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Brother, Sister, Rival, Friend: How Siblings Shape One Another’s Lives

New research is bringing an unexpected truth to light: Brothers and sisters may have as powerful an effect on one another's lives as parents.

Brothers and sisters are, more often than not, children’s first playmates and adults’ oldest friends. But sibling relationships play out in unpredictable ways with unpredictable results. Brotherhood and sisterhood can teach social skills and help us learn to resolve conflicts—or cause life-long social dysfunction. New research indicates that, for many brothers and sisters, sibling relationships yield mixed results. More interestingly, that same research, which represents an early attempt to sort through so-called Sibling Effects, keeps falling back on one key point: the effects of sibling relationships in childhood echo through the rest of our lives.

“Sibling relationships influence children’s adjustment and development about as much as parenting does,” Mark Feinberg, who teaches human development at Penn State University.

Sibling Effects impact a surprisingly broad spectrum of the human psyche. Studies (some more rigorous than others) have identified a handful of consistently positive and negative effects, and even ventured into the fraught science of predicting sibling relationship quality. It’s important work because the key to parenting siblings effectively is understanding what makes this unique relationship tick. “Cognitively, emotionally, socially—there are just a lot of influences that siblings have on one another,” says Laurie Kramer, a clinical psychologist at Northeastern University.

What The Studies Say (And What They Don’t)

There is ample research out there on how siblings impact one another. Studies have shown that younger siblings teach empathy to their older brothers and sisters, and that siblings who report feeling close to one another tend to either both graduate college or both drop out, as a unit. We even know that the best sibling arrangement—tied to the highest educational and economic attainment for all children in the family—is XB-S, the code for when and eldest child of either gender (X) is born two years before a brother who is born five or more years before a sister (S).

Less optimistic research has linked sibling bullying to depression, anxiety, and self-harm.

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Even among studies that highlight significant sibling effects, however, there are serious limitations in what we can confidently conclude. A handful of studies have attempted to demonstrate that only children are developmentally stunted, but researchers agree that most of these disadvantages are short-lived. “By the time we reach adulthood, we have gained enough other formative experiences in the world that any actual differences between siblings and singletons are pretty negligible—overridden by differences in temperament, personality, and personal preference,” says Anderson University psychologist Susan Doughty. “A lack of siblings may still shape your life in some ways, but it is only one influence among many.”

So how do we square the idea that having siblings profoundly affects people with the idea that the effects of having siblings are often negligible from a statistical perspective? Put simply: very volatile relationships have effects that are far from negligible. And one quirk of the sibling bond is that it leads to a disproportionate amount of strong positive and strong negative relationships.

“Moderate to high levels of both positive and negative sibling relationship dimensions are typical,” says Sarah Killoren, who studies sibling relationship dynamics at the University of Missouri. “Most differences in adjustment are seen between siblings who have very positive relationships—high intimacy, low negativity—versus those who have very negative relationships—low intimacy and high levels of conflict.” So while it’s true that sibling relationships are only one influence among many, they still can have profound, lingering effects.

There are, in short, few influences more meaningful than a brother or sister.

The Positive Effects Of Siblings

“Siblings are often a child’s first play partners,” Nina Howe, research chair of early childhood development at Concordia University, told Fatherly. “I think of the sibling relationship as a natural laboratory for learning how to get along with people.”

Very young children with older siblings tend to develop theory of mind (or, the ability to put themselves in someone else’s shoes) a bit earlier than their peers. “If you have siblings yourself, it makes sense,” Doughty says. “No one knows how to push your buttons better—or earlier—than a sibling…. That’s a skill that requires a well-developed theory of mind!”

Because siblings are often our first peers, sibling relationships tend to follow fairly predictable patterns. Younger siblings are fascinated by older siblings, and eager to learn their games and customs; older siblings test out leadership skills and conflict resolution on their younger brothers and sisters. These interactions are largely positive. Older sibling-younger sibling power dynamics melt away over time, Killoren says, when younger sibling hit late adolescence. After that, everyone is equals, which leads to better conflict resolution.  

“Whatever jealousy or anger that siblings may feel toward one another,” Howe says. “There’s pretty good evidence that it doesn’t last very long.

Of course, the positive effects of sibling relationships change over time. In toddlerhood, a sibling serves as “an interesting partner,” Howe says. “Helping one another in language development, social interactions, how to stand up for yourself, learning to share.” As children mature, siblings take on more practical responsibilities, helping one another with schoolwork or with navigating friendships outside the family, Howe says.

Siblings can also serve as sources of comfort in adulthood. “Very often, in older age, as people near the end of their lives, they reconnect with their siblings,” Howe says. “This is the person that you have known longest in your life, and you have a shared history. Remembering, what was mom like? What was dad like?”

In dialogue, siblings settle on family histories that seem plausible and fair.

