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Information about menstrual cycle variables was obtained by questionnaire using 462 female twin pairs. The twins were either members of the Institute of Psychiatry Volunteer Twin Register, or of the Birmingham Population-based Register. The two samples were analysed separately using univariate and multivariate methods so that an independent replication was obtained. Maximum likelihood estimation was used to fit simple models of genetic and environmental variation to these data. The results suggest that age of menarche, menstrual cycle regularity and premenstrual symptom reporting may be heritable, whereas menstrual cycle length is not. The results should be interpreted with caution as not all variables were replicated in the smaller sample, and the method of retrospective menstrual cycle data collection has been questioned.
A simple path model applicable to twins and their parents and involving both cultural and genetic transmission in the presence of phenotypic assortative mating was extended to cover the bivariate case. The model allows for cross assortative mating as well as cross cultural transmission. It was applied to two correlated measures derived from a fear survey questionnaire given to 1000 subjects. In allowing for the impact of more than one variable, the model allows for a much more realistic picture of cultural transmission than provided by the univariate model. The logic of the model and an application are outlined. (The authors are indebted to Professor R.J. Rose for providing the illustrative data.)
Three questionnaires measuring altruistic tendencies were completed by 573 adult twin pairs from the University of London Institute of Psychiatry Volunteer Twin Register. The questionnaires consisted of a 20-item Self-Report Altruism Scale, a 33-item Empathy Scale, and a 16-item Nurturance Scale, all of which had previously been shown to have construct validity. For the three scales, the intra-class correlations for the 296 MZ pairs were 0.53, 0.54, and 0.49, and for the 179 same-sex DZ pairs were 0.25,020, and 0.14, giving rough estimates of broad heritability of 56%, 68%, and 72%, respectively. Maximum-likelihood model-fitting revealed about 50% of the variance on each scale to be associated with genetic effects, virtually 0% to be due to the twins' common environment, and the remaining 50% to be due to each twins' specific environment and/or error associated with the test.
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