A jellyfish is little more than a pulsating bell, a tassel of trailing tentacles and a single digestive opening through which it both eats and excretes - as regrettable an example of economy of design as ever was.

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.

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.

People with anxiety disorders such as OCD know that nothing can be more paralyzing than having too many options. Go to a store to buy a sweater, find four that you like and the odds are pretty good you'll stare and stare... and buy nothing at all.

Becoming food savvy is one thing, but it's amazing how fast savvy turns to snooty, and snooty leaves you preparing three-hour meals that break your budget and that the kids won't even eat.

The truth, of course, is that the only necessary and sufficient condition for human beings to murder one another is the simple fact of being human.

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.