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12 Proven Ways Siblings Help Make You Who You Are

From the people we date to the professions we choose, an increasing body of science says our childhood sibling relationships have a hand in shaping the adults we become.

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The importance of siblings

Whether you grew up with a bossy big sister, a whiney little brother, or a twin you couldn’t live without, National Siblings Day is a great time to consider the roles our sibs play in our lives. “Grownups can have very strong love-hate feelings about their siblings, but adults don’t always recognize how formative those childhood relationships were,” says Laurie Kramer, PhD, a clinical psychologist and Professor of Applied Psychology at Northeastern University. She adds that science has just recently started investigating these dynamics. “There’s been an awful lot of research on how parents—especially mothers—impact the adults their children become, while the influences of siblings has been under-recognized. But when you study siblings you see how powerful those relationships are in terms of shaping the people we end up being and affecting social skills that impact other relationships across our lives.” Find out about 12 of the most famous sibling rivalries throughout history.

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Having a unique influence

Part of the power of sibling relationships comes from the fact that they’re different from all other family and social connections. “It’s the longest-lasting relationship in most people’s lives,” says Susan McHale, PhD, Distinguished Professor of Human Development and Family Studies, and Professor of Demography at Penn State University. “It starts in childhood before people meet a spouse or partner and usually ends in late life after parents are gone, so there’s a lot of time for sibling influence.” In addition, growing up together means sharing intimate knowledge about the interior of your family and each other. “Not many people know you like your sibling does,” McHale adds.

What’s more, a sibling relationship often brings different stages together. “Unlike childhood friendships, siblings—unless twins—aren’t the same age,” says Nina Howe, PhD, Professor of Early Childhood and Elementary Education at Concordia University. “So they’re at different levels in terms of development and knowledge of the world, which can come into play as they interact.” Find out 25 things only middle children understand.

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Providing practice

The fights and friendships between young siblings add up to rehearsal for life outside the nest. “The sibling relationship can be a natural laboratory for learning how to get along in the world,” says Howe. This can include figuring out how to engage in positive interplay, testing authority over younger siblings, and negotiating disagreements. Of course, such practice can involve negative behaviors, too. A 2014 Developmental Psychology paper co-authored by McHale that looked at the social “training ground” between brothers and sisters reported, “If sibling exchanges are predominantly hostile, then negative interaction patterns are reinforced and the child develops a generalized coercive interpersonal style.” Find out how this woman repaired her relationship with her brother after not speaking for six years.

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Predicting your romantic relationships

It turns out whether you grew up with a same-sex or other-sex sibling impacts the nature of your romantic heterosexual relationships in adolescence. “Middle childhood is a period of segregation, when the other sex ‘has cooties,’ so exposure to peers of the opposite gender can be limited,” says McHale. “This means that children with a sibling of the other sex have the advantage of seeing the behaviors and interests that are more common in the other gender.” McHale co-authored a 2015 study in the Journal of Family Issues that found adolescents who had grown up with other-sex siblings had greater “romantic competence,” which included considering themselves better able to relate to an other-sex partner. “We also asked adolescents in romantic relationships to rate their levels of intimacy, conflict, and power, and we found those with other-sex siblings had higher quality romantic relationships,” McHale says.

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Being shaped by parents’ “favorites”

Researchers say a key area of sibling life is the perception of whether mom and dad played favorites. “From a young age, children are very attuned to how parents treat them relative to their sibling,” says McHale, who has published multiple studies in this area. “A great deal of research has shown that children and adolescents who are less favored—especially in terms of warmth, closeness, and support—have more adjustment problems, from depressive symptoms to risky behavior.”

Research showing these connections has found that even adult children are susceptible to the impact of uneven treatment from parents. A 2013 study in the Journal of Marriage and Family found that young adults who said they got less parental support than their sibling reported more depression, and the greater the amount of differential treatment, the less closeness there was between siblings. However, if a parent’s uneven treatment is warranted for some reason, for example, if one sibling has a disability or illness, the other sibling might not like it, but they do consider it fair, which can counteract the effects of differential treatment on children’s adjustment. Here are 11 serious consequences of favoring one child over the other.

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Affecting achievement

Differential treatment from parents can also impact each sibling’s academic achievement, says McHale, who has researched this area well. “If parents see one child as being smarter than the other, the difference in school grades between the two siblings increases over time.” Some studies have even seen parental differential treatment predict differences in college graduation among siblings. This phenomenon may have to do with the ways kids see their place in the family. For example, if little brother gets the message he’s “the athletic one” and big brother gets the message he’s “the smart one,” little brother may be less inclined to try in academic areas. “All this evolves from the parents’ differential treatment, which leads to children hearing messages about who they are and how they compare to who their sibling is.” Be impressed by these 13 stories of famous sibs, who teamed up to achieve greatness.

