Three siblings laughing.
Published on: Mar 11, 2020

When you talk about sibling issues, everyone takes it personally. Whether it’s birth order and the supposed advantages of being the oldest (or youngest, or middle), or the question of having (or being) the favorite child, people tend to respond immediately with their own sometimes very individual and emotional stories.

What I want to talk about today are sibling sex ratios — having a sibling of the other sex versus growing up in all-boy or all-girl sibling configurations. The most evocative phrase I’ve seen for this is “family constellations,” which I like because it suggests that there are lots of interesting — and even beautiful — arrangements, but that differences are real.

But let’s take one step further back: Are there actually parents, or parent pairs, who are more likely to conceive boys or girls? Does the five-daughter family (from “Pride and Prejudice” or “Fiddler on the Roof”) or the seven-son setup (“Seven Brides for Seven Brothers”) just reflect five (or seven) random rolls of the dice, or is there actually something going on from an evolutionary point of view?

The evolutionary theory, which has been advanced to explain sex ratio, goes back to Darwin, but was fully formulated in 1930 by a British scientist named Ronald Fisher, who made the argument that if individuals vary in the sex ratio among their offspring (that is, some are more likely to produce more males or more females), the reproductive advantage in a population will always lie with the rarer sex, and thus the sex ratio will equilibrate toward 1:1.

After all, Fisher argued, half of the genetic material of the next generation must come by way of those who tend to produce males, and half from those who tend to produce females.

But are there such tendencies? I’ve heard people say that having boys “runs in the family,” or that their cousins are almost all girls, that’s the “family pattern.” But a very large study of 4.7 million births in Sweden published in February in the journal Proceedings of the Royal Society argues that there is no evidence of a genetic tendency toward one sex or the other, or a family tendency.

Brendan Zietsch, the director of the Centre for Psychology and Evolution at the University of Queensland, Australia, who was the first author on the study, said in an email, “There were various evolutionary theories suggesting that parents may influence the sex ratio of their offspring,” that is, that evolution may favor strategic deviations in one direction or another, or that parental hormone levels at the time of conception might be important.

“The offspring sex in humans is simply random,” Dr. Zietsch wrote. “People don’t have a tendency to have one or the other sex, as shown by the lack of correlation between the sex of a parent’s offspring and the sex of their next offspring.” The large sample size, he wrote, should remove all doubt about this.

However, even if it’s just a set of rolls of the dice, that family constellation, that sibling sex ratio, can be profoundly important in a child’s development.

“It’s the longest-lasting relationship that most people have,” said Susan M. McHale, a professor at Penn State University whose research focuses on siblings. Although there are not many studies of siblings in later life, she said, those that we have show that people who have closer relationships with their siblings are healthier in older age.

The research shows, she said, that parents tend to spend relatively more time with a child of the same sex as the parent — but that when parents have children of both sexes, the discrepancy shrinks, at least in the U.S. families they have studied.

“Where there’s an ethic of trying to treat your children the same, having a sibling of the other sex can lead to your having more time with your opposite sex parent,” Dr. McHale said. “Younger brothers with older sisters spend more time with mothers than younger brothers with older brothers.”

Like birth order, sometimes sex differences can play into — or be perceived to play into — differential treatment, which is the biggest source of conflict and bad feeling in sibling relationships. In other studies, Dr. McHale and her research team have also looked at parents’ differential treatment of their children. Children who perceive that the other sibling is the favorite, she said, are at risk for depression and risky behavior, but those negative consequences are mitigated when there’s a reason for the differential treatment, and parents explain it so children see it as fair: “Your brother has needs right now that require some special attention.”

In another study, parents were asked if one child was smarter than the other, and then the researchers looked over time at the children’s math and science grades in school. If the children’s grades at the start of the study were held constant, the parental attitudes predicted whether or not differences would develop and increase over time.

As children get older, the influence of the sibling relationship affects adolescent behavior and development. When it comes to heterosexual romantic relationships, adolescents who have an other-sex sibling “grow faster in their romantic competence” than those with a same-sex sibling, Dr. McHale said.

Other research has suggested, Dr. Zietsch wrote, that “males with more older brothers are more likely to be gay,” referencing the so-called fraternal birth order effect, which is thought to be linked to biological processes involving the mother’s immune system during pregnancy.

Having an older brother is also tied to risk-taking behavior in adolescence, especially for boys. “The brother-brother pair can be the at-risk dyad in the sibling literature,” Dr. McHale said. There’s a worry that having an older brother exposes the younger one to risky behavior. “These boy-boy pairs, especially if the brothers are close in age, can be at risk for more delinquency and substances,” she said. In the families of Mexican origin that her team has studied, “Having an older sister is protective; having an older brother is a risk factor.”

The sibling relationship is formative in so many ways, and parents need to take it seriously, and be willing to monitor it directly, Dr. McHale said. Conflict between siblings — often around perceived differential treatment — is the most common kind of family conflict, and it can have very negative consequences for children.

The team has developed and tested an intervention to promote positive sibling relationships, helping parents coach their kids, talk to them about one another’s point of view, and control their emotions. Parents should “set rules,” Dr. McHale said. “Sibling conflict doesn’t have to be commonplace.” Direct supervision is linked to good outcomes, she said, and parents spending time with the siblings together can help them get along better.

Dr. McHale said, “It’s a sibling who’s likely to be there with you at the end of your life, so the parents’ investment in promoting positive sibling relationships is hugely important for them even after the parents are gone.”