I'm a huge fan of Judith Harris' The Nurture Assumption, which powerfully debunks the idea that how your parents raised you has a large effect on how you turned out. Now she's got a new book, No Two Alike, which presents her theory of personality formation.
Harris deliberately sets the bar high: her explicit goal is to explain why even identical twins raised in the same home are STILL different from each other.
Has she succeeded? I'm afraid not. While the book is well worth reading and full of interesting insights, she hastily dismisses several promising approaches, and her own story is poorly fleshed out. Main problems:
1. As far as I can tell, Harris overstates how different identical twins are from each other. Even a single individual will test somewhat differently on personality tests at different times. How do the personality test differences between identical twins tested at the same time compare to the test differences between one individual tested at different times? (I think some data does correct for this problem, but it doesn't seem like Harris makes this distinction).
2. Harris is too quick to dismiss genetic-environmental interaction effects. Yes, the evidence in favor is crummy. But it's also very difficult to get. Contrary to Harris, the jury is still out.
3. Harris is too quick to dismiss socialization as a cause of identical twins' differences. Yes, if there is only one culture, culture can't explain differences. But if there are many cultures and sub-cultures, and twins don't always belong to exactly the same one, socialization could easily explain differences.
4. In the end, Harris zeroes in on the pursuit of status to explain personality differences. But how does this help to explain differences between identical twins? In the end, she has to appeal to random events that build on each other. Fine, but what makes her think that random events relevant to status are so special? Why not just say "random events"?
5. Harris barely tries to relate "the status system" to any of the main personality traits that psychologists study. For example, how does greater or lesser Openness or Neuroticism have anything to do with status? Maybe there's an answer, but it's not in this book.
6. Harris knows a lot of disciplines, but she should have added economics and game theory to the mix. If we think about the distribution of personalities as a mixed strategy Nash equilibrium, then we should expect that - at the margin - all personalities will be equally effective. From this perspective, variation is not very puzzling: Maybe we see a lot of variation because variation is practically free.
Is this a bad book? Hardly. I'd much rather read a book that asks a thoughtful question and fails to answer it than a book that asks a boring question and gets it right. But if you want to see Harris at her best, start with The Nurture Assumption.