Why Are We Different?
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.