Watching Serena Williams play her sister Venus at the U.S. Open last night, it was hard not to imagine what you’d be thinking if your own sibling stood between you and some huge professional goal: Don’t you dare screw this up for me. Serena herself, for what it’s worth, said the best way to play her big sister is to ignore the fact that she is playing her big sister. “She’s the toughest opponent I’ve ever played in my life and the best person I know,” Williams said in an on-court interview. “It’s going against your best friend and at the same time going against the greatest competitor in women’s tennis. When I’m playing her I don’t think of her as my sister.”
Fine, but for the rest of us with grown-up siblings, the sisterly rivalry is good reason to reflect on the many ways our brothers and sister have shaped who we are. Sibling dynamics have for many years been somewhat understudied in psychology literature, but that’s starting to change. The somewhat sad news: The effects of birth order on personality, while really fun to think about (Oldest kids are smarter! Youngest kids are more rebellious!), is mostly exaggerated. Still, there are plenty of other ways brothers and sisters influence one another, many of which last for a lifetime.
Science of Us took a dive into the research to see what science says about the way siblings impact our health and behavior, even as grown-ups. Here’s what we found.
Having a big sister makes women more competitive. (Maybe.) So argue the Japanese authors of a study published last month in the journal Personality and Individual Differences. When given the choice to earn money by solving math problems on their own or entering a tournament, this study found that women with older sisters were about 30 percent more likely to pick the competitive option than the rest of the women in the study. Intriguing — not to mention immediately applicable to the Williams sisters — but it’s just one smallish study, with fewer than 400 participants. Still, the authors of a comprehensive review of the literature on sibling relationships, published in 2011 in the Journal of Family Theory Review, also contend that siblings of the same gender “may be more sensitive to issues of rivalry and competition.”
But siblings are competing with each other, not for parental attention. As a long-held theory goes, sibling rivalry stems from yearning for parental attention and family resources, and, sure, there probably is some truth to this evolutionary explanation. But that’s almost certainly not the way siblings themselves see it. One study asked children what they fought about with their brothers and sisters, and their answers indicated that their squabbles were strictly kid business (such as one getting the other back for an earlier annoyance). This study also found that kids fought with their brothers and sisters an average of five times per day.
In some ways, big brothers and sisters have more influence on their younger siblings than their parents do. Recently, some research has attempted to measure the influence of siblings on a person’s physical health, and the results have been pretty striking. For instance: If you have an older sibling who is obese, your own risk of being obese doubles, according to a study of about 2,000 Americans published last fall in the American Journal of Preventive Medicine; this association was especially strong in families with siblings of the same gender.
By way of comparison, only children with obese parents also have twice the risk of becoming obese themselves, as compared to only children whose parents are not obese; this held true for eldest children as well. But for younger siblings, the association between parental obesity and their own weight disappears. “Younger children look up to their big brother or sister for behavioral cues, often seeking their approval; and siblings may spend more time with each other than parents, often eating and playing sports together,” the lead author of the study, Mark Pachucki of Harvard Medical School, told the Harvard Gazette last year.
Adult brothers and sisters — even the ones who are competitive with each other — often grow up to be close pals. Kids who have good relationships with their siblings grow up to become more prosocial — that is, more generous, compassionate, and altruistic — than kids without siblings, or kids who have brothers and sisters and don’t get along with them. (A “good” relationship means that they fight and make up, rather than act with hostility toward each other.) And the research also indicates that most brothers and sisters do become or stay close as they grow up. Two thirds of the adults in one large, nationally representative study involving nearly 8,000 American adults named their brother or sister as one of their best friends, and about one third said in an emergency they’d call their sibling first. The research has spoken: No one loves you or bugs you quite like your brother or sister.