the broad trait level to characteristic adaptations and beyond.

Keywords: behavior genetics, heritability, twin study, personality, personality traits, characteristic adaptations.

BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 3

Behavior Genetics Research on Personality:

Moving Beyond Traits to Examine Characteristic Adaptations

Personality research broadly investigates patterns in psychological individual differences that may have meaningful predictive power in real life contexts, including health, relationships, and academic and career achievements (Bleidorn et al., 2019). Over the last several decades, personality research has focused heavily on traits, which are relatively stable individual differences in behavior, emotion, and cognition (DeYoung, 2015). Traits have been such a dominant focus that they are commonly equated with personality, rather than as one component of the broader personality system. Indeed, McAdams (1995) highlighted that traits are but one of three levels of personality, along with characteristic adaptations (more contextualized and developmental aspects of personality than traits, such as goals and motivations) and the integrated life story (highly contextualized stories of people’s lives).

Given the relative stability of traits and their implications for life outcomes, there has long been interest in understanding their genetic and environmental origins. This question has been examined via behavior genetics, an area of research that examines patterns of genetic and environmental influences on the development and manifestation of individual differences in psychological and behavioral characteristics (Knopik et al., 2016; Plomin et al., 2016). Like most areas of research, however, the heavy emphasis of behavior genetics research on personality has been with traits (Bleidorn et al., 2014), and thus we have limited insights on genetic and environmental contributions to other levels of personality.

In this paper, we argue for moving the research focus of behavior genetics research of personality beyond traits to include characteristic adaptations. Characteristic adaptations provide important contextual information, and thus large-scale investigations into the sources of BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 4 variability within this level will be valuable to answer practical research questions regarding physical and mental well-being. First, we provide a brief review on behavior genetics designs and the abundance of existing findings on personality traits, which has reached a convergent point at which few further findings are reported beyond the classic distribution of high genetic and non-shared environmental influences with little to no shared environmental effects. Second, we review the sparse existing behavior genetics research on characteristic adaptations that, although limited in volume, has demonstrated patterns that are far less consistent and with different genetic and environmental contributions from the trait level. Finally, we discuss future directions for expanding the investigated constructs while being mindful of important limitations in combining behavior genetics and non-trait personality research.

Behavior Genetics Research on Personality Traits

Conceptualizations of traits have been dominated by the Five Factor Model or Big Five, consisting of Agreeableness, Conscientiousness, Extraversion, Neuroticism, and Openness to

Experience (Costa & McCrae, 1985; Goldberg, 1993), although other trait taxonomies are also used (e.g., HEXACO, Ashton et al., 2004; HiTOP, Kotov et al., 2017). Moreover, these five trait domains are part of a trait hierarchy, with higher-order meta-traits Plasticity and Stability

(DeYoung, 2006; Digman, 1997), lower-order specific facets (McCrae & Costa, 1992), and aspects that lie between domains and facets (DeYoung et al., 2007). As traits move from the highest to lowest levels of the taxonomy, they become increasingly specific such that their expressions are constrained by the context in which they were conceptualized and measured.

Similar to personality research in general, the behavior genetics study of personality has overwhelmingly focused on traits. The classical twin study (CTS) involves the comparison of the similarity of reared-together monozygotic (MZ) and dizygotic (DZ) twins (Martin et al., 1997). BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 5

MZ twins effectively share their genomes; DZ twins are like ordinary full siblings in that they share on average 50% of their segregating genetic material. Greater MZ than DZ similarity is taken as evidence for the contribution of genetic factors. Heritability is defined as the proportion of variance in observed individual differences that are due to genetic variations (Visscher et al.,

2008), and is estimated as twice the difference between MZ and DZ correlations. Notably, this is a statistical construct resulting from variance decomposition, and does not reflect a deterministic judgment of the source of individual differences. Given the potential for misinterpretation between statistical and causal interpretations, we return to this issue in greater depth towards the end of the paper when discussing future directions and limitations.

