Sibling relationships are, statistically, the longest relationships most people will ever have, outlasting marriages, friendships, and often the parent-child bond itself. That’s the core argument behind a growing body of psychological research, including an American Psychological Association podcast episode devoted entirely to the topic, featuring psychologists Laurie Kramer, PhD, and Megan Gilligan, PhD, both of whom have spent careers studying how brothers and sisters shape each other across a lifetime. The pitch isn’t that siblings matter more than a spouse in every case. It’s that the relationship gets far less deliberate attention than a marriage does, despite quietly influencing mental health, social skills, and resilience for decades longer than most romantic partnerships last.
Mark Feinberg, a research professor at Penn State’s Edna Bennett Pierce Prevention Research Center who has spent years studying sibling dynamics, put the distinction plainly in his own institution’s research summary of his work: “The thing about sibling relationships that’s unlike anything else in our life is we don’t choose it, it’s given to us and we deal with it for the rest of our life.” That involuntary quality is precisely what makes the relationship so psychologically distinct. You choose a spouse. You choose your friends. Nobody chooses a sibling, and yet the bond frequently outlasts relationships that were chosen deliberately.
What the research actually tracks
Feinberg’s work, summarized by Penn State’s own prevention research center, links sibling relationship quality to a wide range of adult outcomes: anxiety, depression, substance use patterns, academic achievement, friendship quality, and how people go on to handle their own romantic relationships. The center also notes something easy to overlook: in the U.S., a child is statistically more likely to grow up alongside a sibling than alongside a father. Given that reality, treating sibling relationship quality as a minor family detail rather than a major developmental input looks increasingly out of step with how people are actually raised.
Laurie Kramer, whose research at the University of Illinois focuses on what makes sibling bonds supportive versus corrosive, has found that simply minimizing conflict between siblings isn’t enough to produce a good relationship. In her own university research summary, she explains: “Even if you’re successful at reducing conflict and antagonism, research suggests that you’ll probably be left with little positive interaction between siblings.” In other words, absence of fighting isn’t the same as presence of closeness. The relationships that hold up over decades tend to have three ingredients in place deliberately: the ability to communicate, genuinely enjoyable shared time, and mutual support during hard stretches, not just an absence of conflict.
Why this matters more as people age
The case for prioritizing sibling relationships gets stronger, not weaker, later in life. Parents eventually pass away. Friend groups thin out or scatter geographically. Marriages, even good ones, involve only one person who wasn’t present for a shared childhood. A sibling is often the only living witness to a person’s earliest years, the only one who remembers the same house, the same holidays, the same version of your parents before either of you had any perspective on them. That shared history becomes a specific kind of resource in adulthood, a form of validation and continuity that a spouse, however close, simply cannot replicate because they weren’t there for the formative parts.
Kramer also notes that the skills siblings build together, working through disagreement, taking another person’s perspective, regulating a reaction in the heat of an argument, don’t stay contained within the family. As she puts it in her own research summary, “your kids are developing positive ways of dealing with others that will be useful outside the family as well.” That’s the mechanism by which a childhood sibling relationship ends up shaping adult romantic relationships and friendships too: the sibling bond functions as a training ground most people never think to credit.
There’s a practical dimension to this too, one that tends to surface only once it’s urgent: siblings are frequently the people who end up coordinating a parent’s care in old age, splitting decisions about medical treatment, finances, and living arrangements that no one prepared them to make together. A sibling relationship that’s been actively maintained for decades handles that pressure very differently than one that’s been left dormant since adolescence and is suddenly forced back into daily contact during a crisis. The relationship doesn’t need to be perfect for that coordination to go smoothly, but it does need some foundation, which is exactly what gets built, or doesn’t, in all the ordinary years before a crisis ever arrives.
None of this argues for treating a marriage as less important. It argues for treating a sibling relationship as more than background noise, an actual investment worth the same intentionality people already give to a partnership, precisely because it’s one of the only relationships in your life you’ll never get to choose again after childhood.
