Heritability in the Era of Molecular Genetics: Some Thoughts for Understanding Genetic Influences on Behavioural Traits

Folk Classification and Factor Rotations: Whales, Sharks, and the Problems With the Hierarchical Taxonomy of Psychopathology (HiTOP)

The Hierarchical Taxonomy of Psychopathology (HiTOP) uses factor analysis to group people with similar self-reported symptoms (i.e., like-goes-with-like). It is hailed as a significant improvement over other diagnostic taxonomies. However, the purported advantages and fundamental assumptions of HiTOP have received little, if any scientific scrutiny. We critically evaluated five fundamental claims about HiTOP. We conclude that HiTOP does not demonstrate a high degree of verisimilitude and has the potential to hinder progress on understanding the etiology of psychopathology. It does not lend itself to theory-building or taxonomic evolution, and it cannot account for multifinality, equifinality, or developmental and etiological processes. In its current form, HiTOP is not ready to use in clinical settings and may result in algorithmic bias against underrepresented groups. We recommend a bifurcation strategy moving forward in which the DSM is used in clinical settings while researchers focus on developing a falsifiable theory-based classification system.

Genetic and environmental contributions to IQ in adoptive and biological families with 30-year-old offspring

While adoption studies have provided key insights into the influence of the familial environment on IQ scores of adolescents and children, few have followed adopted offspring long past the time spent living in the family home. To improve confidence about the extent to which shared environment exerts enduring effects on IQ, we estimated genetic and environmental effects on adulthood IQ in a unique sample of 486 biological and adoptive families. These families, tested previously on measures of IQ when offspring averaged age 15, were assessed a second time nearly two decades later ( M offspring age = 32 years). We estimated the proportions of the variance in IQ attributable to environmentally mediated effects of parental IQs, sibling-specific shared environment, and gene-environment covariance to be .01 [95% CI .00, .02], .04 [95% CI .00, .15], and .03 [95% CI .00, .07] respectively; these components jointly accounted for 8 percent of the IQ variance in adulthood. The heritability was estimated to be .42 [95% CI .21, .64]. Together, these findings provide further evidence for the predominance of genetic influences on adult intelligence over any other systematic source of variation.

On the link of self-esteem, life satisfaction, and Neuroticism

OBJECTIVE

Self-esteem (SE) and life satisfaction (LS) are highly correlated but little is known about the sources of this association. Both characteristic adaptations are negatively correlated with Neuroticism (N). We investigated the relationship between SE, LS, and N and the degree to which shared variance was explained by N from a behavior genetic perspective.

METHOD

We analyzed more than 2,000 German same-sex twin pairs and their siblings. Twins were 17 and  23 years old and siblings were M = 21.6 years old. The sample was balanced regarding gender. We applied multivariate twin-sibling Cholesky models to obtain genetic and environmental correlations and estimated the impact N had on genetic and environmental correlations of SE and LS.

RESULTS

The genetic correlation between SE and LS was .99 and 47% of this phenotypic correlation was explained by genetics. About 28% of the common genetic variance between SE and LS was explained by shared genes with N.

CONCLUSIONS

Our findings suggest that N is not a common factor of SE and LS. The results are not in line with theories assuming that N is a basic tendency whose genetic factors account for most of the genetic variance between the assumed characteristic adaptations SE and LS.

The Dopamine D4 Receptor (DRD4) Exon 3 VNTR Contributes to Adaptive Personality Differences in an Italian Small Island Population

The search for evolutionary forces shaping the diversity of human personality traits encouraged studies that have found that islanders are relatively closed and introverted, with little interest in the external world. The ‘personality gene flow’ hypothesis was proposed to explain the mechanism underlying this difference, suggesting that the frequency of alleles that influence islander personality traits might progressively increase in the gene pools on islands because of selective emigration of individuals not displaying these alleles. We genotyped 96 individuals from the Italian mainland and 117 from Giglio Island, whose residents were previously assessed regarding their personality traits. We genotyped three polymorphisms: the dopamine D4 receptor (DRD4) exon 3 repeat region, the serotonin–transporter SLC6A4 5–HTTLPR indel and the dopamine transporter SLC6A3 DAT1 3′UTR repeat region. Only the DRD4 exon 3 repeat was hypothesised to show varying allele frequencies because this polymorphism could be associated with human migration and personality traits such as extraversion, openness and novelty seeking. As predicted, no differences in allele frequencies were found for the SLC6A4 and SLC6A3 polymorphisms, whereas significant differences were observed in the frequency of the DRD4 exon 3 alleles. The DRD4.2 repeat was more common in mainlanders, as expected, whereas the DRD4.7 allele was over–represented among islanders who never emigrated. This last result contradicts the suggested association of this allele with long–distance migrations. We suggest that emigration might have caused gene flow out the island that resulted in somewhat unpredictable changes in the frequencies of specific alleles, thus influencing islander personality traits. Copyright © 2013 European Association of Personality Psychology

