Siblings have historically served as civilizing and socializing influences on each other. All the skills practiced in the playroom—empathy, conflict resolution, conflict avoidance, sharing, negotiation—are later put into play in the schoolyard and, much later, in the workplace and the boardroom. Multiple studies have shown, for example, that siblings who learn how to resolve their differences with each other through words rather than blows tend to be less physically combative with peers as well. A Norwegian study in 2007 also showed that firstborns get something of an IQ bump (about 3 points) from mentoring their younger siblings. And while the little sibs don’t get quite the same intellectual advantage, they benefit simply by having had that mentoring. Brothers who grow up with sisters and sisters who grow up with brothers tend to have a better understanding of the opposite sex when they enter the dating world. One study showed that potential romantic partners recognize that difference and preferred such people even when they didn’t know their sibling background.
Lastly, no matter how good a playmate you think you are for your child, nobody’s better than a sibling. Consider the difference between a human playing with a kitten and two kittens running and wrestling and rolling around together—there’s no contest!
—Jeffrey Kluger, author of The Sibling Effect: What the Bonds Among Brothers and Sisters Reveal About Us