A sibling adoption study of adult attachment: the influence of shared environment on attachment states of mind.

This study extends existing research investigating sibling concordance on attachment by examining concordance for adult attachment in a sample of 126 genetically unrelated sibling pairs. The Adult Attachment Interview (George, Kaplan, & Main, 1985; Main, Goldwyn, & Hesse, 2003) was used to assess states of mind with regard to attachment. The average age of the participants was 39 years old. The distribution of attachment classifications was independent of adoptive status. Attachment concordance rates were unassociated with gender concordance and sibling age difference. Concordance for autonomous/non-autonomous classifications was significant at 61% as was concordance for primary classifications at 53%. The concordance rate for not-unresolved/unresolved was non-significant at 67%. Our findings demonstrate similarity of working models of attachment between siblings independent of genetic relatedness between siblings and generations (i.e., parent and child). These findings extend previous research by further implicating shared environment as a major influence on sibling similarities on organized patterns of attachment in adulthood. The non-significant concordance for the unresolved classification suggests that unresolved loss or trauma may be less influenced by shared environment and more likely to be influenced by post-childhood experiences or genetic factors.