No Two Alike is a followup to her previous work, The Nurture Assumption. In her first book, explained that most of the non-genetic affects on the personalities of adults are a result of their interactions with peers rather than with their parents. She pointed out that many people want to believe (and prove) that parents are the major source of their children's personalities. According to The Nurture Assumption, the field of sociology has been confused for several decades and has been trying to distinguish nature and nurture, when they needed to be either distinguishing the effects of heredity from environment, or disentangling the environmental influences which include both parents and peers.'s book
The best tool, according to, for distinguishing the effects of heredity and environment consists of studies of twins. Comparing twins raised together and twins raised apart controls for genetic affects and allows us to see the effect of gross differences in environment. Comparing fraternal twins raised together with identical twins raised together holds the gross environment constant and makes it easier to see what differences are purely genetic.
In the new book, siamese twins have different personalities, and they share all of their genes, and all of the environmental influences that anyone could hope to treat as responsible.focuses on why twins are so different, in order to isolate the causes of differences that aren't explained by other results. The existing literature says that some proportion of personality differences are due to genetics, and some proportion by each of various environmental causes: parents, wealth, neighborhood, etc. But a significant amount of variation remains that isn't apparently caused by any of these. Her focal example is that even
' conclusion (skipping over most of her argument for the moment) is that there must be something driving each of us to be unique, and that means we have to find a distinction to enhance. The bottom line is that a significant part of personality (who we are) isn't determined by factors that we can examine or control. Each individual starts out with an endowment of heredity, and occupies an environment that isn't fully under their control, but the developing personality is still a negotiation between that individual and their context. If the strongest part of their innate tendencies is best suited for a niche that is already filled, they will look for a second best. When two identical individuals struggle to fill the same niche, some factor (random or not) will eventually determine a winner in each particular event, and at some point the effects of competition, if nothing else, will drive them to exploit different strengths. The different choices and different results in competition will magnify any differences, and over a reasonable lifetime, they will become recognizably different people.
Along the way,spends a good deal of effort (successfully) demolishing other possible explanations (differences in environment, combination of nature and nurture, gene-environment interactions, environmental differences within the family, gene-environment correlations, and transferability of learning between situations). At the end she argues that she has demolished all the other possibilities and provided an argument for the one remaining theory (an innate drive for status), and so it must be true. But her argument for the specific mechanism is a little too weak, and it seems plausible that some variation or related description will fit the data a little better. I'm reasonably convinced that something drives us to differentiate, but it may not be purely a status drive. Two possible variations on her theory include drives for attention or to master something.
I found the style of Harris' presentation sometimes compelling, and sometimes distracting. She fit the presentation into the framework of a detective story. The presentation is salted liberally with examples from popular detective stories to show how attentive the detective has to be to details that have distracted other investigators. This worked for me when I was familiar with the detective in question (Sherlock Holmes, Kinsey Milhone), and didn't work when I hadn't read the stories (Alan Grant). I suspect that Holmes is the only one of these that is widely enough known that other readers would feel that they should get the references even when they don't.
On the whole, I think the book was successful in explaining that fundamental differences in personality are effectively the result of an innate drive that causes us to differentiate. The drive makes use of arbitrary differences in the material it has to work with (genes and environment). Parents do make a difference in the lives their children lead, but the ultimate person each child becomes isn't determined by parenting style at any gross level.