The phenotypic and genetic relationships among measures of cognitive ability, temperament, and scholastic achievement.

The covariance among measures of cognitive ability, temperament, and scholastic achievement was examined in a subsample of 326 (89 Monozygotic, 74 Dizygotic) twins drawn from the Western Reserve Twin Project. Both phenotypic and behavioral genetic models were fit to the data. Univariate analyses indicate significant genetic influences on cognitive, achievement, and temperament variables. Common environmental influences also affected cognition and achievement but not temperament. Multivariate analyses indicate that both genetic and common environmental influences contribute to the covariance among all three variables. Cognition and achievement are highly genetically correlated. In contrast, achievement and temperament are highly correlated for common environmentality, while cognition and temperament are not.