Premarital first births: The influence of the timing of sexual onset versus post-onset risks in the United States.

Motivated by long-standing debates between abstinence proponents and sceptics, we examine how socio-economic factors influence premarital first births via: (i) age at first sexual intercourse and (ii) the risk of a premarital first birth following the onset of sexual activity. Factors associated with an earlier age at first intercourse will imply more premarital first births owing to increased exposure to risk, but many of these same factors will also be associated with higher risks of a premarital first birth following onset. Our analyses confirm previous findings that women from disadvantaged backgrounds are younger at first intercourse and have higher premarital first-birth risks than women from more advantaged backgrounds. However, differences in onset timing have a strikingly smaller influence on premarital first-birth probabilities than do differences in post-onset risks. Our findings thus suggest that premarital first births result primarily from differences in post-onset risk behaviours as opposed to differences in onset timing.