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Brothers and sisters are supposed to show appreciation for one another on National Sibling Day, though whether you’ll actually exchange any outpourings of affection totally depends on whether you like each other or not. While there are plenty of studies outlining the psychological benefits of their constant companionship, the effects of sibling relationships gone awry can persist throughout a lifetime.

Sibling rivalry is commonly observed throughout the animal kingdom: When resources and parental love are limited (and aren’t they always?), siblings will naturally compete with each other because it boosts their chances of survival. Fortunately, it usually doesn’t get nearly as bad in humans as it does in, say, cattle egrets or bald eagles or sharks, species in which competing siblings literally commit siblicide when the parents aren’t around. Rivalry between human siblings manifests in far subtler forms (most of the time), though the effects can still be quite serious.

Noel Liam Gallagher Oasis
Noel and Liam Gallagher of Oasis are one of pop culture's most tempestuous sibling pairs.

According to the work of British developmental psychologist Judy Dunn, Ph.D., who built her career on studying the early relationships between siblings, kids who feel they’re on the losing end of the battle for parents’ attention grow up with a tendency toward anxiety and depression. A more extreme version of this idea was presented in February in a Psychological Medicine article showing that individuals that are actively bullied by their siblings can be up to three times more likely to develop psychotic disorders like schizophrenia in adulthood.

Of course, not all cases of sibling rivalry have clear “winners” or “losers”. One way in which the effects of sibling rivalry seem to be mitigated is through the process of differentiation — shaping one’s personality so that there’s less overlap with a sibling. These processes, write researchers in a review published in the Journal of Marriage and the Family in 2012, “also involve siblings treating one another as sources of social comparison but imply that siblings treat one another as foils, de-identifying from one another by selecting different niches in the family and developing distinct personal qualities.” This, in turn, can “protect siblings from rivalry and jealousy.”

Though the effects of intense rivalry can be quite serious, it’s good to keep in mind that it’s a normal dynamic — and there are plenty of ways in which having a sibling is beneficial, even if that relationship sometimes flares up in squabbles. Siblings, importantly, have been shown to teach one another empathy, no matter how their ages differ.

Perhaps, in the human world, every day is a happy National Sibling Day. Taking into account the fact that some animal siblings — we’re looking at you, baby sand tiger sharks — not only kill their siblings but do so in the womb and then gobble up the remaining dead flesh, even the notoriously violent beef between infamous sibling rivals Noah and Liam Gallagher looks lighthearted in comparison.

Photos via Giphy (1, 2)