In the simplest biometric ACE model (Maes, 2005), the variance in a phenotype, which is often normed to one, is assumed to be an additive function of genetic and environmental factors:

1.0 = a2 + c2 + e2

where a2 is the proportion of phenotypic variance attributable to additive genetic factors (i.e., the heritability), c2 is the proportion attributable to the shared environment (i.e., environmental factors shared by reared-together twins and thus contributing to their phenotypic similarity), and e2 is the proportion attributable to the non-shared environment (i.e., environmental factors that twins do not share and so a source of their phenotypic dissimilarity)1.

Decades of twin studies have provided heritability estimates of personality traits generally ranging from 40-50% (Bouchard, 1994; Nichols, 1976; Polderman et al., 2015;

Vukasović & Bratko, 2015). Further, twins reared apart were, perhaps surprisingly, about as similar in their personality traits as twins who had been reared together (Bouchard et al., 1990).

1In some applications, both additive and non-additive genetic effects are allowed to contribute to trait variance (Finkel and McGue, 1997). However, it is possible to estimate only three variance components in the CTS and the ACE model is typically preferred because there is limited evidence for non-additivity (Polderman et al., 2015). BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 6

The minimal effect of the shared environment on personality as implied by these findings has been demonstrated in many other studies using alternative research designs (e.g., Bouchard &

McGue, 2002; Loehlin & Nichols, 1976; Rowe, 1994). Thus, most of the environmental effects

(which must be substantial as heritability rarely exceeds 50%) results from the non-shared environment; however, little is known about the specific source of these effects (Plomin &

Daniels, 1987; Plomin, 2011).

Behavior genetic findings regarding traits have been so consistent and robust that it led to

Turkheimer (2000) formally proposing three laws of behavior genetics: “First Law: All behavioral traits are heritable; Second Law: The effect of being raised in the same family is smaller than the effect of genes; Third Law: A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.” (p. 160). These laws, as stated, are uncontroversial with respect to the available evidence. We know that they apply to personality traits, but what about other levels of personality?

Indeed, even at the trait level, there is some contradictory evidence in the literature. Here we highlight two lines of such evidence, both of which point to deviations from the three laws.

First, contrary to patterns for most traits, there is evidence for significant effects of the shared environment for traits to which parents may pay particular attention in socialization: some facets of Agreeableness and Conscientiousness (Bergeman et al., 1993; Bouchard & Loehlin, 2001),

Delinquency and Antisocial Behaviors, (Burt, 2009; Rowe et al., 1992; Niv et al., 2013),

Disinhibited Personality and Externalizing/Internalizing Disorders (Buchanan et al., 2009),

Norm-Favoring (Loehlin & Gough, 1990), Religious Orthodoxy (Rose, 1988; Beer et al., 1998 but not Religiosity: Bouchard et al., 1999; Vance et al., 2010), Social Closeness (Tellegen et al.,

1988), Spiritual Involvement (Tsuang et al., 2002), and Traditionalism (Matteson et al., 2013). BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 7

The second line of evidence that highlights the limitations of the broad laws is that of differential heritability, which is the degree to which heritability patterns differ across traits. At the broad trait level, meta-analytic findings indicated little effect of applying different taxonomies or different traits on the pattern of findings, suggesting no evidence of differential heritability (Vukasović & Bratko, 2015). However, for traits lower in the hierarchy, large-scale twin studies provided substantially lower heritability estimates for constructs such as attitudes that may be more similar in nature to characteristic adaptations than to personality traits (Loehlin

& Nichols, 1976; Loehlin, 1982). Fine-grain item clusters (e.g., Worried or Takes Advantage) and social and political attitude scales (e.g., Stereotype Masculinity or Cynicism) were markedly less heritable than broad trait domains and showed consistent differences in variance patterns

(Loehlin, 2012), with heritability estimates ranging widely from 10-45% (Table 1). This notion was supported by research across Canadian and German twin samples, demonstrating multiple genetic and environmental factors that differentially contributed to lower-order facets within each Big Five domain (Jang et al., 2002). These lines of evidence collectively suggested that differential heritability is more frequently observed among narrower facets compared to the broad trait domains. In sum, there is at least some evidence even within the trait literature that the three laws may not apply to all aspects of personality and that there are limitations to relying only on the broad laws in describing psychological individual differences.