Testing an Adaptationist Theory of Trait Covariation: Relative Bargaining Power as a Common Calibrator of an Interpersonal Syndrome

This article provides the first test of an adaptationist ‘common calibration’ theory to explain the origins of trait covariation, which holds that (i) personality traits are often facultatively calibrated in response to cues that ancestrally predicted the reproductive payoffs of different trait levels and (ii) distinct traits that are calibrated on the basis of common input cues will exhibit consistent patterns of covariation. This theory is applied to explain the covariation within a ‘personality syndrome’ encompassing various interpersonal trait dimensions (e.g. extraversion, emotionality and attachment styles). Specifically, it is hypothesized that these traits are inter–correlated because each is calibrated in response to relative bargaining power (RBP)—a joint function of one's ability to benefit others and harm others. Path analyses from a correlational study compellingly supported this theoretical model: Objective and self–perceived measures of RBP–enhancing phenotypic features (physical attractiveness and physical strength) influenced an internal regulatory variable indexing RBP (i.e. self–perceived RBP), which in turn had robust effects on each of the focal personality traits. Moreover, in support of the theory's core postulate, controlling for self–perceived RBP greatly reduced the covariation within the interpersonal syndrome. These novel findings illustrate the promise of an evolutionary psychological approach to elucidating trait covariation. Copyright © 2013 John Wiley & Sons, Ltd.

Highs and lows: Genetic susceptibility to daily events

Why people differ in their susceptibility to external events is essential to our understanding of personality, human development, and mental disorders. Genes explain a substantial portion of these differences. Specifically, genes influencing the serotonin system are hypothesized to be differential susceptibility factors, determining a person’s reactivity to both positive and negative environments. We tested whether genetic variation in the serotonin transporter (5-HTTLPR) is a differential susceptibility factor for daily events. Participants (N = 326, 77% female, mean age = 25, range = 17–36) completed smartphone questionnaires four times a day over four to five days, measuring stressors, uplifts, positive and negative affect. Affect was predicted from environment valence in the previous hour on a within-person level using three-level autoregressive linear mixed models. The 5-HTTLPR fulfilled all criteria of a differential susceptibility factor: Positive affect in carriers of the short allele (S) was less reactive to both uplifts and stressors, compared to homozygous carriers of the long allele (L/L). This pattern might reflect relative affective inflexibility in S-allele carriers. Our study provides insight into the serotonin system’s general role in susceptibility and highlights the need to assess the whole spectrum of naturalistic experiences.

Early childhood deprivation is associated with alterations in adult brain structure despite subsequent environmental enrichment

Millions of children worldwide live in nonfamilial institutions. We studied impact on adult brain structure of a particularly severe but time-limited form of institutional deprivation in early life experienced by children who were subsequently adopted into nurturing families. Institutional deprivation was associated with lower total brain volume in a dose-dependent way. Regionally specific effects were seen in medial prefrontal, inferior frontal, and inferior temporal areas. Deprivation-related alterations in total brain volume were associated with lower intelligence quotient and more attention deficit/hyperactivity disorder symptoms; alterations in temporal volume seemed compensatory, as they were associated with fewer attention deficit/hyperactivity disorder symptoms. We provide evidence that early childhood deprivation is related to alterations in adult brain structure, despite environmental enrichment in intervening years.

