Familial resemblance in infant mental development.

Compared the scores on a scale of mental development (N. Bayley) of 151 sets of 8-mo-old twins and 2,481 other sibling pairs in the Collaborative Perinatal Project. Intraclass correlation coefficients between scores of twins were high (.84, monozygotic; .55, dizygotic), suggesting a substantial genetic influence; however, the average correlation between scores of other siblings was only .22. The discrepancy between the values for dizygotic twins and other sibling pairs may have resulted from the twins' greater environmental similarity. The high heritability estimate derived from the difference between monozygotic and dizygotic correlations was due to the higher concordance for severe retardation in monozygotic pairs. It is noted that since infant twins are a unique sample in terms of retardation frequency as well as birth weight and gestational age, it may be inappropriate to generalize infant twin study results to singleton populations. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)