Tense relationships leads to break-up

Women who reported higher levels of marital conflict were more likely in subsequent years to give birth to girls, rather than boys, they noted.

"Many have suggested that girls have a negative effect on the stability of their parents' union. We are saying: 'Not so fast'," said Duke University economist Amar Hamoudi and co-author Jenna Nobles, a University of Wisconsin-Madison sociologist.

According to them, girls may be hardier than boys, even in the womb, and may be better able to survive pregnancies stressed by a troubled marriage.

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.

"More robust female embryos may be better able to withstand stresses to pregnancy, including stresses caused by relationship conflict," the study argued.

"Girls may well be surviving stressful pregnancies that boys cannot survive. Thus, girls are more likely than boys to be born into marriages that were already strained," Hamoudi contended.

"It is time for population studies to shine a light on the period of pregnancy. The clock does not start at birth," he added in a paper that appeared in the journal Demography.