Western psychologies have traditionally given greater importance to self-development than to interpersonal relatedness, stressing the development of autonomy, independence, and identity as central factors in the mature personality. In contrast, women, many minority groups, and non-Western societies have generally placed greater emphasis on issues of relatedness. This article traces the individualistic bias and recent challenges to this view. It is proposed that evolutionary pressures of natural selection result in two basic developmental lines: interpersonal relatedness and self-definition, which interact in a dialectical fashion. An increasingly mature sense of self is contingent on interpersonal relationships; conversely, the continued development of increasingly mature interpersonal relationships is contingent on mature self-definition. Conclusions include implications for social policy and for facilitating more balanced development of both dimensions in all members of society.