Using a longitudinal twin study of Moscow children, we have studied the development of psychometric intelligence during the transition from preschool (age 6) to school (age 7). Children were tested using the Wechsler Intelligence Scale for Children (WISC). Simplex models were applied to explore the relationship of different sources of phenotypic variance. The following sources of variation were considered: genetic effects, common or shared family environment and unique environment. At age 6, genetic influences were much greater than those of shared environment but the magnitude of genetic influences decreased and the magnitude of shared environment influences increased substantially by age 7.