No Two Alike: Human Nature and Human Individuality - Hardcover

Harris, a former textbook author turned popular writer, dives right in, sharpening her focus by looking at identical twins. After subtracting the share contributed by their mutual genes—about 45 percent—studies show that adult identical twins are no more alike in personality than people plucked at random from a crowd, even though the siblings were raised in the same home, by the same parents, with identical schooling.

Where, then, do personality differences come from? Harris begins, in a savage fashion familiar to readers of her Nurture Assumption, by recounting factors that do not contribute to personality differences. She debunks dozens of studies by psychologists—especially the "developmentalists" and "interventionists" who believe that better parenting or school environments can affect how children turn out—by pointing out where they have fudged numbers and twisted results. She rejects the basis of psychoanalysis, stating there is no evidence that talking about childhood experiences has therapeutic value. She also maintains that learned behaviors do not readily transfer from one situation to another, noting that even babies behave differently to fit different environments.

To answer her opening questions, Harris then develops a complex scheme based on "the modular mind," a framework set forth by Harvard University evolutionary psychologist Steven Pinker and others. (Harris herself has no doctorate and is housebound by systemic sclerosis and lupus, two autoimmune disorders.) She describes three modules—the relationship system, the socialization system and the status system—and explains how each contributes its part to making us who we are. The relationship system starts in the cradle as infants study and learn the faces and voices of the people around them, collecting information that helps form personality. The socialization system adapts people to their culture. The status system takes all the information collected during childhood and adolescence and shapes and modifies our personalities in accord with our environments.

Harris’s last chapter lays out her theory in tabular form, explaining how each module interacts with the others to produce our distinct personalities. It is lavishly footnoted, like the rest of the book, shoring up her strategy of pointing out the failings of other models and then proposing her own. Her goal, she writes, is to explain the variations in personality that cannot be attributed to variations in people’s genes. After saying she believes she has succeeded, she throws down her gauntlet: "I will leave it to other people to test my theory."

Jonathan Beard