GENETIC ETIOLOGY OF BEHAVIORAL-INHIBITION AMONG 2-YEAR-OLD CHILDREN

Inhibited behavior in 157, 24-month-old twin pairs was studied in a situation in which two pairs of twins, unfamiliar with each other, played together in an unfamiliar room. An intraclass correlation comparison between monozygotic (MZ) and dizygotic (DZ) twins showed that the heritability for inhibited behavior, for the entire sample, was significantly different from zero: r(MZ) = .82, r(DZ) = .47. The genetic influence on extreme inhibition was assessed using a relatively new technique which employs a multiple-regression model to predict the co-twin's inhibition score from the proband's score and the degree of genetic relationship (1.0 for MZ twins and 0.5 for DZ twins). The partial regression of co-twin's score on relationship, an index of the extent to which the deviant scores of probands are due to heritable influences, was large and statistically significant. This result provides evidence favoring a genetic contribution to behavioral inhibition in children.