Book review: No Two Alike: Human Nature and Human Individuality by Judith Rich Harris
This book provides a clear theory of what causes the personality differences between people that can’t be explained by genetic differences. She focuses a fair amount on identical twins, because the evidence that their environmentally caused personality differences are the same as ordinary siblings, and the same whether they’re reared together or apart, rules out many tempting theories.
Amazon reviewer Sioran points out an inconsistency – she claims early on that random chance can’t explain all of the variation, but her explanation ends up amounts to saying the causes are ultimately random. I find her early arguments against randomness unconvincing. And her explanation’s reliance on randomness doesn’t imply that her explanation is useless – she rules out most kinds of randomness as a cause, narrowing down the class of random causes to those which affect the person’s view of her status in society (e.g. differences in who outside family the person interacts with, and physical differences such as being tall due to better nutrition).
The most surprising prediction she makes is that mindblind (i.e. most) animals won’t have persistent personality differences that can’t be explained by genetic differences. I’m unsure whether to believe this – it seems that animals should only need to remember differences in how others treat them (rather than have a theory of mind) in order to produce the results we see. She would probably predict that autistic people have no persistent environmentally caused personality differences, but she isn’t clear about that (it may depend on the degree of autism).
One interesting result that she mentions is that autistic children are unable to use the fusiform face area (which in most people is specialized to do good face recognition), and instead seem to recognize faces the same way they recognize ordinary objects. I’m wondering how much this explains about why autism impairs many parts of the mind that deal with relationships.
I’m annoyed by how many pages she spends recounting the reaction to her prior book (The Nurture Assumption, a better book than this). If you’ve read that, most of the first half of this book will be a waste of time.
One interesting piece of evidence she mentions is this paper from the Journal of Political Economy which says that one’s height as a teenager is a better predictor of wages as an adult than adult height.
One small quibble: she says being a firstborn is unimportant (often not even known) outside the home in “contemporary societies — at least those not ruled by monarchies”. Korean society appears to be a clear exception to that claim.