Studies investigating evolutionary theories on the origins of national differences in intelligence have been criticized on the basis that both national cognitive ability measures and supposedly evolutionarily informative proxies (such as latitude and climate) are confounded with general developmental status. In this study 14 Y chromosomal…

Many explanations have been proposed for the evolution of our anomalously large brains, including social, ecological, and epiphenomenal hypotheses. Recently, an additional hypothesis has emerged, suggesting that advanced cognition and, by inference, increases in brain size, have been driven over evolutionary time by the need to deal with…

Self-perceived abilities (SPA), which play an important role in academic achievement, have been recently reported to be fully attributable to genetic and non-shared environmental influences. To replicate and extend this finding, 732 Croatian twins (15-22 years old) were assessed on cognitive ability, self-assessed intelligence (SAI), and Five…

Correlations on subscales of Wechsler intelligence quotient tests and the Revised Beta Examination were obtained for biologically related and unrelated individuals in 181 adoptive families in the Texas Adoption Project. Generally higher correlations for biologically related individuals support the importance of genetic influence in intellectual…

To study whether individual differences in general cognitive ability differ in males and females, full-scale IQ data from 426 child twin pairs were fitted to a structural equation model of sex limitation. Individual differences in general cognitive ability appear to be substantially due to common genetic influences in males and females. (SLD)

A large number of national and geographic population samples were used to test the hypothesis that the variation in mean values of skin color in the diverse populations are consistently correlated with the mean measured or estimated IQs of the various groups, as are some other physical variables, known as an ecological correlation. Straightforward…

Genetic and environmental correlations among 11 Wechsler Intelligence Scale for Children-Revised (WISC-R) subtests were estimated from a sample of 143 twin pairs using the methodology of multivariate behavioral genetics. Correlations due to within-pair environmental influences were generally small, whereas those due to shared environmental…

Genetic and environmental etiologies of individual differences in tests of intelligence and school achievement were investigated in a study from the Colorado Adoption Project. A multivariate conditional path model was fitted to general cognitive ability and reading performance data from 119 adoptive and 120 nonadoptive families. (SLD)