Sisters have ways of socializing brothers into the mysteries of girls. Brothers have ways of socializing sisters into the puzzle that is boys.

Every batch of sperm represents an opportunity for genetic typos - called de novo mutations - to be passed on. A 20-year-old man and woman will each pass on about 20 de novo mutations to a baby they conceive. By the time the couple is 40, a woman's total has remained at 20, while a man's has jumped to 65 - and it keeps climbing from there.

The vexing thing about human behavior is that when we say we know we should do something, we really and truly do know it. It's hard to be 50 lbs. overweight or smoke a pack a day or feel miserable every moment you spend at work and not understand in a deep and primal way that change is in order - and that in some cases it could even save your life.

Kids are anarchy writ large.

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.

Overspending is as certain a part of the holiday season as overeating. But pushing away from both the table and the cash register at least a little bit sooner can make the post-holiday hangover hurt a little bit less.

My own life has in some ways been a decades-long tour of the sibling experience. I have full sibs, I have half-sibs, and for a time I had step-sibs.