When you learn conflict-resolution skills in the playroom, you then practice them on the playground, and that in turn stays with you. If you have a combative sibling or a physically intimidating, older sibling, you learn a lot about how to deal with situations like that later in life. If you're an older sibling and you have a younger sibling who needs mentoring or is afraid of the dark, you develop nurturing and empathic skills that you wouldn't otherwise have.

I was born in the U.S., my wife was born in Mexico and emigrated here when she was in college, and my daughters were born in New York City. That makes them passport-carrying, natural-born, eligible-to-run-for-president Americans. But they're also Mexicans and they like that just fine.

Identical twins are ideal lab specimens for studying the difference between learned and inherited traits since they come from the womb preloaded with matching genetic operating systems. Any meaningful differences in their behaviors or personalities are thus likely to have been acquired, not innate.

More and more NFL players have been willing their bodies to science so that their brains can be studied even if they die of other causes.

Kids whose puberty begins too soon face not just psychological risks, but physical ones too, with an increased likelihood of cancer, as well as skeletal changes that could prevent them from attaining their full adult height.

Older siblings get more total-immersion mentoring with their parents before younger siblings come along. As a result, they get an IQ and linguistic advantage because they are the exclusive focus of their parents' attention.

Indeed, the best way to think of willpower is not as some shapeless behavioral trait but as a sort of psychic muscle, one that can atrophy or grow stronger depending on how it's used.