Previous research has revealed only modest concordance in attachment security between siblings during childhood and adolescence. The first goal of this dissertation was to estimate sibling concordance in adult attachment security and identify factors contributing to divergence. The Adult Attachment Interview (AAI) was administered to young adult siblings to assess security as well as inferred childhood experiences with parents. Parental behaviors were also assessed via questionnaires. Participants' AAI security was unrelated within sibling pair, and differential experiences with parents partly accounted for AAI divergence. Furthermore, less supportive parenting compared with one's sibling puts individuals at relative risk for attachment insecurity. Findings are consistent with the view that within-family attachment divergence can be explained in light of nonshared environmental experiences, particularly with parents. An additional goal of this work was to link adult attachment with sibling relationships. Security was associated with self-reported and observed relationship quality. In addition, sibling dissimilarity on security predicted non-collaborative behaviors during sibling interactions. The study adds to similar findings during childhood, as well as extends the predictive significance of the AAI to this unique interpersonal context. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]