Behavior Genetics Research on Characteristic Adaptations

Beyond the trait level, characteristic adaptations broadly include “motives, goals, plans, strivings, strategies, values, virtues, schemas, self-images, […] and many other aspects of human individuality that speak to motivational, social-cognitive, and developmental concerns”

(McAdams & Pals, 2006). Characteristic adaptations differ from traits in several ways. First, BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 8 there is no organizational taxonomy such as the hierarchical trait model, and thus no clear bounds on what is included. Although promising research is ongoing to devise a taxonomy of situational characteristics (e.g., DIAMONDS: Rauthmann et al., 2014), there exists no framework that organizes different characteristic adaptations into a unified structure. Beyond characteristic adaptations, McAdams and Pals (2006) described integrated life stories as the most contextualized and culturally influenced level of personality. On the other hand, DeYoung

(2015) has argued that these personal narratives should be considered a special case of characteristic adaptations instead of a unique level. This view continues to be debated (see Syed et al., 2020); however, it is not our intention to draw clear distinctions between the levels.

Rather, we argue for the general need to expand our constructs of interest beyond universal traits.

Although characteristic adaptations are the focus of the current review due to their relative proximity to traits, researchers should eventually broaden their scope to all psychological individual differences, of which life narratives are a crucial component.

Second, unlike the universal dispositional traits, characteristic adaptations are by definition reflective of specific situations and life demands. As a result, constructs in this level are expected to be more constrained by the contexts in which they are conceptualized and observed. Here we review behavior genetic research for a few characteristic adaptations that have been investigated from behavioral genetic perspectives: identity processes, adult attachment, coping and defense mechanisms, and educational attainment (see Table 1 for a summary of findings). Although this selection of constructs may appear disjointed at first glance, they can all be understood as goals, interpretations, and/or strategies that are contextualized in specific situational demands (DeYoung, 2015). There are, of course, many other important constructs in this level that are excluded from the current review simply due to their absence in BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 9 the literature, yet they still fall under the broad aforementioned definition (e.g., agency, personal projects, ethnic identity, ultimate concerns).

Identity processes

Identity refers to how people reflect consciously on who they are and how they fit in the world (Erikson, 1968). The dual-cycle model of identity development (Luyckx et al., 2006;

Luyckx et al., 2008; Marcia, 1966) consists of a set of processes related to exploration, or the degree to which people engage in processes of discovery (exploration in breadth, exploration in depth, ruminative exploration), and a set related to commitment, or the degree to which identity elements have been incorporated into the sense of self (commitment making, identification with commitment, and reconsideration of commitment). Identity development processes are highly related to well-being in the domains of family functioning (e.g., Schwartz et al., 2009), well- being (e.g., Waterman et al., 2013), internalizing/externalizing symptoms (e.g., Schwartz et al.,

2011), and health (for a review, see: Schwartz et al., 2013).

To date, the Longitudinal Israeli Study of Twins (LIST; Knafo, 2006) is the only study to provide information on the genetic and environmental contributions to adolescent identity processes. Using this sample, Markovitch and colleagues (2017) examined the sources of individual differences in different dimensions of exploration and commitment. For all but one dimension, there was evidence for partial genetic influence (18-45% – Table 1), with the majority of variance due to non-shared environmental effects. As discussed, the pattern of greater non-shared environmental than genetic contributions is pervasive across personality traits

(Turkheimer, 2000), although heritability estimates for identity dimensions were somewhat lower than has typically been found at the trait level. In-depth exploration through discussion with others was the exception, showing no genetic effects yet evidence for meaningful shared BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 10 environment effects. This suggested the need for further investigation into the social aspects of the identity formation processes. However, the lack of corroborating research, along with the young age of the sample (11-year-old twins), requires additional considerations. Specifically, effects of the shared environment and its interaction with genetic factors tend to decrease in importance with age, particularly after leaving the home environment (Bouchard & Loehlin,

2001; Briley & Tucker-Drob, 2014). Thus, this young sample may overestimate lasting effects of the home environment.

