Physical aggression and expressive vocabulary in 19-month-old twins.

In the prevention of physical aggression, possible etiological links with language development are rarely taken into account. Indeed, little is known about when language and aggressive behavior become linked during development and which mechanisms are responsible for this association. This study investigated the association between physical aggression and language in late infancy with a genetic design that involved 562 19-month-old twins. A modest but significant correlation (r = -.20) was found between physical aggression and expressive vocabulary. Substantial heritability was found for physical aggression. Quantitative genetic modeling suggests that the correlation between expressive vocabulary and physical aggression cannot be explained by shared etiologies. However, phenotype-to-phenotype models indicate that the covariation can be entirely accounted for by a significant phenotypic path from expressive vocabulary to physical aggression. The implications of these results for early prevention of chronic physical aggression are discussed.