You have probably noticed, if you have spent any time around adult siblings in their forties or fifties, that some of them appear to genuinely like each other and some of them do not. The ones who do tend to stand out. They text without an occasion. They call to complain about something small. They defend each other in a way that suggests they have been practicing since childhood. They laugh at private jokes older than most of the people in the room. What is less obvious, from outside, is what specifically about their childhood made any of this possible.
The instinct most people have, when they notice a pair of adult siblings who actually like each other, is to assume the family they came from must have been unusually harmonious. The parents must have been particularly loving. The household must have been particularly peaceful. When you actually interview these adult siblings about their childhoods, what you tend to hear is different. They describe the usual mess. The financial pressures, the arguments, the difficult grandparents, the ordinary levels of disappointment and confusion. What they describe instead, when pressed on what their parents did differently, is a specific and quite narrow set of choices about how the siblings were treated in relation to each other.
The pattern is well documented in the research. In a 2013 study of 151 pairs of young adult siblings, Alexander Jensen and colleagues at the University of Michigan looked at how differential treatment by parents affected sibling relationships in early adulthood. The finding, in the researchers’ own summary, was that “greater amounts of differential treatment were associated with less sibling intimacy.” The sibling who had received less support, relative to the other, also reported more depressive symptoms. The comparing produced measurable damage that was still showing up decades after the childhood had ended.
The Jensen finding is not an outlier. A 2024 meta-analysis pooling 26,451 participants across 43 samples found the same pattern. When parents treated their children differently, in either the affection they showed or the way they disciplined them, the differential treatment predicted more emotional and behavioural difficulties in the children. The pattern held across cultures, samples, and decades of studies.
What parents in these families didn’t do
What the parents in the “genuinely like each other” households seem to have avoided, on the research and the interview literature, is a specific set of comparisons. “Why can’t you be more like your brother.” “Your sister never had trouble with this at your age.” “He’s the smart one.” “She’s the athletic one.” “You’re the moody one.” These sentences, and the thousands of variations on them, are the ones the adult siblings who like each other tend to describe never really hearing growing up. The parents were not, in most cases, deliberately restraining themselves. They simply did not tend to think of their children as versions of the same product that could be measured against each other. Each child was, more or less, a separate person the parents were separately curious about.
What they did instead
The second thing these parents seem to have done, on the interview accounts, is treat difference between the siblings as ordinary rather than as a problem to be solved. If one child was quiet and the other was loud, this was, in the household’s working picture, just how the two children were. Not something that needed to be fixed. Not something that needed to be worried about. Not something that needed to be interpreted through some family narrative about who took after whom. The quiet child was allowed to be quiet. The loud one was allowed to be loud. Neither had to translate themselves into the other’s shape to be recognised by the parent as fully belonging in the family.
A note on what this is
We write about research here, not from a family therapist’s office. The patterns described come from a substantial body of sibling relationship research, including the parental differential treatment work that has been running since the 1980s. They do not describe any specific family. Some adult siblings who like each other grew up in households that did not match this description exactly. Some adult siblings who do not like each other grew up in households that did. What the research documents is a strong average pattern, not a rule.
Why comparing does the damage it does
The reason comparisons between siblings do the damage they do is, in the research, relatively straightforward. A comparison converts the sibling from a person into a measuring stick. The child hearing “why can’t you be more like your sister” is being told two things at once. First, that their own way of being is deficient. Second, that their sister is the standard by which the deficiency is being measured. Neither piece of information is neutral. The first produces the more obvious wound, which is a slow lifelong sense that one’s basic self is inadequate. The second produces the less obvious wound, which is a specific complication in the relationship with the sibling being held up as the standard. The sibling has, by no fault of their own, become associated with a particular kind of pain the child would rather not be near.
Over ten or fifteen years of childhood, these small comparisons accumulate. By the time the siblings are in their twenties, the pattern is set. The one who was held up as the standard often carries a low-grade guilt about a privilege they did not ask for. The one who was compared against carries a low-grade resentment they cannot direct at the sibling without seeming petty, because the sibling did not do the comparing. The relationship, from the outside, looks fine. From the inside, it runs on a specific set of small avoidances that neither sibling has ever fully named.
What this suggests
The cultural conversation about producing good sibling relationships still tends to focus on grand gestures. Family holidays. Shared traditions. Enforced togetherness. The research suggests something less dramatic and more useful. What produces adult siblings who like each other is not, in most cases, the presence of anything specific. It is the absence of a particular set of comparisons most parents make almost without thinking, and a tolerance for a level of sibling difference that most families still, quietly, treat as a problem to be worked on. The good news, for parents currently raising more than one child, is that the practice this points at is not complicated. It is stopping the sentence in your head that begins “Why can’t you be more like…” and letting each child be, more or less, the different person they already are.