In an earlier study of blood pressure variation in middle aged parents and their young adult twin offspring, the greater blood pressure variation observed in the parent sample was accounted for in terms of an increasing influence of individual environmental experiences with increasing age and a commensurate reduction in the impact of heredity. In the present study, the sample size was enlarged to provide a more powerful test of these effects. Maximum likelihood model-fitting techniques were applied to blood pressure covariation in balanced pedigrees, consisting of 85 families (40 MZ and 45 DZ twin pairs). As before, our analysis indicated that a developmental effect was a salient factor in the older age group.
