In the U.S., couples with daughters are somewhat more likely to divorce than couples with sons. Many scholars have read those numbers as evidence that daughters cause divorce.
But new research from Duke University suggests something quite different may be at play: Girls may be hardier than boys, even in the womb, and may be better able to survive pregnancies stressed by a troubled marriage.
Previous studies have argued that fathers prefer boys and are more likely to stay in marriages that produce sons. Conversely, the argument runs, men are more likely to leave a marriage that produces daughters. That scholarly claim has been around for decades, and has gained a following in popular culture.
"Many have suggested that girls have a negative effect on the stability of their parents' union," said Duke economist Amar Hamoudi, who co-authored the new study with Jenna Nobles, a University of Wisconsin-Madison sociologist. "We are saying: 'Not so fast.' "
Their study appears online in the journal Demography.
Hamoudi, who teaches in Duke's Sanford School of Public Policy and is a fellow of the Duke Center for Child and Family Policy, points to a very different potential explanation for differing divorce rates: the robustness of female embryos.
Throughout the life course, girls and women are generally hardier than boys and men. At every age from birth to age 100, boys and men die in greater proportions than girls and women. Epidemiological evidence also suggests that the female survival advantage actually begins in utero. These more robust female embryos may be better able to withstand stresses to pregnancy, the new paper argues, including stresses caused by relationship conflict.
Based on an analysis of longitudinal data from a nationally representative sample of U.S. residents from 1979 to 2010, Hamoudi and Nobles say a couple's level of relationship conflict predicts their likelihood of subsequent divorce.
Strikingly, the authors also found that a couple's level of relationship conflict at a given time also predicted the sex of children born to that couple at later points in time. Women who reported higher levels of marital conflict were more likely in subsequent years to give birth to girls, rather than boys.
"Girls may well be surviving stressful pregnancies that boys can't survive," Hamoudi said. "Thus girls are more likely than boys to be born into marriages that were already strained." Hamoudi and Nobles also make a broader point that reaches beyond the issue of divorce. Population studies typically begin at birth, Hamoudi said. Yet if demographers and other social scientists want to fully understand how family dynamics affect populations, they need to consider the months before birth as well.
"It's time for population studies to shine a light on the period of pregnancy," Hamoudi said. "The clock does not start at birth."