"What is the missing ingredient — not genes, not upbringing — that shapes the mind?"

We know that genes play an important role in the shaping of our personality and intellects. Identical twins separated at birth (who share all their genes but not their environments) and tested as adults are strikingly similar-though far from identical-in their intellects and personalities. Identical twins reared together (who share all their genes and most of their environments) are much more similar than fraternal twins reared together (who share half their genes and most of their environments). Biological siblings (who share half their genes and most of their environments) are much more similar than adopted siblings (who share none of their genes and most of their environments).

Many people are so locked into the theory that the mind is a Blank Slate that when they hear these findings they say, "So you're saying it's all in the genes!" If genes have any effect at all, it must be total. But the data show that genes account for about only about half of the variance in personality and intelligence (25% to 75%, depending on how things are measured). That leaves around half the variance to be explained by something that is not genetic.

The next reaction is, "That means the other half of the variation must come from how we were brought up by our parents." Wrong again. Consider these findings. Identical twins separated at birth are not only similar; they are "no less" similar than identical twins reared together. The same is true of non-twin siblings — they are no more similar when reared together than when reared apart. Identical twins reared together — who share all their genes and most of their family environments-are only about 50% similar, not 100%. And adopted siblings are no more similar than two people plucked off the street at random. All this means that growing up in the same home — with the same parents, books, TVs, guns, and so on — does not make children similar.

So the variation in personality and intelligence breaks down roughly as follows: genes 50%, families 0%, something else 50%. As with Bob Dylan's Mister Jones, something is happening here but we don't know what it is.

Perhaps it is chance. While in the womb, the growth cone of an axon zigged rather than zagged, and the brain gels into a slightly different configuration. If so, it would have many implications that have not figured into our scientific or everyday way of thinking. One can imagine a developmental process in which millions of small chance events cancel one another out, leaving no difference in the end product. One can imagine a different process in which a chance event could derail development entirely, making a freak or monster. Neither of these happens. The development of organisms must use complex feedback loops rather than blueprints. Random events can divert the trajectory of growth, but the trajectories are confined within an envelope of functioning designs for the species defined by natural selection.

Also, what we are accustomed to thinking of as "the environment" — namely the proportion of variance that is not genetic — may have nothing to do with the environment. If the nongenetic variance is a product of chance events in brain assembly, yet another chunk of our personalities and intellects would be "biologically determined" (though not genetic) and beyond the scope of the best laid plans of parents and society.

Steven Pinker, research psychologist, is professor in the Department of Brain and Cognitive Sciences at MIT and author of Words and Rules.