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Anybody who has worn a hand-me-down, shared a bathroom or survived a long car trip with a brother or a sister knows that siblings can affect your life in nearly every way possible.

Researchers, however, are just starting to unspool the ways those relationships affect health.

"It's kind of an underdeveloped area," said Dr. Susan B. Sisson, a professor of nutritional sciences at University of Oklahoma Health Sciences in Oklahoma City who has studied sibling-health connections. But research is hinting at the power of siblings – or their absence.

Evidence suggests that siblings have "a pretty strong influence" on health behaviors related to obesity, physical activity, nutrition and more, Sisson said.

The research can be challenging. For starters, "families are pretty complex," Sisson said. Siblings can be connected by birth, adoption or stepparents.

"It's really tough to make any kind of blanket statement about sibling relationships and influence, because there's just so many factors that need to be considered," said Dr. Keith Vakafatu Osai, an assistant professor of child and family studies at Weber State University in Ogden, Utah.

But Osai, whose own research has looked at how siblings affect one another's , said there's no denying the powerful role siblings have in shaping one another.

"We typically say that this is the most enduring relationship, out of all relationships," he said. "We're with our parents when we're born – but our siblings are typically there," too. Spouses and significant others may come and go, but siblings are still there. "And then our parents pass away," he said, "but our siblings are still there."

Here are some of the links researchers have found between siblings and various health factors.

How we eat

Sisson was co-author on a , published in 2019 in the Journal of Nutrition Education and Behavior, that compared the habits of 43 children with siblings with 27 only children. It showed that young children with siblings ate healthier diets than children with no siblings.

"Some of that's probably just a proxy of busy parents," she said. It might be easier for parents to afford fast food with one child than with several, for example. And families that have multiple children tend to have more family meals, she said, which have been linked to healthier eating.

Indeed, have healthier diets and watch less television, according to a study published in Social Science and Medicine in 2017, which found that every additional sibling correlated with a decline of 2.6 percentage points in the likelihood of obesity in early adolescence. Other also have associated having siblings with lower levels of obesity than in people with no siblings.

But that's not always the case. A study that looked at 326 families with two adolescent siblings linked sibling conflict with an increased risk of being overweight as an adolescent. In that study, published in 2014 in , the correlation was highest in brother-brother and older brother-younger sister relationships and lowest in sister-sister relationships.

How we move

Children with siblings have higher levels of physical activity than only children, Sisson said. An that she helped write suggested that children who had more siblings were more active.

A of 161 children at a preschool in Portugal, published in Children in 2021, found that children in homes with siblings tended to have higher motor skills than children without siblings.

A sibling can be "a built-in playmate," Sisson said. A parent might be more willing to send a child outside to play if they have an older brother or sister to keep an eye on them, she suggested, or children might be more likely to be allowed to walk home if they can do so as a pair rather than alone.

Genes, of course, play a big role in fitness as well as obesity, she said. "But genetics doesn't necessarily influence whether or not you have somebody to kick the ball back to you."

And then there's chores. "When there's more people in the household, there's more work to be done in the household," Sisson said, and it's more likely the parents will delegate that work.

Osai's own research into youth sports has found that siblings who have warm relationships with older siblings tend to follow their interest in sports. He knows this from his personal life, too.

"My siblings really helped to shape the way that I engaged with sport and physical activity," said Osai. He's the youngest of eight – four boys and four girls, the oldest of whom was 17 when he was born.

His brothers were his first unofficial coaches, he said. In the street, in the backyard, at parks and in church gyms, "they were the first ones to teach me how to catch a ball, how to shoot a ball." When a sister with a son who played youth football saw that Osai was eager to join, she brought the idea up with their parents, then drove him to practices and cheered him on. He ended up playing all the way through junior college.

The risks we take

Siblings influence our choices related to delinquency and risky behaviors, such as smoking and drugs and alcohol use, Osai said. Siblings who are similar in terms of age or biological sex feel more socially connected or report warmer relationships, and are more likely to follow an older sibling who engages in risky behavior, he said.

Mental health

Research suggests siblings can affect us for better or for worse.

A study published in December in the that looked at the data of about 9,400 eighth graders in China and about 9,100 from the U.S. linked more siblings with poorer mental health.

The Harvard Study of Adult Development, which has been , linked poorer relationships with siblings before age 20 to the risk of major depression in adulthood.

On the other hand, a study of Latina women in Southern California, published in , said that having sisters and more frequent communication with siblings was associated with fewer depressive symptoms during pregnancy.

And a study published in the Journal of Family Psychology in 2019 found that among older adults, a was associated with lower levels of loneliness.

Given all the potential ways siblings can influence health, what should someone do? Sisson suggested that for parents, just being aware of the role that siblings and families play is important.

She encourages parents to "do what you can when you can to promote shared family meals, healthy eating, healthful choices of food, limited screen time and more movement." Parents of only children might want to be extra conscious of family mealtimes or creating opportunities for active play.

Given the potential lifelong influence of a sibling, Osai said, nurturing sibling relationships is important.

"It is an intimate relationship," he said. Siblings can be allies who share secrets, and they also can cause a lot of hurt. So "leaving room for forgiveness" and taking an interest in one another's activities can pay off over time, he said.

Osai acknowledges that sibling relationships can be filled with conflict, rivalry and jealousy. But he recognizes the support he received from his siblings and how it contributed to the success he has had both in and out of athletics.

"Sibling relationships can be tough at times," he said. "But these relationships – they can be so warm and so beneficial in a variety of ways."