The degree to which individual differences in child – parent attachment were mediated by genetic, shared environmental, and nonshared environmental influences was investigated. One hundred and ten preschool-aged twin pairs (N= 220) were assessed in the Strange Situation and coded using conventional four-way classifications and a continuous measure of attachment security. The degree of sibling similarity in attachment was substantial, with an overall concordance rate of 67% at the secure/insecure level. The degree of concordance was equally high in monozygotic and dizygotic twin pairs, 70% and 64%, respectively, suggesting little genetic influence but a moderate degree of discordance. Twin similarity on the continuous measure of attachment security was r(57)= .48 and r(53)= .38 for monozygotic and dizygotic twin pairs, respectively, also consistent with a modest role for genetic influence but a significant effect for shared and nonshared environment. The implications for genetic influences on the environment and for understanding nonshared and shared environmental influences are discussed.