Genetic and environmental variation in Eysenck Personality Questionnaire scales measured on Australian adolescent twins

The Eysenck Personality Questionnaire was administered to 1400 Australian twin pairs aged 11 to 18, and the data were analyzed by a multivariate normal model using the software FISHER. For each scale, attempts were made to transform to normality, about a mean modeled separately for each sex as a quadratic function of age. Variances and covariances were estimated for each sex-zygosity group as a monotone function of age. Evidence for genetic sources of variation were assessed in part by fitting models which allowed for age-dependent, sex-specific, and correlated additive genetic factors, and age-dependent and sex-specific environmental factors, under the assumption that effects of environmental factors common to twin pairs are independent of zygosity. Evidence for genetic factors independent of age and sex was most compelling for Psychoticism and Neuroticism. For Extraversion, if genetic factors exist they would be mostly sex-specific and age-dependent. For the Lie scale there was evidence for, at most, a small component of genetic variation.