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