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.

There are a lot of ways to make people not like you, but one of the most powerful - if least fair - is to be really, really successful. Nobody resents the guy who just lost his job. But the guy whose Internet start-up made him a billionaire at 25? That's a whole different kettle of envy.

There's only one thing harder than living in a home with an adolescent - and that's being an adolescent. The moodiness, the volatility, the wholesale lack of impulse control, all would be close to clinical conditions if they occurred at another point in life. In adolescence, they're just part of the behavioral portfolio.

Certainly, people can get along without siblings. Single children do, and there are people who have irreparably estranged relationships with their siblings who live full and satisfying lives, but to have siblings and not make the most of that resource is squandering one of the greatest interpersonal resources you'll ever have.

When we're awake, cortisol can fragment memories - one reason eyewitness crime scene accounts are so unreliable. But at night that very fragmentation allows creative recombinations of ideas.

We are all born with an innate understanding of interpersonal equity - the idea that if you lend me your rake today, I'll respond in kind when you come to borrow my shovel tomorrow. Or nearly all of us are born with that. Psychopaths aren't.

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.