Early childhood deprivation is associated with higher rates of neurodevelopmental and mental disorders in adulthood. The impact of childhood deprivation on the adult brain and the extent to which structural changes underpin these effects are currently unknown. To investigate these questions, we utilized MRI data collected from young adults who were exposed to severe deprivation in early childhood in the Romanian orphanages of the Ceaușescu era and then, subsequently adopted by UK families; 67 Romanian adoptees (with between 3 and 41 mo of deprivation) were compared with 21 nondeprived UK adoptees. Romanian adoptees had substantially smaller total brain volumes (TBVs) than nondeprived adoptees (8.6% reduction), and TBV was strongly negatively associated with deprivation duration. This effect persisted after covarying for potential environmental and genetic confounds. In whole-brain analyses, deprived adoptees showed lower right inferior frontal surface area and volume but greater right inferior temporal lobe thickness, surface area, and volume than the nondeprived adoptees. Right medial prefrontal volume and surface area were positively associated with deprivation duration. No deprivation-related effects were observed in limbic regions. Global reductions in TBV statistically mediated the observed relationship between institutionalization and both lower intelligence quotient (IQ) and higher levels of attention deficit/hyperactivity disorder symptoms. The deprivation-related increase in right inferior temporal volume seemed to be compensatory, as it was associated with lower levels of attention deficit/hyperactivity disorder symptoms. We provide compelling evidence that time-limited severe deprivation in the first years of life is related to alterations in adult brain structure, despite extended enrichment in adoptive homes in the intervening years.

A Life History Approach to the Female Sexual Orientation Spectrum: Evolution, Development, Causal Mechanisms, and Health

Women's capacity for sexual fluidity is at least as interesting a phenomenon from the point of view of evolutionary biology and behavioral endocrinology as exclusively homosexual orientation. Evolutionary hypotheses for female nonheterosexuality have failed to fully account for the existence of these different categories of nonheterosexual women, while also overlooking broader data on the causal mechanisms, physiology, ontogeny, and phylogeny of female nonheterosexuality. We review the evolutionary-developmental origins of various phenotypes in the female sexual orientation spectrum using the synergistic approach of Tinbergen's four questions. We also present femme-specific and butch-specific hypotheses at proximate and ultimate levels of analysis. This review article indicates that various nonheterosexual female phenotypes emerge from and contribute to hormonally mediated fast life history strategies. Life history theory provides a biobehavioral explanatory framework for nonheterosexual women's masculinized body morphology, psychological dispositions, and their elevated likelihood of experiencing violence, substance use, obesity, teenage pregnancy, and lower general health. This pattern of life outcomes can create a feedback loop of environmental unpredictability and harshness which destabilizes intrauterine hormonal conditions in mothers, leading to a greater likelihood of fast life history strategies, global health problems, and nonheterosexual preferences in female offspring. We further explore the potential of female nonheterosexuality to function as an alloparental buffer that enables masculinizing alleles to execute their characteristic fast life history strategies as they appear in the female and the male phenotype. Synthesizing life history theory with the female sexual orientation spectrum enriches existing scientific knowledge on the evolutionary-developmental mechanisms of human sex differences.

Interpreting Behavior Genetic Models: Seven Developmental Processes to Understand

Behavior genetic findings figure in debates ranging from urgent public policy matters to perennial questions about the nature of human agency. Despite a common set of methodological tools, behavior genetic studies approach scientific questions with potentially divergent goals. Some studies may be interested in identifying a complete model of how individual differences come to be (e.g., identifying causal pathways among genotypes, environments, and phenotypes across development). Other studies place primary importance on developing models with predictive utility, in which case understanding of underlying causal processes is not necessarily required. Although certainly not mutually exclusive, these two goals often represent tradeoffs in terms of costs and benefits associated with various methodological approaches. In particular, given that most empirical behavior genetic research assumes that variance can be neatly decomposed into independent genetic and environmental components, violations of model assumptions have different consequences for interpretation, depending on the particular goals. Developmental behavior genetic theories postulate complex transactions between genetic variation and environmental experiences over time, meaning assumptions are routinely violated. Here, we consider two primary questions: (1) How might the simultaneous operation of several mechanisms of gene-environment (GE)-interplay affect behavioral genetic model estimates? (2) At what level of GE-interplay does the 'gloomy prospect' of unsystematic and non-replicable genetic associations with a phenotype become an unavoidable certainty?

Negative results are needed to show the specific value of a cultural explanation for g

Burkart et al. suggest that social learning can explain the cognitive positive manifold for social animals, including humans. We caution that simpler explanations of positive trait intercorrelations exist, such as genetic load. To test the suggested explanation's specificity, we also need to examine non-social species and traits, such as health, that are distal to cognitive abilities.