Adult attachment

Attachment theory (Ainsworth et al., 1978; Bowlby, 1969) refers to individual differences in the formation and maintenance of interpersonal bonds. In individual differences research, adult attachment styles have been studied categorically with distinct types: Avoidant,

Anxious-Ambivalent, and Secure (Hazan & Shaver, 1987; Bartholomew & Horowitz, 1991; for a review, see: Gillath et al., 2016). Alternatively, dimensional approaches examine attachment- related Anxiety and Avoidance (Fraley & Shaver, 2000), and attachment security has also been studied with narrative approaches and conceptualized as narrative coherence in relationship discourse (Borelli et al., 2013). Attachment research provides important implications for various areas of well-being and interpersonal relationships, such as social competence (Fass & Tubman,

2002), psychopathology (Ogilvie et al., 2014; Paetzold et al., 2015; Theule et al., 2016), psychotherapy (Shorey & Snyder, 2006), substance abuse (Fairbairn et al., 2018), and romantic relationship satisfaction and distress (Collins & Read, 1990; Li & Chan, 2012; Petersen & Le,

2017). Attachment styles, while strongly related to traits, were more predictive of relationship outcomes due to higher specificity compared to broad trait domains (Shaver & Brennan, 1992). BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 11

Behavior genetic research into the dimensions of adult attachment2 (Picardi et al., 2011) and a combination of dimensional and categorical examinations in adolescent twins (Fearon et al., 2014) demonstrated that the majority of variance for all attachment phenotypes was attributable to the non-shared environment, with the balance of variance due to genetic contributions. There was limited evidence for a contribution of the shared environment.

However, the Dismissing-Avoidant categorical attachment type showed an exception, with significant shared and non-shared environmental contributions and no evidence of genetic contribution (Brussoni et al., 2000). These results suggested that the environment might be particularly important for attachment styles, specifically with the shared home environment for the solitary, self-enhancing Dismissing style. This pattern has also been seen in research on infant attachment, where estimates of shared and non-shared environmental contributions are far greater than genetic effects for all attachment phenotypes (e.g. Roisman & Fraley, 2008). The difference in results between infant and adult attachment is unsurprising, however, because developmental trends in the attachment literature (Picardi et al., 2020) and personality literature at large (Briley & Tucker-Drob, 2014) suggest that genetic influences typically increase with age, at the expense of the shared environment.

Coping and defense

Coping strategies refer to adaptive defense patterns in response to specific life stress or trauma (Frydenberg, 2017; Lazarus & Folkman, 1984). They are often examined with different dimensions of problem-focused, emotion-focused, and avoidance-focused strategies (Carver et al., 1989). Coping has been studied widely from the context of daily well-being (Stevenson et al.,

2 It is important to note that broad attachment constructs are often measured in a trait-like manner and describes the general tendency across different relationships. On the other hand, relationship-specific attachment (e.g., current romantic relationship) is much more clearly a characteristic adaptation. A more thorough discussion of measurement considerations is included in the Limitations section. BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 12

2019) to trauma (Littleton et al., 2007) or serious illnesses (Fasano et al., 2020; Moskowitz et al.,

2009).

Behavior genetic research on coping demonstrated differential heritability among different coping factors and strategies: Besides the overall strong influence of non-shared environmental effects, patterns of genetic and shared environmental influences differed for all three orthogonal factors of Turning to Others, Problem Solving, and Denial (Kendler et al.,

1991). Differential patterns were also reported among the narrow and specific coping styles: whereas some strategies showed marked genetic contribution, others showed effects of shared environmental factors, and the non-shared environment remained the most significant contributor

(Busjahn et al., 1999). More recent research has generally confirmed this pattern of modest genetic, large non-shared environmental, and minimal shared environmental contributions

(Kozak et al., 2005), noting that although coping styles may share some genetic influences with personality traits, their environmental influences are distinct (Jang et al., 2007).

Educational attainment

Educational attainment is sometimes described as a “model behavioral phenotype” in behavior genetic research (Lee et al., 2018), due to its prevalence in many datasets, ease of objective measurement, and relationship with important health outcomes. Educational attainment positively predicts many important adult outcomes, including income, employment, health behaviors, and depression (Conti et al., 2010). This gradient effect on health, in which higher levels of education are associated with an increase in health behaviors and an elevated health status, has been explored and determined to be at least partially causal (for a review, see: Cutler

& Lleras-Muney, 2006). BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 13

Educational attainment, similar to most behavioral phenotypes, is moderately heritable

(Behrman & Taubman, 1989; Silventoinen et al., 2004). A meta-analysis compiling studies across different nations and cohorts showed large contributions from genetic, shared, and non- shared environmental sources, with the non-shared environmental effects being the smallest

