The heritability of inhibited and uninhibited behavior: A twin study.

Young children who respond to the unfamiliar with wariness and avoidance belong to a temperamental category called behaviorally inhibited; sociable children who approach the unfamiliar are called uninhibited. This study assessed the heritable nature of these 2 categories. The longitudinal sample consisted of 178 same-sex twin pairs seen at 14 months and 162 pairs seen at 20 and 24 months. An aggregate inhibition index was constructed at each of the 3 ages by averaging standard scores for observed laboratory behaviors indicative of wariness