No Two Alike: Human Nature and Human Individuality Hardcover – March 17, 2006

Judith Rich Harris has devised a solution to the second half of the puzzle she set herself a decade ago. John Locke proposed three centuries ago that a baby enters the world as a blank slate, tabula rasa in Latin, upon which worldly experience writes, and in doing so forms the adult. Per Harris' title, each of us is quite unique. The process by which we attain our uniqueness has occupied philosophers and scientists for centuries.

I begin with a caution to the reader. Do not search your mental library for a template to pre-apply to Harris' book. You don't have one. It is not Nature vs. Nurture redux. Not genetic determinism. This is a fresh assemblage of ideas, pulled together from a number of realms of contemporary science, to be approached with a fresh and open mind.

The history of the science upon which she draws is full of strong personalities and opinions, drawing on and shaping the intellectual currents of their times: Galton, with his belief that human ability and character were inherited; Skinner and Watson, who believed in the potency of environmental interventions and the malleability of human nature (a belief shared by generations of educators). This is the baggage Harris asks you to throw aside. Neither Galton's nor Skinner's model comes close to explaining human nature as we experience it. What, then, does?

45% of individual differences - Harris focuses on personality, but other researchers find it to be true of other traits such as cognitive ability - are inherited. This was one of the principal arguments of Harris' 1998 "The Nurture Assumption." The second major argument concerned what did not account for the other 55%. After controlling for genetics, parents' influence on children's personalities, language and values is minimal. These are testable hypothesis: simply look at twins raised together and apart, and blood siblings vs. adoptive siblings under the same roof. The book posed a serious threat to the dominant model. It exculpated generations from the parental guilt imposed by Freud and his followers, and challenged parents' and educators' sense of their own importance in forming a child. Harris got into some nasty academic mudslinging, and takes delicious pleasure in recounting her victories in "No Two Alike." It is exactly the kind of gossip upon which humankind, per her thesis, thrives, and I relished it.

That still leaves 55% of the variance in human personality unaccounted for. What are the culprits? Harris crafted the book like a mystery novel, lining up the usual suspects one by one and dismissing them: (1) home environments, (2) child rearing practices, (3) gene-environment interactions, (4) birth order, and (5) gene-environment correlations. 1,2 and 4 are self-evident. Concepts (3) and (5) take some understanding of genetics and statistics. (3) might say that an environmental factor - harsh discipline, say -

might make tall children less aggressive and short children more aggressive, with the result that on average there is no effect. (5) would posit that individuals who love to cheer would naturally gravitate towards an environment where they can do so, such as sports. But in the end none of these hypotheses hold water. They do not account for any meaningful fraction of human personality differences. Now for what does!

Three things: Our

o Relationship system

o Socialization system, and

o Status system

The first question Harris must address is, exactly what are these? These terms were not found in the Nature-vs-Nurture discussion. She draws extensively on recent science, especially Stephen Pinker's "How the Mind Works" and "The Language Instinct." Pinker's driving insight is that far from being a tabula rasa, the brain is extensively prewired for the tasks of vision, speech, hearing, manipulation of the hands and the other functions that make us human. Researchers have the ability to see which areas of the brain "light up" in each function.

Harris' hypothesis is that we have evolved to be born prewired with mental systems for the most important social functions in primate society. Our relationship system is a mental Rolodex of individuals, with slots for name, face, abilities, personality traits and every other feature that is important for us to know in maintaining relationships. Newborns immediately start to populate their Rolodexes with a who's who of voices, smells and expectations.

We are social animals. We need to belong. The socialization system fits us into groups. We observe groups, abstract the traits of members of the group into a stereotype, and attempt to mold our behavior to that stereotype. A boy playing baseball does what kids playing baseball do: toss the ball back and forth, take play swings, and talk the talk. A kid who doesn't, like Hobbes' friend Calvin, is acutely aware of being an outsider. Each of us belongs to a great many groups. Harris points out the fallacy of the "nurture assumption" that the family group is the major influence on a child. Wrong - the kid can be a totally different person at home, in school, at summer camp, and on a team. As parents often hear.

Status, the third system, operates in dynamic tension with socialization. We conform to groups, but we each occupy a unique niche within the group pecking order. The higher our status, the more social goodies we enjoy: material stuff, mating opportunities, company and leisure. After striving for acceptance by the baseball team, we compete for the position of captain, pitcher and other prestigious positions.

Harris closes by admitting that her theory is complex and that testing it will involve monumental challenges. To what degree are the three constructs independent of one another? How does one observe and measure things like relationships or status to subject them to statistical analysis? What alternative, derivative or simplified theories might equally well explain what we observe? These issues will be a long time in the resolution, but whatever the outcome, Harris' will be remembered as the one who framed the right question.