The Negative Effects Of Sibling Relationships

Good sibling relationships are the norm, but bad sibling relationships happen and can have strong negative effects. “Difficult, conflictual, and even violent sibling relationships interfere with development,” Feinberg says. “Children learn coercion, develop peer problems, and become exposed to negative influences with a range of outcomes: depression, substance abuse, low educational attainment.” Indeed, Feinberg cites one study that found that sibling relationships are among the most critical factors influencing adult well-being—and disturbing evidence that 10 percent of family homicides (and 1.5 percent of all murders) are attributable to sibling conflict.

Indeed, sibling relationships are also the most violent relationships between family members. And while a lot of that is normal sibling roughhousing, therapists and scientists agree that parents should treat sibling aggression as potentially harmful, especially when there’s a significant age difference. Sibling bullying is a real problem, with some studies suggesting that up to 80 percent of children report being bullied by their brothers or sisters. In extreme cases, sibling bullying can lead to depression and self harm—or teach victims to bully others, in turn.

One of the best ways to discern normal sibling conflict from problematic sibling conflict is to watch its trajectory. In most cases, sibling conflict “tends to increase over childhood to early adolescence, and then decrease around mid-adolescence,” Feinberg says. If it persists, that’s a red flag.

What compounds sibling relationship problems? Parental favoritism (perceived or actual) for one. “When parents treat kids differently, in ways that kids feel are unfair, that’s associated with worse sibling relationships and lower self-concept,” Kramer says. “It’s not just the act of treating them differently, but doing it in ways that kids feel are unjustified and unfair.”

Predicting Sibling Relationship Health

Given the benefits of a good sibling relationship and the dangers of a bad one, trying to predict how outside factors might influence the interactions between brothers and sisters is a priority. One of the major factors at play is age difference. “If siblings are born more than about 6 or 7 years apart, in a lot of ways they are essentially two only-children,” Doughty says. “They are in such different developmental places that they don’t relate to one another the same way.”

Gender is also an important factor. Studies suggest that individuals with a sibling of the other gender express the highest levels of romantic competence, but that boys with brothers rate themselves the highest of all. Other work has shown that boys with older sisters tend to endorse more egalitarian gender roles, perhaps reflecting their experience “growing up with a female peer who was always older, bigger, faster, stronger, and smarter than you,” Doughty says.

There is limited evidence that adversity helps bring siblings closer to one another. “After the period of divorce, which is a terribly stressful time for everybody, siblings in some cases actually become closer,” Howe says. “Because they join together as a team, particularly if they’re going back and forth between parents.” Poverty may have similar cohesive effects. “There is some literature suggesting that siblings help each other with schoolwork when the parents themselves are not well-educated, or cannot help because they don’t speak the language.”

It doesn’t take a lot to imagine that, in cases of great adversity, siblings may pull together.”

Kramer is fascinated by the science of predicting sibling relationship quality. She has dedicated much of her career to identifying predictors and helping parents implement positive changes. One of her long-term, longitudinal studies that followed children from birth through high school found that, while gender and age gaps made some difference, the single greatest predictor of positive sibling relationships were positive social interactions with unrelated peers.

“The quality of a relationship that a preschooler has with a friend is a strong predictor of what they’ll do with their siblings,” Kramer says. “If they coordinate their behavior, play games, and don’t freak out when there’s a conflict, those are really positive predictors of sibling relationships.” The trend held through high school. “The qualities of friendship turned out to be even more important predictors than the relationship kids had with their fathers and mothers.”

In a word, the best way to figure out whether a child or teen will make a good sibling is to look at how they treat their peers—not their parents. Getting along with others is a transferable skill.

Parents Can Help Siblings Be Good To Each Other

Since reaching these conclusions, Kramer has incorporated what she learned into an online program that teaches parents and children how to optimize sibling relationships. “The most important thing is teaching kids how to look at a situation not only in terms of what they want, but also from a sibling’s point of view, to appreciate that there are different perspectives that are equally valid.” Right back to that theory of mind business. “They’re really important social and emotional competencies, and they can transfer those skills to many other relationships.”

But going to school for parenting is not always necessary, and there are a few basic changes parents can make that will help foster the healthiest sibling relationships. First, set a good example. “Demonstrate how to resolve conflicts peacefully, and speak positively about others in the family,” Feinberg says. And set high expectations—do not make the mistake of considering sibling bullying inevitable, and stress that you expect your children to maintain close friendships with one another throughout their lives. “Expect that siblings are going to treat each other well,” Feinberg says. “Make it clear that physical and verbal aggression are unacceptable.”

Crucially, try to coach siblings to resolve their problems independently, amongst themselves. “Help children define the problems that they are having with each other, think about solutions together, and agree upon a way to resolve the issue,” Feinberg says. Because that’s what the sibling relationship is for, afterall. It’s a learning laboratory—and the lifelong journey toward understanding others and interacting positively with one’s peers often starts right at home.