“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.