DZ twins comprise nearly three-fourths of the realized sample for ERP study, rather than the two-thirds of the full population sample from which the sample was selected. Means and variances of adolescent RAPI scores of the 358 ERP-studied twins, and, more important, their mean absolute intrapair RAPI differences and the variances of those differences are substantially elevated from the full sample of twins assessed at that age. The mean absolute difference in RAPI scores at age 18½ for twin pairs selected for laboratory study approached 1.7 times that of the full sample of twins (N = 5,080) from which it was drawn, and the variance of those absolute differences among selected twin pairs was elevated 1.4 times above that of the full sample. Twins selected for RAPI-discordance (with an intrapair difference ≥10 points on the 66-point possible range of RAPI scores) approximate the upper one-sixth of the distribution of intrapair RAPI differences across all ∼2,500 twin pairs in the epidemiological sample from which they were drawn. Similar, but substantially smaller, effects were found for individual RAPI and Mm-MAST scores and