Whenever I read a science book that analyzes hard data to make radical claims, I immediately rush to read all the responses. The manner of the responses is what convinces me Harris's books are correct. So far the criticisms have been ideological, emotional, and focus on Harris's lack of credentials. (What doctorate means: You are hereby allowed to make money claiming this belief system is true.) Harris has no dog in this fight. Her rigor hit a nerve and made psychologists self-conscious about their rigor. So far nobody has dared argue against her data in any detail.
Writing it in the style of a detective story is a serviceable way to propel the narrative. It reads like a personal memoir of a no-nonsense lady stubbornly in search of truth, with Joan as her trusty lance-bearer, while they battle against the stubborn momentum of popular myths.
Harris took a cogently argued polemic and turned it into a meditation on the very methods by which science overcomes the confirmation bias, and the methods by which it does NOT.
I am convinced there is no evidence that children's relationships with their parents has any affect on their adult relationships, and there has never been any evidence shown that birth order has an affect. If somebody has evidence that it does, show it. As Harris says, "Show me the data." If psychologists can't respond with data, the argument rests on the conclusion that Freudian psychology is based on a hunch.

One of the more fascinating aspects of being human is that we're all built from the same parts, yet we all have different personalities. What makes you you and me me? Confronted with this question, most people would respond that it's some blend of nature (the genes you inherited from your parents) and nurture (the environment you were raised in). Judith Rich Harris debunks that answer and replaces it with an elegant hypothesis about the true sources of human uniqueness.

In the first part of her book, Harris takes developmental and social psychologists to task for over-emphasizing the influence of early childhood experiences on personality formation and not giving enough credit to what happens outside the home. She also maintains that developmental studies that don't screen for genetic influences are prone to confuse cause and effect. She shows how research that's been inadequately digested makes its way into mainstream culture, where it's regurgitated as pop truth. According to Harris, most of the conventional wisdom about birth order, home environment, parenting style, and the interaction of genes and the home environment does not adequately explain why we're different from one other.

Having eliminated the usual suspects, Harris turns to Stephen Pinker's How The Mind Works as her starting point for solving the mystery of human personality. Pinker posits that our brains are organized into "mental modules" that perform discrete tasks. Some examples of modular systems are facial recognition, language acquisition, and our ability to postulate what other people are thinking (a theory of mind). All of these had evolutionary value to small tribes of wandering hominids, and evolved to better serve our survival needs.

Harris' theory is that once we've figured out what part of a personality is accounted for by genes (a little less than half if you average the various studies), the remainder of a personality is created through the interplay of three mental systems: the relationship system, the socialization system, and the status system. Each of these systems evolved to deal with a pressing survival issue. The relationship system helps us attach to, and later relate to, specific individuals. The socialization system enables us to figure out the norms of our group so we can fit in. The relationship system is with us at birth, and the socialization system develops quite early in life. The slowest system to emerge is the status system, which we use to set ourselves apart from the other members of our group. The status system takes longest to develop because we need to be intellectually sophisticated enough to figure out who we are and what we're good at.

These systems can issue contradictory directives. For instance, the generalizations of the social system war with the specificity of the relationship system. This explains why a person can dislike an ethnic group while getting along perfectly well with a neighbor who happens to be a member of that group. The socialization and status systems are often at odds: do I sacrifice my personal goals for the good of the group, or do what's best for me even if it harms the others? Most of a life's high drama involves trade-offs among these systems. What makes you unique among all the humans who have ever trod the planet is the way your mental modules process the stimulus provided by your particular environment.

I wish we had more detail on how Harris' mental systems work. For instance, language acquisition theorists continue to go round on how much of our language facility is innate versus how much is instantiated by experience. Just as we have to memorize a lot of specific words to speak a language, we have to organize a lot of data - particular faces, facts about the people attached to those faces, rules, norms and prototypes of groups, specific information about our own skills and how people view us - to populate Harris' mental modules. Does the framework - eg grammar rules or the relationship module - precede the data, or does it emerge as the data is acquired? If the three systems are more innate than experiential, does this mean that more of our personality is influenced by genes than we currently believe? If it's experiential, by what process do we generate the proper responses to a specific situation? What triggers the appropriate neural assemblies and how do we make trade-offs between specific information and general rules?

As Harris herself states, her theory needs to researched, tested and validated. The really exciting breakthroughs will come when we're able to correlate observational studies of human behavior with the genes and genetic switches that activate those behaviors. Behavioral geneticists are advancing into this new territory, which will help us lift the analysis of human personality out of the realm of metaphor and into the realm of hard science. Meanwhile, Harris has given us an elegant hypothesis, rich in implications, written in a clear and entertaining manner. As a theory, it explains an immense amount about why you're you and I'm me.