35. Robinson, J. L., Kagan, J., Reznick, J. S., & Corley, R. (1992). The heritability of inhibited behavior: A twin study. Developmental Psychology, 28, 1030-1037.

Temperamental styles are hypothesized to be organized patterns of behavior which appear early in life, persist into childhood, and have a partial biological basis. One style, the tendency to respond to the unfamiliar with wariness or avoidance has come to be known as "behavioral inhibition". Previous developmental research of behavioral inhibition, emerging during the second year of life, has shown it to be fairly stable through age five--about one-half preserve the phenotype. Research findings from several laboratories have suggested that there is a biological contribution to this behavior. In this study of twins during the second year of life, we hypothesized that the behaviorally inhibited style would show a significant heritable component. The sample consisted of 100 MZ and 100 DZ twin pairs who completed assessments through 20 months in the MacArthur Longitudinal Twin Study. At each age, 14 and 20 months, twins were tested separately in a laboratory paradigm similar to that employed in the Harvard studies of behavioral inhibition. An index was constructed by averaging the standard scores from videotape scored latencies to approach and total times spent proximal to mother during three novel stimuli as well as the latency to leave mother during the first stimuli. At both ages, the composite measure showed relatively high internal consistency. Estimates of heritability were significant at both ages, lending support to our hypothesis. Estimates of common environmental influence were near-zero. Moderate stability was observed in the inhibition indices across the two ages, suggesting both some continuity and considerable change in the behavioral style during the second year of life.