12 proven ways siblings help make you who you are

Whether you grew up with a bossy big sister, a whiney little brother, or a twin you couldn’t live without, we don’t often 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 recognise how formative those childhood relationships were,” says Laurie Kramer, PhD, a clinical psychologist and Professor of Applied Psychology. 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-recognised. 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.”

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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. “So they’re at different levels in terms of development and knowledge of the world, which can come into play as they interact.”

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 practise can involve negative behaviours, 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 generalised coercive interpersonal style.”

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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 behaviours 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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Researchers say a key area of sibling life is the perception of whether mum and dad played favourites. “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 favoured – especially in terms of warmth, closeness and support – have more adjustment problems, from depressive symptoms to risky behaviour.”

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.

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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 tertiary 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.”

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 realised that mums who had positive childhood sibling relationships might assume kids just get along, so these mums 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.

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Howe and her colleagues have been laughing more lately because they’ve started to study humour between young siblings, from potty jokes to goofy movements. “Siblings are a natural audience for one another, so they can explore that humour 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 humour, she adds. “Those kinds of interactions don’t go away. While the bathroom humour may disappear in adolescence, developing a good-natured sense of humour is a really important part of getting along with people – it just makes life go better.”

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For some, being the firstborn, middle chil 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.

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Young siblings are famous for getting in trouble together, and research is showing how such negative behaviours 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 behaviours, from disobeying parents to off-colour jokes, and they can reinforce these non-compliant behaviours 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 behaviours puts little brother at greater risk for those behaviours as he grows. “Risky behaviours like playing with matches in primary school often predict risky behaviours like underage drinking in adolescence,” McHale says.