Before we have friends, partners or even a clear sense of ourselves, we have siblings — our first allies, our first rivals and, often, our longest-running relationships.
Yet, for something so fundamental, we tend to reduce the sibling dynamic to a handful of clichés. Birth order becomes shorthand: the responsible oldest, the often-forgotten middle, the free-spirited youngest. As the oldest sibling in my family, countless online memes tell me I’m overly self-sufficient and have a hard time asking for help because I essentially served as a third parent (not exactly on the mark, but that’s what the internet says). The youngest is often considered the wild child — the idea being that by the time they came along, their parents were simply too exhausted to enforce the rules. And then there’s middle child syndrome, which suggests that middle children sometimes feel overlooked, quietly sandwiched between their siblings on both sides — never quite the trailblazer and never quite the baby.
It’s a tidy way to explain something far more complex. While these are just stereotypes, interest in how sibling relationships influence who we become has grown in recent years. Siblings, after all, are the people who have known us the longest and most intimately. Unlike friendships, which we choose, or romantic relationships, which we seek out, sibling bonds are assigned to us — and yet they often turn out to be among the most enduring and influential connections of our lives. The patterns we establish early on with our brothers and sisters tend to follow us into adulthood, with growing evidence linking these dynamics to emotional development, conflict resolution styles and long-term social behavior.
According to an article published in the Journal of Family and Marriage, growing up with siblings means spending a lot of time with another person, much of it without parents nearby. Countless hours of unsupervised play, arguments and shared experiences help shape development.
By being present in each other’s daily lives, siblings learn from one another — modeling behaviors, reinforcing habits and shaping how each navigates the social world.
This cuts both ways: A sibling relationship can be a genuine source of support and growth, but it can also be a pathway to picking up problematic behaviors or attitudes. Siblings aren’t just people you grew up with; they’re some of the earliest and most consistent influences on who you become. As Susan McHale, professor emeritus of human development and family studies at Pennsylvania State University, explained in a 2022 interview, “Parents don’t stay with you your whole life, your romantic partners come and go (and you don’t meet them until later in life), friends come and go, but siblings are always there through the lifespan.”
What does that look like in practice? According to a 2025 Psychology Today article, birth order theories often ring true. Older siblings sometimes take on a caregiver role — an experience that can benefit everyone involved, helping the older sibling develop responsibility and leadership while giving younger ones a sense of security and support. Research from the University of Cambridge suggests that this dynamic can boost a younger child’s emotional growth, helping them develop stronger empathy and social skills and serving as an early model for navigating relationships beyond the family home.
The home environment siblings are raised in can also shape their relationship, according to McHale. The broader family system — including parenting style, stress levels and available emotional support — helps set the tone for daily interactions and long-term relational patterns. A stable, nurturing household tends to amplify the positive aspects of sibling relationships, giving brothers and sisters the space to support one another through challenges.
In a house full of conflict and distress, however, things shift. “They can be protective of one another,” McHale noted in the same interview, “provided that they both understand what’s going on, it’s recognized and the favored child can be supportive of the less favored child to help make up for parents’ behavior.”
Even sibling rivalry has its benefits. According to another Psychology Today piece, bickering teaches kids to negotiate and solve problems. Disagreements, however frustrating in the moment, are actually low-stakes rehearsals for the conflicts children will inevitably face in the wider world with classmates, colleagues and partners. What looks like chaos from the outside is often just two kids learning, in real time, how to coexist.
It comes as no surprise that, according to a study published by Cambridge University Press, twins tend to influence each other even more significantly than other siblings, due to their shared genetics and closely aligned developmental timelines. This closeness is most pronounced in early childhood, when twins do nearly everything together. As they grow older and develop their individual identities, however, their relationship begins to resemble that of typical siblings.
As we move through life, sibling relationships continue to shape our mental health and emotional well-being.
For a study published in the Journal of American Psychology, researchers followed families for roughly 20 years to see how warm or tense sibling relationships were at different points between ages 15 and 31. They found that the quality of sibling relationships in your early 20s can foreshadow how anxious or depressed you might feel in your 40s; the closer siblings were at 23, the better off they tended to be emotionally later in life. The effort we put into these relationships — even as adults — can make a major long-term impact.
A separate study published by Cambridge University Press found that siblings may also play a role in the likelihood of teenage substance abuse. Researchers concluded that siblings tend to model for each other — watching and copying what the other does — and to reinforce each other’s behavior. These patterns can later influence how teens choose friends and interact within those friendships.
The psychology of sibling relationships is a fascinating one that experts seem to agree needs further examination. In an episode of the American Psychological Association’s “Speaking of Psychology” podcast, Laurie Kramer, a professor of applied psychology at Northeastern University, said, “These are our lifelong partners, and I think it’s really sometimes overlooked how important these relationships are. Your siblings, your brothers and sisters, they’re with you throughout this life journey. They are people who really understand you, both your flaws as well as your strengths and assets, and they use that in many ways. But they’re truly the individuals that we learn with. We develop so much of our character, of learning how to negotiate various social situations and other life challenges, and they teach us so much in so many ways.”