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. The index had relatively high internal consistency at all ages and was moderately stable across adjacent ages. Heritability was a significant factor at all ages. Analyses of data from selected twin pairs suggest that extremely inhibited behaviors are heritable. Results of longitudinal genetic correlation analyses provide evidence for substantial genetic contributions to continuity and change from 14 to 24 months.