A model of developmental change in hierarchical phenotypes with application to specific cognitive abilities.

A hierarchical longitudinal path model is described for analysis of twin and sibling data. The model combines multivariate and longitudinal methodologies for assessment of continuity and change in the relationships among characters over time. Additionally, the model permits assessment of shared and independent etiologies for groups of measures at single and multiple occasions. The procedure is illustrated by application to specific cognitive ability data from 103 adopted and 109 nonadopted sibling pairs at ages 3, 4, 7, and 9 years, and 50 pairs of monozygotic and dizygotic twins at ages 3 and 4 years. The results suggest that much of the observed continuity in general intelligence measures is attributable to genetic influences common to specific abilities and indicate differential etiologies for specific abilities at different occasions in childhood.