Abstract
Judith Rich Harris is an independent scholar and evolutionary psychologist who is well-recognized for her thought-provoking earlier book, The Nurture Assumption, in which she reexamined the notion of parenting effects on child development. In No Two Alike, Harris explores the fascinating landscape of personality and individual differences. An avid reader of mystery novels, Harris enthusiastically investigates the mystery of personality differences, with a focus on humans, but occasionally considering individual differences in other species. With the tenacity of a relentless detective, she systematically and with intellectual passion raises important questions, examines scientific evidence, and finally delivers a compelling theory to explain why even identical twins raised in the same home develop different personalities.
In the first four chapters, Harris investigates and eliminates five tenable suspects for the causes of personality differences. In chapter two, she examines the first two suspect causes of these differences: the environment, or a combination of genes (nature) and environment (nurture). Harris highlights the contributions of behavioral genetics studies and is critical of the traditional developmental approach. She follows the clues provided by behavioral genetics research and argues that genes are partly responsible not only for similarities but also for differences in personality. Heritability accounts for all the similarities in personality between identical twins, but is responsible only for some of the similarities and some of the differences between fraternal twins. Moreover, heritability does not account for any similarities in personality between adoptive siblings, whereas it accounts for half of their differences. She concludes that in any of these sibling pairs the percentage of differences not accounted for by genetic differences is about 55%. According to Harris, the source that is responsible for the differences in personality between identical twins is probably responsible for the differences between ordinary siblings as well as between any two individuals. She argues that developmental noise is responsible only for a very small percentage of the remaining variance. Harris's goal in No Two Alike is to investigate the sources of this unexplained variance in personality.
In chapter three, Harris eliminates another suspect. Because identical twins have the same genotype, gene-environment interactions cannot be responsible for the differences in their personalities. In chapter four, she investigates the research on birth order, and concludes that birth order only affects behavior within the family. In the same chapter, Harris eliminates yet another suspect: gene-environment correlations, or “indirect genetic effects.” According to Harris, indirect genetic effects make twins more similar by evoking similar reactions from their environment, whereas she is interested in what makes them different: “the mystery of individuality.”
Harris builds up her model by critiquing core assumptions made by traditional developmentalists. Chapter five takes a closer look at these assumptions and biases—including a bias to generalize behavior across different contexts and a bias to attribute behavioral consistency to learning. Harris argues that children might generalize learning across different situations, but only when the situations are effectively equivalent. For example, an infant whose mother is suffering from depression will behave in a somber manner in the presence of his mother under any situation, such as home or the laboratory; however, the same infant behaves in a normal or lively manner in the presence of other familiar but non-depressed caregivers. The infant builds a mental concept of the relationships with his mother that motivates him to behave consistently in her presence even in different situations. There are other complex systems involved in determining personality differences across different situations and contexts. In the remaining chapters, Harris gives a thorough explanation of these systems.
Before Harris presents her own theory of three evolved psychological systems responsible for the “mystery of individuality,” she reviews the tenets of evolutionary psychology and modularity of mind in chapter six. Her purpose here is to introduce the specialized systems that underlie social development. In the remaining chapters, Harris presents and explains these distinct systems as the products of evolution that account for the previously unexplained variance in personality: the relationship system, the socialization system, and the status system.
Harris is a proponent of Steven Pinker's work. She borrows the term “mental lexicon” from Pinker to build her model of the relationship system. In chapter seven, Harris argues that humans have a mental lexicon that functions as a “people-information acquisition device.” This system collects and stores information on individuals and helps humans to distinguish between specific individuals and to behave accordingly. This information-collection system alone cannot be responsible for socialization, because it only adjusts behavior in short-term encounters with different individuals.
Children are socialized by adjusting their behavior to fit the social prototypes to which they are exposed. Harris argues against the conventional theory that children are socialized exclusively by their parents. In a simple yet logical argument, she uses an evolutionary perspective to explain why children must be socialized, not only by parents but also by the culture and the environment outside of home. For example, the child's aggressive behavior might be tolerated by parents, but the child soon learns that aggressive behavior is not accepted by people besides parents, such as the child's peers. People outside the network of kinship are less likely to accept non-conforming behavior and, therefore, children must learn to adjust their behavior accordingly. Self-categorization provides the motivation for this behavioral adjustment. Children are motivated to behave in a way that fits a context-dependent prototype. According to Harris, this is why socialization makes children in the same subculture more alike and not more different. This is the realm of the socialization system.
In chapter nine, Harris offers her solution to the mystery of personality differences by introducing the third system: the status system. Whereas the relationship system and the socialization system collect and store information about others, the status system collects and stores information about the self. And whereas the socialization system encourages conformity, the status system encourages competition with rivals for status. Harris proposes that the status system has long-term effects on personality and that childhood and adolescent experiences with this system can change personality over the long run.
Finally, Harris returns to the central question of her book: Can the status system explain the personality differences between identical twins raised in the same home? According to Harris, it can—through complex social feedback. She argues that developmental noise produces small but crucial differences between identical twins, which in turn can produce social feedback that has lasting consequences on personality development. Humans are inclined to see people as distinct individuals, and twins are no exception, so once they are treated as distinct individuals, differences in their personalities emerge. According to Harris, “Human differences in personality, to the extent that they are not biological, are the outcome of a collaboration between the relationship system in your head and the status system in mine” (p. 240).
No Two Alike elucidates the mystery of individuality based on these three systems. Harris argues that humans are armed with dispositions that make us ready to join social life by adjusting our behavior in different social contexts. The relationship system arms us with tools to collect information about individuals and to build prototypes of particular classes of individuals. The socialization system motivates us to be more alike, and the status system encourages us to compete and to become less alike.
In conclusion, Harris emphasizes the importance of behavioral genetics studies in providing a critical avenue for understanding differences in human behavior and personality. She argues that research on personality and individual differences must control not only for the effects of genes, but also for the effects of context. Harris comments that a goal in writing her book is raising a “healthy skepticism in regard to research.” No Two Alike has achieved this goal through Harris's transparent writing style, meticulous examination of conventional theories of personality, and step-by-step investigation of the evidence. Harris proposes a solution to the mystery of individuality and leaves it to future researchers to collect the evidence. We recommend this book to any personality, developmental, or evolutionary psychologist who is intrigued by the mystery of individuality and we suggest careful empirical investigation of the three systems that she proposes.