A genetic analysis of the trait of neuroticism and symptoms of anxiety and depression in 3,810 pairs of adult MZ and DZ twins is reported. Differences between people in these measures can be explained simply by differences in their genes and in their individual environmental experiences. There is no evidence that environmental experiences that are shared by cotwins, such as common family environment or social influences, are important. There are differences between the sexes in gene action affecting neuroticism, and genetic effects become more pronounced with age in females. The lack of evidence for dominance variance affecting neuroticism contrasts well with the detection of considerable genetical nonadditivity for extraversion in the same sample and reinforces the view that these two traits are not only statistically, but also genetically, quite independent. An analysis of the causes of covariation between anxiety, depression, and neuroticism shows that additive gene effects are more important causes of covariation than environmental factors. Genetic variation in symptoms of anxiety and depression is largely dependent on the same factors as effect the neuroticism trait. However, there is also evidence for genetic variation specific to depression.

Data gathered in Australia and England on the social attitudes of spouses and twins are largely consistent with a genetic model for family resemblance in social attitudes. There is substantial assortative mating and little evidence of vertical cultural inheritance.The facility with which humans learn and their great investment in mate selection, parental care, and education make the human species a model system for the study of cultural inheritance. Until comparatively recently, however, genetic models for family resemblance such as those devised by Fisher (1) were superior to cultural models because the former were quantitative and led naturally to statistical estimation and hypothesis testing. The emphasis of theoretical analysis has changed over the last 10 years, with the formulation of many quantitative models for the contribution of cultural inheritance to individual differences and family resemblance (2-8). Such models have explored vertical transmission between parent and child, horizontal transmission between siblings, and one-to-many oblique transmission between teacher and students.This seminal theoretical work on cultural inheritance has not been matched by the collection of informative data. For example, illustrate their models of vertical transmission with data on interests and attitudes from a small sample of nuclear families (n = 203) and pairs of friends (n = 98) ascertained from Stanford University undergraduates. The authors themselves admit that the nuclear family design, comprising only parents and children, may illustrate models of vertical transmission but is incapable of resolving biological and cultural inheritance. Thus, while their study focused on measures in which the a priori likelihood of cultural inheritance was greatest, the power of their analysis was constrained by their experimental design and small sample size.Over the last 15 years, data on very large samples of monozygotic and dizygotic twins and spouses have been collected in the attempt to provide more powerful resolution of the basic elements of cultural and biological inheritance. In the case of personality measures, mate selection is virtually random and the resemblance between relatives is almost entirely genetic in origin (9-12). There is little evidence that cultural inheritance contributes to individual differences in personality in the populations studied so far.Social attitudes present a marked contrast to personality measures. They too show substantial family resemblance (8,13,14), but the similarity between mates for social attitudes is also considerable (14, 15). Secular changes in attitudes are so rapid (16) that frequent revision of test instruments is necessary. On the face of it, such findings lend support to a purely cultural model for family resemblance. However, studies of attitudes so far have not tested the assumption that vertical transmission is cultural. They have not addressed the alternative hypothesis that individuals are influenced by their genotypes in their acquisition of particular ...

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Abstract.A genetic analysis of alcohol consumption in 3810 pairs of adult twins is reported. When no correction was made for age, individual environmental variance, including non-repeatable errors of reporting, accounted for approximately 44% of variation in both sexes. In females, there was no evidence of shared environmental effects and 56% of the variance was genetic in origin. In males, only 36% of the variance was genetic and common environmental effects accounted for the remaining 20% of individual differences.For females, the results for younger (30 years and under) and older (over 30) twins were similar. For males, however, the effect of age was striking. In younger male twins over 60% of the variance was genetic in origin, with the remaining variance due to environmental influences unique to the indiviudal. In older twins genetic differences do not appear to be important, with approximately 50% of the total variance due to individual environmental differences and the remaining 50% due to the effect of the common family environment. Our results suggest that both age and sex need to be considered when analysing the causes of variation in alcohol consumption.