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AbstractThis article discusses findings of two recent studies conducted in collaboration with the East Flanders Prospective Twin Survey in the field of cognitive ability. The first study examined the effect of chorion type on heritability estimates of intelligence in children. The second study investigated the causes of association between child psychopathology and lower cognitive ability. Findings of these studies are discussed in the light of the current view on cognitive ability (or 'g') and recommendations for future research are made.

AbstractTestis size is an important feature of male pubertal development. The genetic and environmental contributions to variation in human testis size have hardly been studied. We estimated the heritability of human testicular size in a group of mono- and dizygotic twins and their non-twin brothers (145 twins and 20 brothers from 95 families). Participants were 18 years old on average and all had reached Tanner development stage 4 or higher. Dizygotic twins and their siblings had a larger mean testis volume than monozygotic twins and their siblings. There was significant familial resemblance, with higher correlations in monozygotic twin pairs (0.59) than in dizygotic twin and sibling pairs (0.34). Heritability was estimated at 59% (95% CI = 37–75%), but a model that excluded genetic influences and attributed all familial resemblance to shared environment, fitted the data only marginally worse. The finding of larger mean testis volume in dizygotic twins may be of interest for future research into the mechanisms underlying dizygotic twinning.

The questions whether and how much genetic factors affect psychological dimensions and disorders represent important first steps in understanding the origins of individual differences. Because it is now widely accepted that genetic influences contribute importantly to individual differences throughout psychology, genetic research is moving beyond merely estimating heritability to asking questions about how genetic mechanisms work. We focus on two examples of ways in which genetic research is going beyond heritability. The first is to use genetically sensitive designs to identify specific environmental influences, taking into account two of the most important findings from behavioral genetics: nonshared environment and genotype-environment correlation. The second is to use the new tools of molecular genetics to identify specific genes responsible for the substantial heritability of a variety of behavioral traits.

One of the strongest regularities in the empirical political science literature is the well-known correlation in parent and child partisan behavior. Until recently, this phenomenon was thought to result solely from parental socialization, but new evidence on genetic sources of behavior suggests it might also be due to heritability. In this article, the authors hypothesize that genes contribute to variation in a general tendency toward strength of partisanship. Using data collected at the Twins Days Festival, the authors compare the similarity of partisan strength in identical twins who share all of their genes to the similarity of partisan strength in nonidentical twins who share only half of their genes. The results show that heritability accounts for almost half of the variance in strength of partisan attachment, suggesting we should pay closer attention to the role of biology in the expression of important political behaviors.

Behavioral genetics and cultural evolution have both revolutionized our understanding of human behavior-largely independent of each other. Here we reconcile these two fields under a dual inheritance framework, offering a more nuanced understanding of the interaction between genes and culture. Going beyond typical analyses of gene-environment interactions, we describe the cultural dynamics that shape these interactions by shaping the environment and population structure. A cultural evolutionary approach can explain, for example, how factors such as rates of innovation and diffusion, density of cultural sub-groups, and tolerance for behavioral diversity impact heritability estimates, thus yielding predictions for different social contexts. Moreover, when cumulative culture functionally overlaps with genes, genetic effects become masked, unmasked, or even reversed, and the causal effects of an identified gene become confounded with features of the cultural environment. The manner of confounding is specific to a particular society at a particular time, but a WEIRD (Western, educated, industrialized, rich, democratic) sampling problem obscures this boundedness. Cultural evolutionary dynamics are typically missing from models of gene-to-phenotype causality, hindering generalizability of genetic effects across societies and across time. We lay out a reconciled framework and use it to predict the ways in which heritability should differ between societies, between socioeconomic levels and other groupings within some societies but not others, and over the life course. An integrated cultural evolutionary behavioral genetic approach cuts through the nature-nurture debate and helps resolve controversies in topics such as IQ.

We aimed to determine (1) the prevalence of premenstrual syndrome in a sample of twins and (2) the relative contribution of genes and environment in premenstrual syndrome. A group of 193 subjects inclusive of same gender twins (n = 176) and females from opposite sex twin sets (n = 17) entered the study. Heritability analysis used same gender twin data only. The probandwise concordance rate for the presence or absence of premenstrual syndrome was calculated and the heritability of premenstrual syndrome was assessed by a quantitative genetic model fitting approach using MX software. The prevalence of premenstrual syndrome was 43.0% and 46.8% in monozygotic and dizygotic twins, respectively. The probandwise concordance for premenstrual syndrome was higher in monozygotic (0.81) than in dizygotic twins (0.67), indicating a strong genetic effect. Quantitative genetic modeling found that a model comprising of additive genetic (A) and unique environment (E) factors provided the best fit (A: 95%, E: 5%). No association was found between premenstrual symptom and the following variables: belonging to the opposite gender twin set, birth weight, being breast fed and vaccination. These results established a clear genetic influence in premenstrual syndrome.

Background: The normal development of cortisol regulation during childhood is thought to be influenced by a complex interplay between environmental and genetic factors. Method: The aim of this study was to estimate genetic and environmental influences on basal cortisol levels in a sample of 151 twin pairs aged 9–16 years. Salivary cortisol was collected on two consecutive days when the children attended school — immediately after awakening, 30 min post-awakening and at bedtime. Results: Heritability was highest (60%) for cortisol levels about 30 min after awakening. For samples taken immediately at awakening heritability was less pronounced (28%) and in the evening low (8%). Conclusion: The limited genetic influence on evening levels, moderate on cortisol at awakening and high on awakening response, might imply two genetic regulation patterns, one specifically for awakening response and one for the circadian rhythm proper. These findings could explain divergent results in previous studies and highlight the importance of taking the circadian rhythm into account in studies of cortisol levels in children.