Thursday, April 27, 2006

Why Are We Different?

In an earlier post, I discussed Judith Harris’s first book, The Nurture Assumption. She has now written another one, and it too is interesting.

The subject of No Two Alike is a simple and striking puzzle. Human personality, so we believe, is the product of genetics and environment. It would seem to follow that two individuals with the same genetics and the same environment should have the same personalities.

They do not. Identical twins raised together end up significantly different—about as different as identical twins raised apart. Identical twins physically attached to each other, never separated in their lives, have substantially different personalities.

About two-thirds of the book is devoted, not to solving the puzzle, but to knocking down solutions, eliminating red herrings. It is an interesting account, especially the story of one prominent book in the field that Harris pretty clearly believes—although she does not quite say so—to be fraudulent. The strongest evidence for that conclusion is its author’s response to criticism: Legal threats to try to prevent publication and a refusal to provide anyone with the data needed to check the book’s factual assertions.

The final third of Harris's book sketches a theory of how human beings, from infancy on, deal with their social environment. From that theory she derives a conjecture about the source of human differences. It is an interesting conjecture and may well be true, but those chapters would be more satisfactory if the author suggested ways of testing her conjecture and offered some evidence in its support beyond the failure of alternative explanations.

That said, it is an interesting, intelligent, thoughtful book, and I recommend it.


Anonymous said...

Two different individuals with the same genetics and environment would turn out the same. But twins don't share the same environment, physiologically or physically - only a similar one. It does sound very interesting though that the similarities of environment between twins raised together have no effect.

Anonymous said...

Did she consider the possibility that people actively seek to be somewhat different from those around them? (Perhaps evolution features personality specialization / differentiation?)

If this theory is correct -- that people strive to be different in certain ways -- then we might expect that identical twins raised together would end up very slightly *more* different than identical twins raised apart.

Anonymous said...

Your assessment of the book seems to be about par for the course. "Strong start with a weak finish."

Jeff Brown said...

I'm happy to read this. I've thought that a lot of one's personality is generated through the stochastic process of internal rumination. I've got in mind the butterfly-changing-the-weather phenomenon.

For instance, suppose I and my twin Guy are attached at the shoulder. We both want to go to a movie, but our dad says we can't, so we both start to sulk. Then I catch a glimpse of a parachuter, and my thoughts turn to that instead. As a result, it might be that Guy's anger festers, making him a little more sour than me, for no particularly meaningful or foreseeable reason. Then again, it might not -- but as we go through millions of experiences, certainly SOME of them will have different effects on us, just by random chance.

[All that would be true even if there were no phenomena such as inter-twin disputes, which are certain to have different effects on the winner and the loser.]

To believe that identical genes and simliar conditions leads to nearly identical people would be to believe in some damping force that nixes those myriad deviations. I would instead expect them to recursively knock things around in a manner that forbids any sort of "equilibrium personality".

I would expect that because I think the process of beoming a person depends largely if not mostly on the thoughts of the person doing the becoming. Those thoughts are recursive and partly random, which is a sure recipe for the nonexistence of a single equilibrium.

(I know, I'm using math ideas without defining my terms precisely. I, um, won't.)

David Friedman said...

Jeff offers a conjecture about the reason for differences.

That isn't exactly Judith Miller's version. Hers depends on the idea that individuals are filling niches within their social group. If one of the niches to be filled is "dominant male in the group," then only one of the (identical, nonattached) twins can fill it, so the other finds another niche he fits into and adapts his behavior and personality accordingly.

Similarly for any case where two otherwise identical people can't fill identical roles because there aren't two identical roles available to be filled.

Anonymous said...

But doesn't that assume that the roles in the group are somehow given? Isn't the economic point of view rather that the roles are created by the individuals in the group (methodologic individualism)? It certainly is the one I'd favour, and it fits quite nicely with Jeffs idea.

Anonymous said...

I'm curious what she has to say about the "birth order" theories of Sulloway and a few others. Is this perhaps the theory that she hints (without saying so) is fraudulent?

David Friedman said...

Christopher asks about Harris's view of birth order theories. Sulloway is in fact the author who appears, from her accounts, to be a fraud--in particular, who refused to provide the information needed to check his factual claims and threatened legal action against a journal that was about to publish an article critical of him. It's an interesting story. You can find a summary at:

The more general issue of birth order is one Harris goes into in some detail in her earlier book. By her account, lots of studies find some relation between birth order and personality--but then, there are a lot of possible relations, and if you look for enough relations you will, by chance, find one. If you pool all the studies, no consistent relation remains, except (I think) when personality is evaluated by the person's family members.