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.

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.

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.

The families of many athletes - incensed at the sports leagues and hoping to make games safer overall - are increasingly making the brains of players who die prematurely and suspiciously available for study. Some athletes are even making the bequest themselves.

A cockroach likely has no less brainpower than a butterfly, but we're quicker to deny it consciousness because it's a species we dislike.

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.

Spare a thought for the poor introverts among us. In a world of party animals and glad-handers, they're the ones who stand by the punch bowl. In a world of mixers and pub crawls, they prefer to stay home with a book. Everywhere around them, cell phones ring and e-mails chime and they just want a little quiet.