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

Inhibited behavior in 157, 24-month-old twin pairs was studied in a si tuation in which two pairs of twins, unfamiliar with each other, playe d together in an unfamiliar room. An intraclass correlation comparison between monozygotic (MZ) and dizygotic (DZ) twins showed that the her itability for inhibited behavior, for the entire sample, was significa ntly different from zero: r(MZ) = .82, r(DZ) = .47. The genetic influe nce on extreme inhibition was assessed using a relatively new techniqu e which employs a multiple-regression model to predict the co-twin's i nhibition score from the proband's score and the degree of genetic rel ationship (1.0 for MZ twins and 0.5 for DZ twins). The partial regress ion of co-twin's score on relationship, an index of the extent to whic h the deviant scores of probands are due to heritable influences, was large and statistically significant. This result provides evidence fav oring a genetic contribution to behavioral inhibition in children.