5-HTTLPR polymorphism is associated with nostalgia proneness: The role of neuroticism

Nostalgia, a sentimental longing for the past, is a self-relevant and social emotion. Nostalgia proneness is associated with alleviation of distress or instability (e.g., neuroticism). Although nostalgia proneness is heritable, the specific molecular contributors to this heritability are unknown. We focused on a polymorphism in the promoter of the serotonin transporter gene (5-HTTLPR) as a possible biological basis of nostalgia proneness, because the serotonin system has been associated with sensitivity to negative experience. Participants (N = 397 adults) who had reported levels of nostalgia proneness were genotyped. A subsample also completed a measure of neuroticism. Participants with the 5-HTTLPR short allele were higher on nostalgia proneness than those without this allele. Neuroticism mediated the relation between 5-HTTLPR and nostalgia proneness. These findings enrich our understanding of the genetic and personality underpinnings of nostalgia.

Genetic and environmental influences on household financial distress

Heterogeneity of household financial outcomes emerges from various individual and environmental factors, including personality, cognitive ability, and socioeconomic status (SES), among others. Using a genetically informative data set, we decompose the variation in financial management behavior into genetic, shared environmental and non-shared environmental factors. We find that about half of the variation in financial distress is genetically influenced, and personality and cognitive ability are associated with financial distress through genetic and within-family pathways. Moreover, genetic influences of financial distress are highest at the extremes of SES, which in part can be explained by neuroticism and cognitive ability being more important predictors of financial distress at low and high levels of SES, respectively.

Nostalgia and Self-Enhancement

We addressed phenotypic and genetic research questions regarding nostalgia and self-enhancement. At the phenotypic level (178 university students; Study 1), we found that nostalgia was moderately associated with self-enhancement. At the genotypic level (232 twin pairs; Study 2), we found that nostalgia, self-enhancement, and their relation were largely heritable. Our findings shed light on two heavily investigated traits and open up exciting research directions.

Personality in the Age of Industry: Structure, Heritability, and Correlates of Personality in Middle Childhood from the Perspective of Parents, Teachers, and Children

Middle childhood is a crucial juncture in the lifespan where children work towards achieving a sense of competence foundational for future development. However, middle childhood has historically been underrepresented in the personality literature. The current study provides a comprehensive examination of personality in middle childhood using a large (N = 2510), longitudinal sample of 10- to 12-year-old twins. The structure, heritability, and correlates of personality in this period were investigated using personality ratings of parents, teachers, and children. Results showed that personality in middle childhood has a coherent structure, is heritable, and is relevant for developmentally salient outcomes such as externalizing behavior, substance use, and academic engagement. Results emphasize the importance of investigating personality in middle childhood across multiple informants.

Genetics and Organizational Behavior

Articles on the genetics of complex human behaviors and psychological traits provided in past volumes of journals published by Annual Reviews tended to adopt a pathological perspective and focused heavily on the disorders of human affect and behaviors. In our review, we expand our focus to the more general, nonclinical population, and in particular on the advances in the understanding of the genetics of attitudes and behaviors in work settings. We review the recent and emerging literature using a behavioral genetics approach to examine the influence of genetics on a wide array of important constructs in organizational behavior (OB) research and provide unique theoretical insights offered by this approach. We discuss practical implications and future research directions from a broad person-environment interactionist perspective by taking a genetics approach.

Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?

Positive associations between human intelligence and brain size have been suspected for more than 150 years. Nowadays, modern non-invasive measures of in vivo brain volume (Magnetic Resonance Imaging) make it possible to reliably assess associations with IQ. By means of a systematic review of published studies and unpublished results obtained by personal communications with researchers, we identified 88 studies examining effect sizes of 148 healthy and clinical mixed-sex samples (>8000 individuals). Our results showed significant positive associations of brain volume and IQ (r=.24, R(2)=.06) that generalize over age (children vs. adults), IQ domain (full-scale, performance, and verbal IQ), and sex. Application of a number of methods for detection of publication bias indicates that strong and positive correlation coefficients have been reported frequently in the literature whilst small and non-significant associations appear to have been often omitted from reports. We show that the strength of the positive association of brain volume and IQ has been overestimated in the literature, but remains robust even when accounting for different types of dissemination bias, although reported effects have been declining over time. While it is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as an isomorphic proxy of human intelligence differences.

