Genetic, environmental, and gender effects on individual differences in toddler expressive language.

In this article, the authors examined the genetic and environmental factors influencing expressive language development in a sample of 386 toddler twin pairs participating in the Wisconsin Twin Project. Expressive language was assessed using 2 measures from the MacArthur Communicative Development Inventories-Short Form: Total Vocabulary and Two-Word Combination Use (L. Fenson et al., 2000). A sex-limitation structural equation model estimated the contribution of genetics, shared environment, and nonshared environment to individual variation. For vocabulary, heritability was higher for boys than for girls (20% vs. 8%). For word combination use, heritability was higher for girls (28% vs. 10%). However, the majority of individual variation in both boys and girls could be attributed to shared environment (54%-78%).