Genetic and environmental influences on MMPI factor scales: joint model fitting to twin and adoption data

In general, the shared family environment appears to play a negligible role in determining individual differences in personality and interests. Nevertheless, scattered reports of significant shared environmental influence on such variables appear in the literature. Using data from the Texas Adoption Project (TAP), the current study attempted to replicate twin study findings of significant shared environmental variance on four of nine Minnesota Multiphasic Personality Inventory (MMPI) factor scales (Rose, 1988). Conventional behavioral genetic analyses of the adoption data agreed in affirming a significant shared environmental influence on individual differences in Religious Orthodoxy only. Subsequent simultaneous modeling of Rose's twin data and TAP adoption data resulted in three scales (Extraversion, Inadequacy, and Religious Orthodoxy) showing significant shared environmental influence. Again, effects were most substantial for Religious Orthodoxy, where the shared environment accounted for nearly 50% of the variance. It is argued that assortative mating cannot explain this finding.