(Branigan et al., 2013). Strikingly, a recent analysis of adult twins based on 28 cohorts across 16 countries showed substantial average shared environmental effects of 31%, which did not differ across geographical regions (Silventoinen et al., 2020). This particular pattern is surprising, given the typically negligible effect of the shared rearing environment in the literature

(Turkheimer, 2000), and potential reasons have been discussed, including assortative mating

(Baker et al., 1996) and influences of parenting and residential locations (Freese & Jao, 2017). In addition, there is significant heterogeneity in variance component estimates with higher heritability for males and more recent birth cohorts (Lee et al., 2018; Table 1). Reduced heritability among women and for cohorts where education access may have been limited could arise because genetic effects are often suppressed if there are environmental constraints on the phenotype (Branigan et al., 2013). Educational attainment is also unique in its developmental timespan: the level of schooling achieved is typically established in adolescence and young adulthood, which coincides with the timespan when siblings still live together, and shared environmental effects when they are found appear to be maximal. The heritability estimates, historical trends, and heterogeneity across cohorts highlight the need to include more similar phenotypes – contextual variables that are environmentally contingent such as characteristic adaptations – to produce more informative and heterogeneous findings beyond those obtained with the universal trait domains.

Future Directions and Limitations BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 14

Several themes emerged from our reviews of behavior genetics research on personality:

The pattern of findings shows near uniformity across traits at broad levels and relatively more inconsistency within the lower-order facets. Results from large scale meta-analyses and corroborating findings from various study designs all affirm the three established laws in behavior genetics (Turkheimer, 2000)3. However, the current review highlights an area in personality – characteristic adaptations – for which there might not be such universal laws.

Although the behavioral genetic literature on characteristic adaptations is limited and somewhat inconsistent, there are some suggestive differences from standard behavioral genetic findings.

First, characteristics adaptations appear to be less heritable than personality traits. Second, the shared environment, which appears to have little effect on most personality traits, appears to contribute to individual differences. To expand our current understanding of the genetic and environmental contributions to psychological individual differences, research programs ought to expand beyond the current boundaries to include personality at all levels– a truly holistic approach. Thus, our recommendations for researchers are to include behavioral characteristics outside of the trait level and to favor more contextual and environmentally contingent constructs.

Whereas the decontextualized trait domains have dominated due to their consistency across situations and ease of assessment, context is crucial to understanding real-world processes and individual differences.

Expanding research to characteristic adaptations

3A fourth law has been proposed, stating that most phenotypes are influenced by numerous genetic variants, each with a minuscule contribution (Chabris et al., 2015). Large scale, hypothesis-free genome-wide association studies have established this principle for many psychological constructs (e.g., Neuroticism: Nagel et al., 2018) including those outside of the trait domains (e.g. educational attainment: Lee et al., 2018). Although it is outside of the scope of this review, it is important to note that, at least at the molecular level, this law currently applies to all typical variables of individual differences. BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 15

Behavior genetic research incorporates genetically informative populations and techniques to offer insights into the sources of variations in psychological individual differences.

Such insights, although not deterministic in nature, are particularly important as part of the programmatic effort to identify relevant individual predispositions and environmental impacts that may ultimately contribute to well and ill-being. As noted previously (e.g., Kendler et al.,

1991), the lack of further investigation into constructs beyond traits is surprising, because they provide strong contextual information and are related to many aspects of mental and physical well-being. The current paper further highlights this deficit in the literature and encourages researchers to fill this gap with a more direct focus on the environment.

We provided a brief review of the few characteristic adaptations that have been studied in the behavior genetic literature: identity development, adult attachment, coping styles, and educational attainment. It is clear that emerging findings from these research programs are promising; nonetheless, much more research needs to be done to provide sufficient converging evidence, especially given the likelihood that initial findings on small samples often provide biased and anomalous results (winner’s curse: Ioannidis, 2008). Results from singular initial studies, no matter how promising, are hardly sufficient to draw firm conclusions. Although behavior genetics research for some characteristic adaptations (e.g., identity: Markovitch et al.,

2017) are relatively preliminary with few individual studies, other constructs such as educational attainment (e.g. Branigan et al., 2013; Silventoinen et al., 2020) have shown more consensus using large samples and meta-analytic methods. Thus, we ought not to be content with the current state of research, because there is not sufficient evidence to confidently conclude either that (1) different levels of personality follow different patterns of heritability and shared BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 16 environmental contribution; or that (2) findings for characteristic adaptations conform to the pattern shown repeatedly in behavioral genetic research on personality traits.

