“Genetic and Environmental Contributions to the Covariance between Occupational Status, Educational Attainment, and IQ: A Study of Twins”, 1989-03 (; similar):
Scores of occupational status, educational attainment, and IQ were obtained for 507 monozygotic and 575 dizygotic male twin pairs born 1931–4193589ya and 1944–16196064ya.
A multivariate genetic analysis with statistics from different cohorts showed heterogeneity between cohorts, and analyses were performed in 4 separate cohorts.
The only set of results which departed clearly from the rest was found for the group born 1931–4193589ya, where the ratio of environmental to genetic effects exceeded those of the other groups. Typical heritability values in the 3 youngest groups (weighted means) were 0.43, 0.51, and 0.66 for occupation, education, and IQ, respectively. The values in the oldest group were 0.16, 0.10, and 0.37, but this sample is small and the estimates are unstable. Genetic variance influencing educational attainment also contributed ~1⁄4th of the genetic variance for occupational status and nearly half the genetic variance for IQ. The values for the between-families variances (reflecting family environment and assortative mating) varied 2%–35% in the 3 youngest groups but were higher for education (62%) and IQ (45%) in the oldest groups. All the between-families variance was common to all 3 variables. For educational attainment and IQ, the bulk of this between-families variance is probably genetic variance due to assortative mating. The common-factor environmental within-family variances were generally small, and the specific estimates seemed to contain mainly measurement error.