Larry:

OK. I'm more awake now and ready to try another response to your assertion that our personalities don't change much after age twenty because we live in ruts.

My stance is that the reason for this phenomenon is nature, not nurture. That is, our personalities are a function of our brains, and stop growing when our brain stops growing - around age twenty. That is not to say we don't grow and change after twenty. It's just our style that becomes fixed.

So maybe personality can be defined as a set of data processing tendencies that result from well-worn and thus preferred pathways in our neural networks. By age twenty, each brain has developed a characteristic shape which endows each person with a characteristic way of behaving. This is not a rigid unwavering program, but a set of tendencies, a weighting of the pathways.

This "hardware limitation" may actually make a good deal of evolutionary sense. In his magnum opus, The Society of Mind, Marvin Minsky muses on a related topic. Why is it that after adolescence, most people who acquire a second language never quite master the pronunciation to such a degree that they can pass for a native speaker? Kids can do it. But even when adults master another language they can't seem to get rid of their "foreign accent." Minsky proposes the following rather ingenious theory:

"Why should we be able to learn many different speech sounds before the age of puberty but find it so much harder to learn new ones afterward? I suspect that this link to puberty is no coincidence. Instead, one or more of the genetically controlled mechanisms that brings on sexual maturity also acts to reduce the capacity of these particular agencies to learn to recognize and make new sounds! But why did this particular disability evolve? What evolutionary survival advantage would favor individuals whose genes reduce, after that age, this particular ability to learn? Consider this hypothesis:

"The onset of the childbearing age is the biological moment when a person's social role changes from learner to teacher. The "evolutionary purpose" of suppressing speech-sound learning may simply serve to prevent the parent from learning the child's speech - thus making the child learn the adult's speech instead!"

"Wouldn't parents want to teach the children their language anyway? Not necessarily. In the short run, a parent is usually more concerned with communication than with instruction. Accordingly, if we found it easier to imitate our children's sounds, that's what we'd do. But if parents were inclined and able to learn to speak the ways their children do, those children would lose both incentive and opportunity to speak like adults, and - if every child acquired a different set of language sounds - no common, public language would ever have evolved in the first place!"

I rather like this distinction between learner and teacher, and I think the idea could be broadened to cover a whole range of behavior, not just language. The more I think about it, the more I see how vital it is that we change from learners to teachers. Our survival as a species depends on this ability. And the only way to accomplish this metamorphosis is for our brains to "harden" at some point. And puberty, the point at which we start to become parents, is the logical point for this to happen.

And when our brains harden, our personalities become fixed. And THAT is why our personalities don't change much after age twenty. It has nothing to do with ruts.