What Makes Nations Intelligent?

Modern society is driven by the use of cognitive artifacts: physical instruments or styles of reasoning that amplify our ability to think. The artifacts range from writing systems to computers. In everyday life, a person demonstrates intelligence by showing skill in using these artifacts. Intelligence tests and their surrogates force examinees to exhibit some of these skills but not others. This is why test scores correlate substantially but not perfectly with a variety of measures of socioeconomic success. The same thing is true at the international level. Nations can be evaluated by the extent to which their citizens score well on cognitive tests, including both avowed intelligence tests and a variety of tests of academic achievement. The resulting scores are substantially correlated with various indices of national wealth, health, environmental quality, and schooling and with a vaguer variable, social commitment to innovation. These environmental variables are suggested as causes of the differences in general cognitive skills between national populations. It is conceivable that differences in gene pools also contribute to international and, within nations, group differences in cognitive skills, but at present it is impossible to evaluate the extent of genetic influences.

Comparing the Developmental Genetics of Cognition and Personality over the Life Span

Empirical studies of cognitive ability and personality have tended to operate in isolation of one another. We suggest that returning to a unified approach to considering the development of individual differences in both cognition and personality can enrich our understanding of human development. We draw on previous meta-analyses of longitudinal, behavior genetic studies of cognition and personality across the life span, focusing particular attention on age trends in heritability and differential stability. Both cognition and personality are moderately heritable and exhibit large increases in stability with age; however, marked differences are evident. First, the heritability of cognition increases substantially with child age, while the heritability of personality decreases modestly with age. Second, increasing stability of cognition with age is overwhelmingly mediated by genetic factors, whereas increasing stability of personality with age is entirely mediated by environmental factors. Third, the maturational time-course of stability differs: Stability of cognition nears its asymptote by the end of the first decade of life, whereas stability of personality takes three decades to near its asymptote. We discuss how proximal gene-environment dynamics, developmental processes, broad social contexts, and evolutionary pressures may intersect to give rise to these divergent patterns.

Self-Reported Personality Pathology Has Complex Structure and Imposing Simple Structure Degrades Test Information

Although simple structure is often used as a theoretical guideline for rotation of exploratory factor structures, its empirical foundation is sometimes not investigated. We examined the degree of simple structure in 2 self-report inventories advocated for the description of personality pathology. The multivariate complexity of the inventories was compared against simulated factorial solutions with increasing degrees of simple structure. Results revealed a high degree of complexity in both inventories, regardless of whether focusing on a few broad or many narrow factors and whether items or scales were analyzed. It is concluded that Thurstone's (1934) simple structure criteria are rarely fulfilled in multivariate personality pathology solutions, and, therefore, that alternate rotations represent equally justifiable ways to operationalize the space. Furthermore, using iterative test development to impose simple structure where none originally existed increased standard errors around the factor scores in the interstitial areas between imposed simple structure dimensions. In both simulated and real data, multidimensional item response theory test information functions based on complex solutions demonstrated that test information was uniformly distributed across the space. In contrast, in simple structure solutions, information was weak in areas of the space where nonconforming items had been removed. Implications are discussed.

GE covariance through phenotype to environment transmission: an assessment in longitudinal twin data and application to childhood anxiety

We considered identification of phenotype (at occasion t) to environment (at occasion t + 1) transmission in longitudinal model comprising genetic, common and unique environmental simplex models (autoregressions). This type of transmission, which gives rise to genotype-environment covariance, is considered to be important in developmental psychology. Having established identifying constraints, we addressed the issue of statistical power to detect such transmission given a limited set of parameter values. The power is very poor in the ACE simplex, but is good in the AE model. We investigated misspecification, and found that fitting the standard ACE simplex to covariance matrices generated by an AE simplex with phenotype to E transmission produces the particular result of a rank 1 C (common environment) covariance matrix with positive transmission, and a rank 1 D (dominance) matrix given negative transmission. We applied the models to mother ratings of anxiety in female twins (aged 3, 7, 10, and 12 years), and obtained support for the positive effect of one twin's phenotype on the other twin's environment.

