Book review: No two alike

I just finished reading Judith Rich Harris’s new book No Two Alike: Human Nature and Human Individuality. Harris is an independent scholar whose controversial The Nurture Assumption: Why Children Turn Out the Way They Do, published in 1998, proposed that based on the evidence, it doesn’t appear that parenting styles have much to do with how kids turn out.

No two alike follows up on The nurture assumption to some degree by investigating the question of what it is that drives the personality differences between individuals. Even identical twins (with their shared genes) who are reared in the same household differ from one another; Harris takes as an extreme case a pair of conjoined twins who were definitely quite distinct personalities.

I enjoyed reading this book; Harris has a conversational and engaging style that nonetheless manages to convey quite a bit of information clearly. This book gave me a better understanding of how evolutionary psychology works (i.e., how a hypothesis can be proven or disproven) better than anything else I’ve read so far. Perhaps this is because she focuses not so much on imagined scenarios of Paleolithic life, but on questions like “If a mental system worked like this, how would we expect people to behave, and how can we test whether they do behave that way (and rule out other explanations)?” I’m really encouraged, by the way, by how much Harris has been able to accomplish as an independent scholar. I liked her story about receiving an award named for the guy at Harvard who had written the letter many years before that rejected her as a Harvard grad student.

Harris uses the metaphor of a mystery story to structure her search for the causes of individual differences. She begins by methodically and thoroughly investigating five red herrings that could explain individual differences but in fact do not. Her analysis includes some eye-opening investigations of work that has seeped into the popular press in a distorted form. I had heard of Stephen Suomi’s research that supposedly showed that high-reactive (anxious, fearful) monkeys can become confident, even high-status individuals if they are raised by a low-reactive (calm) mother, which certainly seems to support the idea that parenting influences personality. However, Harris dug deep to try to find details about this research and found only what she described as the scientific analog of vaporware. She dissects some other work that has gone unquestioned into the media, with similar results.

Then with the stage cleared, Harris begins describing her model for three mental mechanisms that together could cause and perhaps even strengthen differences between people. The relationship mechanism evolved to help us manage our interactions with other individuals. It discriminates between different people and helps us deal with each separate relationship appropriately. The socialization system helps us to learn the customs, language, and attitudes that are considered appropriate for the groups of people to which we belong. This system relies more on averages and generalizations. One thing I noted is Harris’s observation that children are socialized by people outside the immediate family, the ones they will have to spend their adulthood with. I had always thought it a little strange that of myself and my six siblings, none of us is a practicing Catholic or even, as far as I know, attends any church at all regularly, despite the fact that my parents raised us in a devoutly Catholic home. But it makes sense that we would learn to fit into the outside world where we have to live our lives now, which is quite different from the religious milieu in which we grew up.

The status system tells us where we stand in the hierarchies and groupings of the society we live in, so that we can figure out our relative strengths and weaknesses and how we can best compete with others. We learn from how others treat us, and to some degree we use our skills at understanding what others are thinking to gauge their reactions to us and form a mental self-image accordingly, so that we can shape our behavior based on what we learn. (E.g., if people tell me “Don’t quit your day job” when I sing or tell jokes, I’ve learned something about how I measure up in my ability to entertain others.)

I liked Harris’s observation that the socialization system and the status system have different motivations (to be like everyone else and to be better than everyone else respectively). This is one of the central conflicts of being human, and it was interesting to see it in the context of a discussion of how we grow into the people we are.

As Harris explains it, the status system is at the heart of the process that leads to individual differences. Our relationships with others, which feed information into our status systems, are based on who we are as individuals, so even identical twins do not get quite the same feedback from the people with whom they interact. The small differences that allow people to tell the twins apart also enable them to treat the twins differently, and of course the same process works for ordinary siblings, who are easier to tell apart. These differences in feedback from the outside world would cause the status system to devise a slightly different competition strategy for each person. Furthermore, random events can cause a systematic change in how we behave if they affect how people see us; also, even minor differences in status in a group can widen an initially small difference in behavior and personality. Only one person can be the brightest or prettiest or strongest or funniest, for example, so if you can’t quite fill that role in a group, you may seek another role.

The evidence Harris produces to support the existence of these three mechanisms is not only persuasive but fascinating to read. She goes into a lot of details about how children are socialized and some of the submodules that the three systems use, and this gets into some aspects of memory and cognition (e.g., how the brain forms and uses categories). She closes the book with a little speculation about why we need consciousness and the role that individual differences play in the division of labor in human societies, which she describes as an emergent property of human groups. She also suggests some ways her hypothesis might be tested; I suspect she’s right when she says that it will probably wind up being even more complicated and nuanced than the picture she presents here. I hope that people follow up on her suggestions.

For more about the book, including links to book reviews, you can visit the No two alike web site.


  1. I’m trying to start a neuroblogging carnival and would like you to be a contributor and co-host. I hope to post the first edition on my blog on 1st July. The carnival would initially be monthly.

    If you’d like to contribute, please email me links to up to three recent neuroscience posts on your blog, or post a comment on my blog. (mo187uk at yahoo dot com.)

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