The Sequenced Inventory of Communication Development: An Adoption Study of Two and Three-year olds

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Abstract
The etiology of individual differences in communicative development is explored using data from 226 adoptive and 224 nonadoptive families in the Colorado Adoption Project. Scores from the Sequenced Inventory of Communication Development (SICD) (Hedrick, Prather, & Tobin, 1975) in 2-and 3-year-old children are compared to concurrent and longitudinal measures of verbal and general cognitive ability and to verbal and general cognitive ability in their parents. Sibling correlations for the SICD and cross-correlations for the SICD and I.Q. in related and unrelated pairs are also examined. Sibling correlations indicate that performance on the SICD is genetically influenced and that the relationship between the SICD and I.Q. at 2 and 3 years of age is in part genetically mediated. At 2 and 3 years of age, SICD scores are related to parental intelligence and verbal ability and this relationship is partially determined by family environmental factors at both ages. At 3 years of age, the SICD scores of adopted children correlates significantly with their biological mothers' general cognitive ability.