Over the last three decades many authors have addressed the demographic transition from the perspective of evolutionary theory. Some authors have emphasized parental investment factors such as the costs of raising children, others have emphasized the effects of mortality and other forms of risk, and others have emphasized the biased transmission of cultural norms from people of high status. Yet the literature says little about the relative strengths of each of these types of motivations or about which ones are more likely to serve as the primary impetus for large-scale demographic change. In this paper, I examine how each of these factors has influenced the demographic transition in urban South India during the course of the 20th century using two measures of fertility transition: number of surviving children and age at marriage. I find that investment-related, risk-related, and cultural transmission predictors all have significant individual effects on the outcome variables, which persist when they are entered in combination. When the three types of predictors are compared, however, investment-related models appear to provide more robust explanations for patterns in both fertility and age of marriage. Am. J. Hum. Biol., 2009. © 2009 Wiley-Liss, Inc.