Little girls may be sugar and spice and everything nice, but having a daughter might boost a couple's risk of divorce, according to past census data.
Not only did researchers find that couples with sons are more likely to stick together, unmarried pregnant couples were more likely to have shotgun weddings if the baby was going to be a boy and divorced mothers of boys are more likely to remarry and stay remarried.
Does this mean that daughters are matrimonially risky and sons are marriage saviors? Not so fast, psychologists say.
In the original 2003 research on the topic, economists Gordon Dahl, from the University of California-San Diego, and Enrico Moretti, at UC Berkeley, found that couples with a first-born girl were about 5 percent more likely to divorce than parents of a first-born boy. When there are as many as three daughters that difference spiked to 10 percent.
Given that the researchers drew from data on more than 3 million adults from U.S. Census data, it's likely this effect is not just a statistical fluke, but the hows and whys of this phenomenon are open to debate.
From one perspective, there could be something about boys that makes parents want to stick it out, either because they enhance marital relations or make the prospect of a fatherless home more frightening.
More recently, however, psychologists have debated whether daughters might make mothers more willing to leave a bad marriage because they provide social support that empowers their mom.
"One dynamic I've seen is that women don't want to put up with a controlling or abusive husband because they're afraid to model this as an acceptable form of marriage to their daughters," said Susan Heitler, a Denver-based clinical psychologist and author of "Power of Two." "There is a lot of individual variation, though; it could go both ways."
Traditionally, researchers have looked at this phenomenon from a sort of "pro-boy" perspective: Either boys are somehow increasing the quality or likelihood of marriage or they are making divorce seem less attractive.
To support this idea, Dahl and Moretti cite a number of statistics that point to a son ensuring that a father will commit to the mother and the family.
"Fathers are significantly [3 percent] less likely to be living with their children if they have daughters versus sons," they wrote in a 2007 paper on the subject. "We estimate that in any given year, roughly 52,000 first-born daughters younger than 12 years (and all their siblings) would have had a resident father if they had been boys."
Couples who conceived a child out of wedlock were most likely to get hitched by the time the baby was born if the mother was having a boy (and had had an ultrasound that most likely predicted this). This would support the notion that a man may be more inclined to commit to a marriage and childrearing if the first born is a boy, they said.
In this scenario, having a son to throw around the football with makes a marriage seem more attractive to a biological father, and the fear of having their son grow up without a male role model might play a role as well.
Focusing on what "keeps a father around" may be a mistake, however, said Anita Kelly, professor of psychology at the University of Notre Dame, because it fails to acknowledge that in the United States an "overwhelming majority of divorces are initiated by women" -- 73 percent, statistics show.
So instead of asking why parents of daughters split up, it's worthwhile to ask, "Why do mothers of daughters leave their husbands more often than mothers of sons," Kelly said.
Another way of looking at the daughter-divorce effect, Kelly said, is to look at what about daughters might make a woman more likely to leave her husband.
Past statistics show that adult daughters living at home reduce the household workload (which falls largely on the mother's shoulders), while adult boys increase it, she said, so it's possible that women can anticipate support from their daughters more so than from their sons, making them "less willing to tolerate negative behavior in a spouse."
Heitler agrees, adding that women within a family -- mothers and daughters -- form a family within a family, offering social support to each other that a son might not offer a mother.
With this kind of emotional support and protection from being lonely (a huge concern expressed by many women considering divorce), "a husband who is being really difficult becomes dispensable," she said.
As far as Moretti and Dahl's observation that mothers of daughters are less likely to remarry and stay remarried following a divorce, a more mother-focused explanation would be that daughters make a new husband less necessary because they are offering social support, Heitler said.
Alternatively, a mother may be more wary of bringing a possible predator into her daughter's life by dating again, Kelly said.
According to Heitler, sticking with an unhappy marriage may seem less appealing if a woman feels she is teaching her daughter the wrong things about love and marriage, a concern she said divorced women have expressed in her practice.
"They think, it's hard to go through a divorce, but at the same time I don't want this to be a model of what marriage should be to my daughter," she said.
She said that often there is a multigenerational pattern of abusive husbands these women are trying to break.
So do girls drive parents apart or just encourage mothers to leave dysfunctional relationships? Do boys actually help marital relations by increasing the devotion and commitment of the father or make parents wary of splitting?
Unfortunately, the census data that allowed for Moretti and Dahl's work does not offer a definitive answer as to why couples with girls are more likely to break up, but in future research, these are questions that Kelly said she hopes to explore.