Attention is needed not only to the few examined constructs previously discussed, but also broadly to all environmentally-contingent conceptualizations of personality. Characteristic adaptations are ideal for these investigations because they are more variable (McAdams, 1994) and, by definition, they concern the specific contents of our motives, plans and goals, life strategies, etc. which are more reflective of cultural and situational influences (McAdams & Pals,

2006). For instance, we discussed research on the general identity development process; however, domain-specific identity constructs such as ethnic identity and sexual identity are also particularly important to consider. Ethnic identity describes people’s sense of self in regard to their ethnic group (Umaña-Taylor et al., 2014) and sexual identity is a dimension of sexual orientation, along with attraction and behavior (Brener et al., 1995). Both of these constructs may vary considerably in salience and meaning across subgroups and historical contexts (e.g., Ridolfo et al., 2012). By investigating constructs at this level, researchers will also indirectly gain valuable information about the way people react and adapt to specific situational demands, without explicitly studying their environment. In other words, characteristic adaptations are ideally situated at the intersection between person and situation4. Universal, decontextualized trait domains are certainly essential to understanding human nature, but we hardly exist in a universal, decontextualized world. Rather, understanding how people react to specific situational

4 In their discussion of characteristic adaptations, McAdams and Pals (2006) noted an earlier definition of the term (McCrae & Costa, 1994) and clarified that characteristic adaptations are not simply by-products of the person-by- situation interaction but are stand-alone constructs reflective of specific situational and life demands that may be influenced by dispositional traits. They affirmed that the malleable and contextual nature does not decrease their significance. The present paper argues that these features in fact enhance their utility. In addition, DeYoung (2015) made the distinction between cultural and situational influences, and that characteristic adaptations are distinguishable from traits only due to the former, as situational fluctuations are also seen in traits. BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 17 demands and the sources of these individual differences will better prepare behavior genetics research on personality to answer more practical and specific questions.

An area of much debate and confusion in personality research is the distinction between traits and characteristic adaptations (Henry & Mõttus, 2020), with a lack of an organizing framework and consensus on which constructs would constitute a characteristic adaptation. This imprecise distinction creates a perfect opportunity to incorporate behavior genetics research methods to provide a potential marker to differentiate the two. As the reviewed literature suggested, we may expect more deviation from the laws of behavior genetics for characteristic adaptations, namely lower heritability estimates and higher estimates of the shared environment compared to the trait level. This pattern would provide a useful marker and also encourage researchers to be more intentional in considering their measurement and conceptualizations of psychological constructs.

Limitations

Along with our recommendation to bring together the research areas of non-trait personality and behavior genetics, it is important to also note some limitations within these fields. First, the concept of heritability has been subjected to much confusion, primarily the blurred line between the statistical definition and the real-life causal implications. Heritability is a statistical construct that represents the proportion of variance in a particular phenotype that is statistically attributable to genetic differences among individuals. It is a population characteristic and not applicable at the individual level. That is, we cannot decompose a specific individual’s personality or phenotype to show 40% that was produced by genes and 60% by environmental impacts. BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 18

Second, these findings and interpretations are conditioned upon a set of assumptions and, when appropriate, researchers should examine potential assumption violations and their consequences to the findings. Estimations of the narrow-sense heritability (i.e., the heritability due to additive genetic effects) assume negligible non-additive effects of dominance and epistasis. Dominance refers to effects of a particular combination of alleles at a specific locus from the same parents. Epistasis refers to effects of a particular configuration of alleles across multiple loci. In general, dominance and epistasis would lead MZ twins to be phenotypically more than twice as similar as DZ twins. Although this additivity assumption holds in many cases

(Polderman et al., 2015), there are some notable exceptions. For instance, different coping factors showed strong dominance effects that often overshadowed additive genetic effects

(Busjahn et al., 1999), and a large meta-analysis showed a significant contribution of dominance to heritability estimates of traits Neuroticism and Extraversion (van den Berg et al., 2014).

