A 2003 paper in this journal reported results from a large sample of twins assessed at 2, 3 and 4 years of age on parent-administered tests and reports of their verbal and nonverbal ability. We found clear evidence for phenotypic general cognitive ability (g) that accounted for about 50% of the variance, for modest genetic influence on g (about…

In previous papers [Johnson, W., & Bouchard Jr., T. J. (2005a). Constructive Replication of the Visual-Perceptual-Image Rotation (VPR) Model in Thurstone's (1941) Battery of 60 Tests of Mental Ability. Intelligence, 33, 417-430.] [Johnson, W., & Bouchard Jr., T. J. (2005b). The Structure of Human Intelligence: It's Verbal, perceptual, and image…

Recent work with the 42 mental ability tests administered to participants of the Minnesota Study of Twins Reared Apart (MISTRA) has suggested that there are important dimensions of mental ability that function independently of "g". Two of these dimensions, rotation-verbal and focus-diffusion, appear to involve trade-offs: greater…

The study of genotype-environment interaction (G x E) has been dominated by two competing hypotheses, one that heritability is greater in high-risk environments (diathesis-stress) and the other that heritability is greater in permissive environments. The current study examined relationships between verbal and nonverbal abilities and 10 measured…

Fifteen monozygotic and 22 dizygotic twin pairs, who were found to be lower in verbal ability than matched singletons at age two years, were compared at age nine years. Although twins still showed lower verbal ability, more variance in verbal ability was accounted for by the mothers' education and birth weight than by being a twin. (SLD)