Behavioral genetic methods were used to estimate genetic and environmental contributions to (a) attachment-related anxiety and avoidance and (b) the overlap between these attachment dimensions and the Big Five personality traits. Two major findings emerged from these analyses. First, individual differences in attachment-related anxiety and avoidance were heritable, and second, much of the overlap between adult attachment and the Big Five traits was due to shared genetic influences. Discussion focuses on the implications of these results for attachment theory and research and provides a speculative account of the interplay between temperamental dimensions and internal working models of relationships across the life span.