of identified families; for details on nonparticipation see Heath et al., 2002) with complete data on all variables were included in the present analysis [831 monozygotic (MZ) pairs, 615 dizygotic (DZ) pairs]. 13% of the sample classified themselves as minority and almost exclusively as African-American, reflecting the minority composition of the Missouri population. Self-reported maternal education levels included 9.8% ‘without high school diploma,’ 39.5% ‘high school diploma without any college education,’ 29.2% ‘some college education,’ and 21.4% ‘degree from 4-year college or more.’
