Abstract—J. Bowlby’s (1969/1982) theory of attachment, focused as it was on the survival function of attachment behaviors that the ethology of the time emphasized, led to the expectation that there would be no sex differences in patterns of attachment. Modern evolutionary thinking, however, building on insights of life history theory, parental investment theory, and sexual selection, yields an alternative prediction—that adaptive sex differences in attachment should emerge in middle childhood and be present in adults, consistent with sexual differentiation of reproductive strategies. This article reviews the theoretical basis of this expectation, including the recent proposal that a hormonally driven reorganization of attachment occurs at the beginning of middle childhood. Available data and various methodological issues involved in empirically testing the proposed model are discussed. It argues that life history theory offers a powerful organizing principle for understanding the emergence of individual differences, providing developmental researchers with exciting opportunities for empirical discovery and theoretical synthesis.