Scientists have long recognized that marriages that produce firstborn daughters are also more likely to end in divorce than marriages that produce firstborn sons. And until recently, sociologists and economists chalked up the link between daughters and divorce to a polarizing theory: Fathers relate to boys better, so they're more likely to stick around and work on a marriage if they have a son.
But a new study shows that the connection isn't so simple. Instead, a surprising developmental difference between male and female fetuses -- something that occurs far ahead of a daughter's birth or her parents' divorce -- may explain the disparity.
Researchers Amar Hamoudi and Jenna Nobles found evidence that suggests female embryos are better able to withstand maternal stress in utero than male embryos. So if a mother is experiencing a rocky relationship from the outset, her female fetus has a better chance of surviving the full term. This theory of characteristic female survival advantage -- meaning girls and women of all ages are more likely than boys or men to make it to their next birthday -- is a widely accepted concept in research, but Hamoudi and Nobles' findings are particularly noteworthy because they show that this survival advantage may start at the moment of fertilization, not just at birth.
The notion of a pre-birth female survival advantage is what drew Hamoudi, an Assistant Professor of Public Policy and Economics at Duke University, to take a closer look at the correlation between daughters and divorce.
"What I wondered was: Why are we starting the story at 40 weeks after fertilization?" Hamoudi told The Huffington Post. "Why don't we start at fertilization? Then it's a much more complicated story."
The researchers set out to determine if the links between stressful pregnancies, pregnancy outcomes and divorce are strong enough to account for the daughter-to-divorce association. Stress hormones have been proven to negatively affect pregnancies in otherwise healthy women; there have even been studies that found a link between a pregnant woman's self-reported well-being and the probability that she carries a child to term. Knowing this, Hamoudi and Nobles decided to see if female embryos have a better shot at making it through a stressful pregnancy than male embryos.
Using data from the 1979 National Longitudinal Study of Youth, they were able to determine that couples who frequently argued were more likely to have a firstborn daughter rather than a son. The researchers also used this data to create a predictive model of how likely a couple with a firstborn daughter would be to divorce.
Hamoudi and Nobles found that nearly all of the links between daughters and divorce can be accounted for by stress-related dynamics during pregnancy.
"We didn't prove that girls don't cause divorce," he said. "What we proved was that it would be hasty to look at the daughter-to-divorce association and say, 'Aha, girls must cause divorce,' because we now have another explanation for why that association might exist."
The biggest takeaway from these findings? Before social scientists jump to conclusions about the link between daughters and divorce, they need start the clock at fertilization, not birth. Otherwise, they're missing a key piece of the puzzle.