Attachment theory has strong environmental origins, from which it has been the broad consensus that variation in attachment is caused by variation in environments (shared and non-shared), with little role of genetic variation, especially in infants. However, the state of the behavior genetic evidence needs critical evaluation to discern whether such environmentalist claims are warranted. Here, I demonstrate that the majority of the available twin studies are not adequately powered to detect additive genetic effects if present, and that the majority of twin studies in infants are not adequately powered to detect shared environmental effects. Within the published research, I can find only three adequately powered twin studies on attachment: one in infants, one in adolescents, and one in adults. I conclude by presenting five observations from behavior genetic studies that future developmental frameworks of attachment must account for.