A number of relatively small-sample, genetically sensitive studies of infant attachment security have been published in the past several years that challenge the view that all psychological phenotypes are heritable and that environmental influences on child development--to the extent that they can be detected--serve to make siblings dissimilar. Using the twin subsample (N = 485 same-sex pairs) of the nationally representative Early Childhood Longitudinal Study-Birth Cohort, the authors provide evidence that parenting quality and infant attachment security observed at 24 months, as well as their covariation, are a product of shared and nonshared environmental (but not genetic) variation among children. In contrast, genetic differences between infants played a prominent role in explaining observations of temperamental dependency. (Contains 4 tables and 5 footnotes.)