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Impacting the parent you become

Kramer’s research has involved visiting families to observe siblings and talk with parents, and she was surprised to find a backward link in the way that mothers’ memories of their own sibling relationships affected the sibling relationships of their kids. “It was striking that mothers who reported more negative sibling relations during childhood were most likely to have offspring who interacted more positively,” said the resulting paper in the Journal of Social and Personal Relationships. This observation seemed counter-intuitive until Kramer dug deeper. She realized that moms who had positive childhood sibling relationships might assume kids just get along, so these moms were more hands-off with their own kids. By contrast: “Mothers reporting anxious and lonely childhood peer relations took the most active role in their children’s development and voiced the strongest intentions to help their children experience more positive relationships,” the paper found. Discover 13 smart ways for parents to stop sibling rivalry.

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Developing a sense of humor

Howe and her colleagues have been laughing more lately because they’ve started to study humor between young siblings, from potty jokes to goofy movements. “Siblings are a natural audience for one another, so they can explore that humor dynamic in a safe, positive way, which serves us later in life,” she says. What’s more, when you tell a joke, you’re understanding someone else’s point of view, which is an important skill even beyond humor, she adds. “Those kinds of interactions don’t go away. While the bathroom humor may disappear in adolescence, developing a good-natured sense of humor is a really important part of getting along with people—it just makes life go better.” Laugh out loud at these 14 funny sibling stories.

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Getting pegged by birth order

For some, being the firstborn, middle child, or baby of the family affects us long after we’ve left the nest. “I think birth order has some impact on the interactions of young children: Older ones tend to be leaders in play and teaching, so younger ones default to the complementary role of the learner,” says Howe, who adds that older siblings often assume caretaker responsibilities, as well. “Those roles can persist throughout adulthood.” Kramer agrees that birth order can bestow certain traits. “In many families, older children can be expected to act as role models, helpers, and teachers, which could lead some first-born children to develop characteristics of being a leader or helper over time.” However, both experts concede these roles can reverse, either because you rebel against them, or because adult illnesses or injuries among older siblings can force younger brothers and sisters to become caretakers. From being switched at birth to finding long lost sisters, these 11 bizarre real-life sibling stories are hard to believe.

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Increasing risk-taking behavior

Young siblings are famous for getting in trouble together, and research is showing how such negative behaviors can lead to bad choices later in life. A concept called deviance training (nicknamed the “partners in crime” theory) says that siblings can team up to make mischief at home and beyond. “Siblings can get together to engage in risky behaviors, from disobeying parents to off-color jokes, and they can reinforce these non-compliant behaviors by egging each other on with laughter and praise,” says McHale. Some evidence—including the 2014 Developmental Psychology study co-authored by McHale—suggests that such deviance training is more common in brother pairs and that the closer the sibling relationship the greater the influence. In this way, having a big brother who engages in risky behaviors puts little brother at greater risk for those behaviors as he grows. “Risky behaviors like playing with matches in elementary school often predict risky behaviors like underage drinking in adolescence,” McHale says. Here are a few more ways that birth order and gender can affect crime rates.

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Becoming stressed—or strengthened—by disability

Having a brother or sister with special needs can create lots of challenges. “Siblings of children with disabilities are at a greater risk than average of developing emotional issues, anxiety, and stress,” Avidan Milevsky, PhD, wrote in Psychology Today. He explained that these siblings may be neglected by overburdened moms and dads, take on parent-like responsibilities, and grapple with emotions from guilt and embarrassment to fear and jealousy. But it’s also possible that having a sibling with a disability could create opportunities later in life. “These siblings often develop certain positive characteristics such as self-control, cooperation, empathy, tolerance, altruism, maturity, and responsibility as a result of dealing with their family situation,” Milevsky wrote. “In some cases, these siblings use someone’s attitude about special needs as a test for screening friends and mates. Their involvement with their sibling may even lead them to choose future occupations in the helping professions.”

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Providing later-life support

As siblings grow they often form other key relationships with spouses and their own children. But later in life those newer connections can end or change. “A lot of people are going to wind up in senior adulthood without a husband or wife, their kids have scattered to other cities, and the only people left at the dance will be the ones what brung them, which is their brothers and sisters,” Jeffrey Kluger, a TIME magazine editor at large and author of The Sibling Effect: What the Bonds Among Brothers and Sisters Reveal About Us told NPR’s Science Friday. This is the time to take advantage of healthy sibling relationships and retrieve ones that may have been lost. “The argument I make, particularly when it comes to taking care of aging parents, is if you can fix [a sibling relationship], do,” Kluger said. “Your sibs are just such a resource.” If you need to reconnect, try these 11 ways siblings can improve their relationship as adults.

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The absence of siblings

While there may be some stereotypes that say an “only child” is selfish and can’t get along with others, the experts say not to worry. “There is a small amount of research on only children, and for the most part those kids grow to be well adjusted,” says Howe. “Only children often develop close connections with cousins or friends instead.” Kramer agrees. “When it comes to developing social skills, it’s not like you’re doomed for life if you’re an only child,” she says. “Children find other relationships in their lives to develop those competencies.” Read on to discover the bad parenting traits you never knew you had.