I have some comments on Agnostic’s interesting and thought-provoking review of my book No Two Alike. According to Agnostic, my theory is simply a “macro” version of developmental noise. In other words, he claims that I’m saying that the (nongenetic) personality differences between identical twins – and, by extension, nongenetic variations in personality in general – are the product of random forces, operating at one or another level of development.
It’s perfectly true that randomness plays a role in my theory. The differences between identical twins look exactly like random differences – like noise. That’s one of the pieces of evidence I had to account for in constructing my theory. I found no way of getting around it: noise has to play a role.
But my theory is not a macro version of developmental noise. At the end of Chapter 2, I considered the possibility that the nongenetic variation in personality is due entirely to biological and/or environmental randomness, and I rejected that possibility on evolutionary grounds. Here’s what I said on page 48 of No Two Alike:
“Random modifications would have random effects on an individual’s chances of surviving and reproducing. Making personality plastic but leaving it at the mercy of chance, to be buffeted this way and that by unpredictable events, doesn’t make sense from an evolutionary standpoint. Natural selection would favor any alternative to randomness that produced results that were better than chance.”
The three systems I propose in the book did not evolve in order to widen the differences between identical twins – that’s simply a side effect of the way these systems operate. The evolutionary purposes of these systems are to fit people to their social environments and to give them a better chance of succeeding in the competition to survive and reproduce. An individual has a better chance of succeeding if he takes account of his strengths and weaknesses, if he concentrates on competing in areas of his strengths, and if he looks for areas in which there is little or no competition (an unoccupied niche in his group, perhaps).
Operating together, two of the systems I propose – the status system and the relationship system – have the incidental effect of widening the random differences between identical twins. As I explain in the first few chapters of the book, a theory of personality development cannot be successful if it cannot explain the (nongenetic) personality differences between identical twins. But it also has to be able to explain the nongenetic personality differences between people who are not twins, and it has to explain them in a way that makes evolutionary sense.
One more comment, a minor one. According to Agnostic, the three systems I proposed are “the relationship system, a mind-reading device, and a status system.” Actually, the three systems I proposed are a relationship system, a socialization system, and a status system. The mind-reading device is a lower-level module that sends output both to the relationship system and to the status system.
Judith Rich Harris