Understanding Measures of Nonmarital Fertility: The Roles of Marriage and Access to Human Capital

This paper proposes an explanation for several decades of rising U.S. nonmarital birth rates and shares, and for cross-sectional differences in black-white fertility. Significantly, the explanation does not rely on changes over time or differences across races in individual fertility behavior. It is consistent with the rising nonmarital fertility measures observed in the United States since the tnid-1970s, higher measured fertility for unmarried blacks than whites, and differences across races in the timing of childbearing, despite nearly constant total fertility rates and increasingly similar target family sizes for blacks and whites. The explanation relies on a selection effect associated with changes in the marriage rate and on racial differences in access to human capital investment opportunities. We find strong support for the explanation using U.S. data over the period 1957-2002. Our findings suggest caution in interpreting the results of empirical studies of childbearing that examine marital and nonmarital fertility rates separately, as these studies typically ignore the selection effect of marriage. (JEL J12, J13, 138) The problem is that the research to date is often ambiguous about causal linkages between changing patterns of marriage and unmarried childbearing. Simple explanations that emphasize changing proportions of unmarried women and married women are insufficient. They cannot explain the fact that fertility rates among unmarried women also are rising. [Lichter 1995, 137] 1. INTRODUCTION The extraordinarily rapid rise in nonmarital births in the United States over the past half-century has elicited calls of alarm and spurred a vast literature on the role of public policy in fostering changes in marriage and childbearing behavior, as reviewed by Moffitt (2000), among others. As yet, however, no consensus has formed on whether changes in marriage behavior or changes in childbearing behavior have been more important in explaining the rise in nonmarital births. Identifying the interactions among marriage, fertility, and other factors behind the increase in both the birth rate (per 1,000 women) and birth share (the fraction of all births) attributable to unmarried women has proven extraordinarily difficult, particularly so against a backdrop of relatively constant total birth rates (TBRs) in the United States since the mid-1970s. Differences by race in the U.S. data pose further challenges. It is well known that black women experience dramatically lower rates of marriage and higher rates and shares of non-marital births than white women. Less familiar, but nonetheless prominent in the data, is the contrast between higher birth rates for blacks than whites in the teens and early 20s, and lower birth rates for blacks than whites beginning in the mid-20s. Clearly, black women and white women time their childbearing differently. This difference has become more pronounced in recent decades in the United States, despite increasingly similar total fertility rates among blacks and whites. [FIGURE 1 OMITTED] In this paper, we show that the most prominent features of U.S. nonmarital fertility measures since the 1950s can be explained without appealing to changes in the childbearing behavior of individual women over time or to differences in "preferences" with respect to childbearing across races. Rather, they are plausibly explained by accounting fully for the effects of changes in marriage behavior over time and differences across races in access to human capital investment opportunities. The particulars of our analysis are motivated, in part, by U.S. experience from the mid-1970s through the end of the century. As illustrated in Figure 1 for white women aged 25-29, TBRs have been relatively flat since the mid-1970s despite dramatic increases in nonmarital birth rates, declines in the proportion of women married (termed the "single share" in this paper), and little movement in marital birth rates (MBRs). …