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.

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A sample of 149 twin pairs was ascertained by extracting all pairs of candidates with the same surname, birth date and address from the files of a public examination taken by about two thirds of the 15-year-old cohort in South Australia. Of these there were 39 opposite sex pairs and 100 same-sex pairs whose zygosity was determined by postal questionnaire. This diagnosis was checked and found to be accurate by typing 47 pairs for ten independent genetic markers. Several analyses suggest that the ascertained twin samples is fairly representative of the fraction of the cohort who took the examination so there is no evidence to suggest that between families components of variance will be underestimated for this reason.