What Drives Adult Personality Development? A Comparison of Theoretical Perspectives and Empirical Evidence

Increasing numbers of empirical studies provide compelling evidence that personality traits change across the entire lifespan. What initiates this continuing personality development and how does this development proceed? In this paper, we compare six theoretical perspectives that offer testable predictions about why personality develops the way it does and identify limitations and potentials of these perspectives by reviewing how they hold up against the empirical evidence. While all of these perspectives have received some empirical support, there is only little direct evidence for propositions put forward by the five–factor theory of personality and the theory of genotype → environment effects. In contrast, the neo–socioanalytic theory appears to offer a comprehensive framework that fits the empirical findings and allows the integration of other, more specialized, perspectives that focus on specific aspects of personality development like the role of time, systematic differences between categories of social roles or the active partake of the person himself or herself. We draw conclusions on the likely driving factors for adult personality development and identify avenues for future research. Copyright © 2014 European Association of Personality Psychology

The effect of paternal age on offspring intelligence and personality when controlling for paternal trait level

Paternal age at conception has been found to predict the number of new genetic mutations. We examined the effect of father’s age at birth on offspring intelligence, head circumference and personality traits. Using the Minnesota Twin Family Study sample we tested paternal age effects while controlling for parents’ trait levels measured with the same precision as offspring’s. From evolutionary genetic considerations we predicted a negative effect of paternal age on offspring intelligence, but not on other traits. Controlling for parental intelligence (IQ) had the effect of turning an initially positive association non-significantly negative. We found paternal age effects on offspring IQ and Multidimensional Personality Questionnaire Absorption, but they were not robustly significant, nor replicable with additional covariates. No other noteworthy effects were found. Parents’ intelligence and personality correlated with their ages at twin birth, which may have obscured a small negative effect of advanced paternal age (<1% of variance explained) on intelligence. We discuss future avenues for studies of paternal age effects and suggest that stronger research designs are needed to rule out confounding factors involving birth order and the Flynn effect.

On the Nature and Nurture of Intelligence and Specific Cognitive Abilities

To further knowledge concerning the nature and nurture of intelligence, we scrutinized how heritability coefficients vary across specific cognitive abilities both theoretically and empirically. Data from 23 twin studies (combined N = 7,852) showed that (a) in adult samples, culture-loaded subtests tend to demonstrate greater heritability coefficients than do culture-reduced subtests; and (b) in samples of both adults and children, a subtest’s proportion of variance shared with general intelligence is a function of its cultural load. These findings require an explanation because they do not follow from mainstream theories of intelligence. The findings are consistent with our hypothesis that heritability coefficients differ across cognitive abilities as a result of differences in the contribution of genotype-environment covariance. The counterintuitive finding that the most heritable abilities are the most culture-dependent abilities sheds a new light on the long-standing nature-nurture debate of intelligence.

The history of the nature/nurture issue

It is worthy to supplement Charney with two historical issues: (1) There were two rival trends in the rebirth of genetic thought in the 1960s: the universal and the variation related. This traditional duality suggested that heredity cannot be equated with genetic determinism. (2) The classical debates and reinterpretation of adoption/twin studies in the 1980s regarding intelligence suggested that the environment had a more active role in unfolding the genetic program.

Correlation and Causation in the Study of Personality

Personality psychology aims to explain the causes and the consequences of variation in behavioural traits. Because of the observational nature of the pertinent data, this endeavour has provoked many controversies. In recent years, the computer scientist Judea Pearl has used a graphical approach to extend the innovations in causal inference developed by Ronald Fisher and Sewall Wright. Besides shedding much light on the philosophical notion of causality itself, this graphical framework now contains many powerful concepts of relevance to the controversies just mentioned. In this article, some of these concepts are applied to areas of personality research where questions of causation arise, including the analysis of observational data and the genetic sources of individual differences. Copyright © 2012 John Wiley & Sons, Ltd.

Understanding Heritability: What it is and What it is Not

Commentators generally found our exposition of the concept of heritability helpful for psychologists, suggesting that we largely accomplished our primary goal. Many provided supplemental and helpful perspectives on concepts we addressed. A few of the comments indicated that we may not have been completely successful in making clear our secondary goal, which was to outline how heritability estimates confound a plethora of influences. In this response, we thus emphasize that we do not claim that specific kinds of complexity, or, even worse, intractable complexity, pervade the genetics of behavioural traits. Rather, our claim is that genetics is riddled with complexity of many degrees and kinds, and heritability is a poor indicator of either degree or kind of underlying genetic complexity. Copyright © 2011 John Wiley & Sons, Ltd.