A multivariate twin analysis of within-family environmental influences in infants' social responsiveness.

Observational measures of social responsiveness were obtained for twin infants. The structure of within-family environmental influences was assessed by correlating identical twin pair differences for 13 behaviors in seven social settings. Factor analysis of the difference score matrix yielded one interpretable factor. This factor indicated that within-family environmental influences were not completely random but may exert a systematic effect on social behavior in several situations.