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What Was the Most Consequential Sibling Rivalry of all Time?

How to Make Friends, According to Science
BEN HEALY

Sibling Rivalry, a History
PETER TOOHEY

Raised by YouTube
ALEXIS C. MADRIGAL
When a sibling relationship is bad, however, it can be really bad—as in messing-up-your-life bad. Tense sibling relationships make people more likely to use substances and to be depressed and anxious in adolescence. [1] Moreover, sibling bullying makes a kid more likely to engage in self-harm as a teen [6] and to become psychotic by age 18. [7]

Whether a person models herself after her siblings or tries to distinguish herself has particularly important consequences. One study found that siblings who felt positively about each other tended to achieve similar education levels, while those who spent unequal time with their dad and perceived unequal parental treatment had diverging educational fortunes. [8] Not that divergence is necessarily bad: Research suggests that as siblings’ relationships with their parents grow more different over time, their relationship with each other may become warmer. [9] And emulating your sibling can be a mistake, depending on what she’s up to: Girls are more likely to get pregnant in their teens and teenagers are more likely to engage in risky behavior if an older sibling did so first. [1] Younger siblings also may have sex earlier than older ones (partly because their big brothers and sisters introduce them to more experienced potential partners). [10]

One way or another, sibling influence is lasting.