Individual differences in social introversion-extraversion were examined in a large sample of twin girls and found to be highly heritable. Genetic factors accounted for more than half of the within-family variance, measured by Gough's Adjective Check List and the Fels Behavior Scales. The results for this population support previous studies by Gottesman, Eysenck, Vandenberg, Freedman, and others, who reported similar heritabilities for social introversion-extraversion in other populations, tested with other measures. Several longitudinal studies have reported consistent individual differences in sociability, which may be explained by the large genetic contribution to the dimension. Both longitudinal and twin data suggest that social introversion-extraversion is a basic way of responding to the environment, produced by polygenic inheritance and environmental interaction.