Further, interpretations of twin studies assume that MZ twins are no more likely to share trait- relevant features of their environments than DZ twins (equal environments assumption: Felson,

2014). In the ACE model, the contribution of shared environmental factors can be underestimated in the presence of nonadditive genetic effects (e.g., Finkel & McGue, 1997) or when the equal environments assumption is violated (e.g., Tishler & Carey, 2007). On the other hand, the shared-environment might be overestimated if twins are different from non-twins due to twin-specific rearing or prenatal environment (e.g., Koeppen-Schomerus et al., 2003). It is consequently essential that findings from twin studies are confirmed using other research designs, such as adoption studies and family studies, which would provide additional important information about the rearing environments (e.g. Burt, 2009; Hahn et al., 2012). In the case of personality traits, for example, studies of personality similarity in adopted siblings report very BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 19 similar estimates as twin studies, suggesting that model invalidity may not be a major issue

(Matteson et al., 2013).

Another important assumption often violated in behavior genetics research is that of generalizability. The current literature includes a disproportionate representation of participants who are of European descent (Duncan et al., 2019) and there are no established analytic methods to adjust heritability estimates accordingly (Peterson et al., 2019). Recent advances in genome- wide association studies aim to identify numerous important genetic variants and to create weighted polygenic risk scores which may then be used as predictors of different behavioral phenotypes and psychiatric disorders. Unfortunately, these scoring schemes are population- specific, with predictive power reduced in non-European populations due to the limited training data (Duncan et al., 2019). Because characteristic adaptations are more culture-specific compared to personality traits (McAdams & Pals, 2006), the lack of generalizability may even be more problematic at this level.

Third, it is important to be mindful of general measurement considerations. On one hand, particular components that constitute a trait in aggregate may reflect characteristic adaptations in isolation. Even within the common trait measure of the Revised NEO Personality Inventory

(NEO-PI-R; Costa & McCrae, 1992), single items varied widely in whether raters categorized them as more indicative of a trait or a characteristic adaptation (Henry & Mõttus, 2020).

Although these ratings were not associated with different heritability estimates, all items included in this study were extracted from a trait measure, and thus further highlighted the need for an explicit examination of characteristic adaptations. For instance, measures of anxiety across different situations may be aggregated to produce a marker of trait Neuroticism. However, the specific anxiety that emerges during academic exams is a characteristic adaptation – contextually BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 20 dependent and not universal across cultures throughout history. On the other hand, characteristic adaptations may be measured in a very trait-like manner which contributes to the unclear distinction from traits. For instance, twin studies on values (Twito & Knafo-Noam, 2020) primarily used the common Portrait Value Questionnaire with the basic values: Openness to

Change versus Conservation and Self-Enhancement versus Self-Transcendence (Schwartz,

2012), and many socio-political attitudes are often mapped onto broad dimensions of liberalism/conservatism, with strong associations with traits authoritarianism or traditionalism

(Koenig & Bouchard, 2006). Although they do not directly map on to the Big Five taxonomy, these constructs are trait-like in that they represent universal structures beyond specific situational characteristics (Schwartz, 2012). Another example is research on goals. Indeed, motivational units play a crucial role in the conceptualization of characteristic adaptations.

Nonetheless, behavior genetics research has only investigated major life goals that are relatively generalized, showing high non-shared environmental influences (agency- and communion- oriented life goals: Bleidorn et al., 2010) and not the more contextualized mid-level goals such as personal projects (Little, 1983).

Because our definitions of heritability and related constructs are inherently statistical, they are affected largely by measurement approaches. As studies rely on either genetically informative populations or large scale consortia, there is a large cost involved in deep phenotyping that may result in a decision to use shorter measures. Although some constructs are adequately measured by even one item (years of education or height as the model phenotypes), shorter measures usually result in higher error which is subsumed under the unique environment component, leading to an artificial decrease in heritability estimates. This further highlights the need for more research programs that focus on the genetic and environmental patterns in BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 21 characteristic adaptations with careful measurement considerations, because an attempt to synthesize an incomplete body of literature might produce misleading conclusions that were statistical artefacts rather than substantive differences among constructs.

