Some of the most rewarding times my brothers and I have are when all of us get together, and we can see what we've been building genetically and culturally.

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.

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.

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.

There are a lot of obstacles in the way of our understanding animal intelligence - not the least being that we can't even agree whether nonhuman species are conscious. We accept that chimps and dolphins experience awareness; we like to think dogs and cats do. But what about mice and newts? What about a fly? Is anything going on there at all?

When it comes to raising civilized kids there are no hard rules, but there are two things on which most parents agree: Boys are generally wilder than girls, and adolescents are wilder than kids of any other age. If you've got an adolescent boy, you're in the sweet spot for trouble.

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.