Wendy Johnson, Thomas Bouchard, and their colleagues have an important paper in press at Intelligence reanalyzing data from the Minnesota Study of Twins Reared Apart (MISTRA). As many of you may be aware, the Minnesota group previously reported a correlation in IQ of 0.75 for 40+ pairs of identical twins reared apart (Bouchard et al., 1990). This implies a broad-sense heritability of 0.75; that is, three quarters of the variance in IQ is ascribable to genetic factors. MISTRA has continued since then, and this latest paper provides an update.

The sample used for this study included 126 twin pairs (74 monozygotic, 52 dizygotic) only. Most of the pairs were separated early in life, reared in adoptive families, and reunited only in adulthood. A few were separated early in life but reared by different biological family members. The twins were primarily from North America, Great Britain, and Australia, though several came from other countries and a few had been raised in different countries. They ranged in age from 18 to 79 years (mean = 42.7, SD = 13.6). Their educational backgrounds varied from less than high school to post-graduate experience, and occupations ranged accordingly.

These subjects were administered over 40 ability tests during their participation in MISTRA. In this reanalysis heritabilities were estimated for g in particular and also for lower-order factors such as verbal ability and image rotation. The heritability of g was estimated to be 0.77 (95% CI: 0.66-0.84). Estimated heritabilities for lower-order factors were also substantial.

The contribution to the correlation between twins caused by similarity in rearing environments was estimated by multiplying the square of the environment-score correlation by the correlation between twins in the environmental measure. It was found that the contributions to the correlation between twins in g by familial cohesion, expressiveness, conflict, independence, achievement orientation, intellectual-cultural orientation, active-recreational orientation, moral-religious emphasis, organization, and control (all dimensions of the Family Environmental Scale) were all zero to within two decimal places. The contributions by family size, parental occupation, parental education, and possessions in the home (including material, cultural, mechanical, and scientific possessions) ranged from zero to 0.02.

Labels: general intelligence, Genetics, IQ