Insights from a Sequential Hazard Model of Sexual Initiation and Premarital First Births

Lawrence L. Wu, New York University
Steven P. Martin, University of Maryland

Since Davis and Blake (1956), demographers have acknowledged that sexual activity is an important component of fertility, but this insight is typically ignored in formal and empirical models of fertility. In this paper, we investigate one fertility outcome---a premarital first birth---by modeling a woman's entry into sexual activity and her subsequent risk of a premarital first birth. These models yield substantive insights not evident from conventional models analyzing women's age at a premarital first birth. In particular, we find that U.S. women contracept ineffectively following onset of sexual activity, but appear to improve their contraceptive efficacy, not with age, but with duration following sexual initiation. We also find substantial unobserved heterogeneity, correlated with age at sexual onset. For this submission, we provide technical and empirical details (Wu and Martin 2007) but will, as indicated above, emphasize the substantive implications of these models in our PAA paper and presentation.