Web Exclusive: The New Science of Siblings

Aug. 31, 2009— -- Po Bronson is the co-author of "NurtureShock" with Ashley Merryman.
You can read an excerpt from the book by CLICKING HERE.

Growing up, my brothers and I fought all the time. We bickered, competed, and pummeled each other. But we also played together endlessly, from morning to dusk, indoors and out. We played every sport in the backyard, built forts in trees, jumped off ramps on our bikes, swam in the lake, and built elaborate haunted houses in our basement.

According to Dr. Laurie Kramer, one of the world's leading experts on sibling behavior, the fact we played together so often matters more than the fact we fought so often, which is why my brothers and I have such close friendships as adults today.

For over twenty years, Kramer has tracked sibling pairs from infancy through adulthood. She's isolated the secret indicators in childhood which predict siblings' futures. It turns out that the ratio of playing together to fighting together is key; there have to be more good times than bad times. Siblings who don't fight -- but don't play together either -- end up not having warm relationships as adults.

Sibling quarrels are a fact of family life. On average, young siblings argue or fight 3.5 times an hour, which adds up to ten minutes of every hour. In observational studies, siblings make 700 percent more negative and controlling statements to each other than they do to friends.

Why? Because if you treat a friend badly, they'll eventually stop being your friend. Siblings, meanwhile, will be there tomorrow, no matter what. In the words of one scholar, "Siblings are genetically sentenced to live together, with no time off for good behavior." There's no incentive to treat each other better.