Sources of continuity in infant predictors of later IQ

Abstract

The sources of continuity between nine measures of infant cognitive processing and later childhood IQ were examined by application of developmental structural equation models to longitudinal data from a sample of 208 twin pairs measured from 7 to 36 months. The models were designed to assess whether the longitudinal trends observed in the infant tests follow a developmental pattern in which infant skills are persistently transmitted into those measured later, or whether transient discontinuities interrupt the continuity observed over the course of early development. Infant cognitive measures administered at 7, 8, and 9 months included Fagan's Test of Infant Intelligence, Visual Expectation and Vocalization measures, and Bayley's Infant Behavior Record. The Bayley Scales of Mental Development, administered at 12 and 24 months, and the Stanford-Binet IQ test, collected at 36 months, served as dependent measures of general mental ability. Model-fitting results suggest that, in general, the specific skills assessed by the infant measures are not continuously incorporated into later general ability measurements but, instead, appear related to general cognitive ability in a relatively unstable manner over the developmental period examined. Exploration of the genetic and environmental sources of the longitudinal measurements further suggests that, although the outcome depends upon the measure, observed continuity is a result of both genetic influences and features of the infant home environment.