Imitation, vocabulary, pretend play, and socially insightful behavior were investigated in 5,206 same- and opposite-sex 2-year-old twin pairs in the United Kingdom. Individual differences in imitative ability were due to modest heritability (30%), while environmental factors shared between twins (42%) and unique to each twin (28%) also made…

Forty-four Head Start classrooms were randomly assigned to enriched intervention (Head Start REDI--Research-based, Developmentally Informed) or "usual practice" conditions. The intervention involved brief lessons, "hands-on" extension activities, and specific teaching strategies linked empirically with the promotion of: (a)…