A twin analysis was applied to infants' social behavior in standardized situations that permitted the comparison of social responding to mother and a stranger in different contexts. Ninety-two middle-class children (members of 21 identical twin pairs and 25 same-sex fraternal twin pairs, with an average age of 22.2 months) were observed in their homes using time-sampled observations of specific behaviors in seven situations: (a) a 5-minute warm-up period, (b) approach by stranger, (c) play with stranger, (d) play with mother, (e) cuddling with mother, (f) cuddling with stranger, and (g) separation from mother. Comparisons between intraclass correlations for identical and fraternal twins yielded significant differences for social behavior directed toward the stranger but not toward the mother. In addition, a novel measure of attachment using the difference in social responding toward the mother and stranger suggested heritable influences only in the initial warm-up situation. We conclude that in infancy, heredity affects individual differences in social responding more to unfamiliar persons than to familiar persons. The behavioral-genetic analysis also suggests that the substantial influence of environment in the development of social behavior may occur primarily within families (making members of a family different from one another-including systematic as well as stochastic effects) rather than between families (making family members similar to one another and different from other families), as is widely assumed. (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1979 American Psychological Association.
