The Nurture Assumption by Judith Rich Harris is quite a book. Her basic thesis is that children are influenced far more by their peers than by their parents. She first proposed this in 1995, in an article entitled "Where Is the Child's Environment? A Group Socialization Theory of Development," published in Psychological Review. Here's a nice speech she gave to the American Psychological Association in which she gives a good overview of her theory. In 1999 she published "The Nurture Assumption," which was a Pulitzer Prize finalist.

In the first part of the book, she examines the history of what she calls the "nurture assumption." Everybody has heard of Nature vs. Nurture: the theory that our behavioral traits are either determined by our genes (nature) or our environment (nurture). The "nurture assumption" is the modern assumption that parents provide most of the environment in which a child's personality develops. Prior to Freud, very few people placed much importance on the role of the parents; even fewer sat around debating the virtues of "parenting styles." Over time this assumption became engrained in our social consciousness, without ever being explicitly questioned or tested using scientific techniques.

But it's obvious, isn't it? As a parent you spend years loving, and raising, and teaching, and disciplining your child. And you've frequently observed how much children turn out to be like their parents, right? Well, yes and no. If you're a middle-class American raising your child in the same middle-class neighborhood in which you were raised, your child is likely to end up very much like you. But, if you're an immigrant raising your child in this same middle-class (English-speaking) neighborhood, you might discover that your child learns to speak unaccented English, and adopts the styles and customs of the American children around him. He didn't learn this from you.

According to Harris, studies have demonstrated that about half of your behaviour is due to genetic predisposition, and the other half is the effect of the environment. This has been determined by performing "twin studies" in which identical twins, fraternal twins, and adoptive siblings are compared. Because there are three different levels of genetic similarity (100%, 50%, and 0%), the researchers are able to figure out how much similarity between the siblings is due to their genes (or the effects of their genes). The rest must be due to their environment.

If parental influence is supposed to be so great, why can siblings turn out to be so different from one another? Two genetically unrelated kids that are raised in the same house turn out to be no more similar to each other than to other unrelated kids raised in the same neighborhood. And identical twins raised together are no more alike than identical twins reared in different homes (p34). It seems as if the home environment does very little to form the identities of children. Harris bases these statement on her review and interpretation of the literature; you'll have to read the book and decide for yourself whether you believe her conclusions.

So if the parents have so little influence, who is shaping our children? Harris proposes an alternative, which she calls 'group socialization theory.' She demonstrates that children do not automatically bring learned behavior from one context to another. Behaviors they have learned at home are not necessarily exploited in playschool, for example. Children unconsciously examine each context to determine what behaviors are appropriate or effective in that context, and tune their behavior to the context. Many parents, for example, have heard the teacher in a parent-teacher conference describing a child they don't recognize. Children develop their personalities to fit into their groups of peers.

Children are natural mimics. They observe and copy, receiving approval when they have copied correctly. At first the parents are the only available models. But then children discover other children. Parents are in charge: they make and enforce the rules, even though they don't have to follow the rules themselves. But other children are in exactly the same situation, and are playing by the same set of rules. Parents quickly discover that their children would rather play with other children than with their own parents.

As the children get older, the group socialization becomes more sophisticated. Children quickly separate by gender, and commence role-playing games, in which they rehearse acceptable behavior. Older children find a subgroup within school that suits them, and then rapidly adopt the group's styles of speaking, dressing, and behaving. Harris says that the academic level of a child will change to fit the group he or she is hanging out with, which opposes the view that the home environment is the primary determinant of academic achievement.

In fact, group socialization continues throughout life. As adults we are influenced by our friends, our co-workers, members of our social circles, and our neighbors. Think about the different things you do as an adult to help you fit into different circumstances. Children are even more susceptible to this than we are; is it so hard to believe that it exerts a powerful force on them? I believe that as you get older, you care less and less about what other people think, perhaps because you're more secure about who you are; consequently you make less effort to fit in with others. This means that your behavior as you get older is less shaped by social forces, and thus more by genetics, which explains quite handily why we turn into our parents as we age.

There is much, much more in this book, including fascinating descriptions and analysis of the statistical techniques used in child development studies; anthropological explanations for why we behave as we do; examinations of interesting studies (including a professor who raised a chimp alongside his son, only to discover that his son was being socialized by the chimp as much as the chimp was being socialized by the parents); and of course a bit of advice regarding what parents can do to help.

After reading the book, I feel she has a powerful argument -- still, I can't escape the belief that I as a parent of pre-teens do play a powerful role in the success of my children. I helped them develop the foundation upon which everything else is built, and I still exert influence over who they spend time with (which will change). Both children are quite different from each other, but that's something that's been there since they were born. And all the nurture in the world isn't changing those personalities they were born with.