Two of the most striking characteristics of contraceptive practice in the world today are the wide variation in patterns of use across countries and the tendency of the distribution of use by method to persist or narrow, even as new methods become available. The argument advanced in this article is that the disposition to commit to a reduced range of methods results from positive feedback in the process of contraceptive choice, and follows the logic of path dependence. The positive feedback derives, in large part, from social interaction among both the providers and the users of contraceptive methods. The persistence of outmoded contraceptive regimes is illustrated with the experience of Mexico and Brazil. In each case, it is argued that the conditions, events, and policies in the early stage of the adoption process have had a determinant bearing on the contraceptive practice prevailing in the late 1990s.