Abstract
Age is a potential source of variation that contributes to differences between, but not within, twin pairs. In most genetic analyses of twin data, linear and other functions of age are usually removed prior to model fitting. This correction is typically applied only within twin groups of the same sex and zygosity, and no heterogeneity test of age regressions is performed. Here we include age as a variable in the model-fitting procedure and allow for tests of heterogeneity of age regressions across sex and zygosity groups. The LISREL formulation of the approach is illustrated with data collected from Australian twins on subjective impressions of drunkenness following alcohol consumption. The results indicate significant negative covariation of impressions of drunkenness with age. The data support a simple model of additive genetic and unique environmental variation. No evidence was found for sex differences in genetic or environmental components of variation.
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The theoretical work and data analysis described in this paper were made possible by NATO Grant 86/0823 and grants from the Belgian National Research Fund, the State University of Gent, and the Catholic University of Leuven. We are also grateful to Drs. R. Vlietinck and R. Derom for excellent organization of the successful workshop. Data collection was made possible by a grant from the Australian Associated Brewers to N.G.M. and Drs. J. G. Oakeshott, J. B. Gibson, and G. A. Starmer and by grants from the Australian National Health and Medical Research Council. The authors were supported by NIH Grants MH-40828 and AA-06781.