Conclusion

This paper reviewed the behavior genetics literature on personality while highlighting research on characteristic adaptations. Overall, the pattern for personality traits remains quite robust: high contribution of genetic effects, even more so for non-shared environmental effects

(i.e., the environmental effects that contribute to differences among reared together relatives), and little to no evidence for effects of the shared environment (i.e., the environmental effects that contribute to similarities among reared together relatives). Traits at the Big Five levels largely share similar heritability patterns, whereas more specific and narrower facets provided some initial evidence of differential heritability. Unfortunately, the literature is much more limited regarding characteristic adaptations and is far less consistent than trait research. For instance, heritability estimates for characteristic adaptations appear to be lower and less homogeneous than those for personality traits. More striking is evidence suggesting the contribution of the shared environment to characteristic adaptations which in some cases is stronger than that of additive genetic effects. Other than for a few socialized constructs, such as aspects within

Agreeableness and Conscientiousness or some antisocial behaviors, there is little evidence of shared environmental influences on personality traits. Nonetheless, given the limited number of relevant studies with characteristic adaptations, it is difficult to draw firm conclusions about the differential role of the shared environment at this time. Furthermore, the constructs discussed in this paper, and many more beyond the trait level, are strongly linked to specific life outcomes, such as relationship status and quality and physical and mental well-being. This connection to BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 22 contextualized life outcomes, the preponderance of established and robust research on traits, and the lack of non-trait research in the literature, all support the need for behavior genetic research on personality to shift beyond the trait level to characteristic adaptations.

BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 23

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Running head: BEHAVIOR GENETICS PERSONALITY BEYOND TRAITS 1

Table 1. Estimates of additive genetic, shared environment, and non-shared environment effects.

Construct a2 c2 e2 Temperament and personality .47 – .53 Personality traits1* Specific personality disorders .45 – .55 Worried .35 – .53 Careless spender .43 – .55 Personality item clusters2 Sensitive .20 – .45 Takes advantage .26 .17 .57 Stereotyped masculinity .10 .35 .55 Intolerance of ambiguity .24 .25 .51 Socio-political attitudes3 Cynical attitudes .33 .12 .55 Intellectual interests .45 .06 .49 Commitment making .45 – .55 Exploration in-breadth .27 – .73 Ruminative exploration .41 – .59 Identity4 Identification with commitment .25 .12 .63 Exploration in-depth (reflection) .18 – .82 Exploration in-depth (discussion) – .21 .79 Attachment security .03 .61 .36 Infant attachment5 Temperamental dependency .43 .11 .46 Attachment coherence .38 – .62 Adolescent attachment6 Attachment security .35 – .65 Secure attachment style .37 – .63 Fearful attachment style .43 – .57 Preoccupied attachment style .25 – .75 Adult attachment7,8 Dismissing attachment style – .29 .71 Anxiety attachment style .45 – .55 Avoidance attachment style .36 – .64 Anxiety-Avoidance attachment style .62 – .38 Turning to others strategy .28 – .72 Problem solving strategy .21 .08 .71 Denial strategy – .19 .81 Coping9,10 Emotion-oriented style .27 .06 .67 Task-oriented style .34 – .66 Distraction .32 .01 .67 Social diversion .36 – .64 All cohorts .43 .31 .26 1900-1909 birth cohort .12 .62 .26 Educational attainment11* 1930-1939 birth cohort .41 .32 .27 1980-1989 birth cohort .60 .13 .26

Notes: a2: additive genetic effects, c2: shared-environment effects. 1Polderman et al., 2015; 2Loehlin, 2012: models included epistasis effects; 3Loehlin, 1982; 4Markovitch et al., 2017; 5Roisman & Fraley, 2008; 6Fearon et al., 2014; 7Brussoni et al., 2000; 8Picardi et al., 2011; 9Kendler et al., 1991; 10Kozak et al., 2005; 11Silventoinen et al., 2020. *denotes meta-analytic 2 reviews. – denotes estimates of zero. Estimates were from ACE models or a = 2(rMZ – rDZ) and 2 c = 2rDZ - rMZ where rMZ and rDZ denote correlations between monozygotic and dizygotic twins.