No Two Alike

by Judith Rich Harris

  Most divergence models also include a positive feedback loop, in which performance of the behavior increases the probability that the individual will perform the behavior again. This self-reinforcement generates divergence even with initially small random differences in behavior and produces a faster and more stable system of divergence…. Division of labor also appears frequently within [other] social species, including humans. As an example, we can imagine an apartment where housemates share tasks. Used dishes pile up in the sink, producing a continuously increasing stimulus. The dishes go unnoticed until the threshold of the one most sensitive to them is met, and he or she washes them. This removes the dishes as a stimulus, further reducing the likelihood that the other group members will ever wash them. The result is a dishwashing specialist (much to his/her dismay), and a set of nondishwashers.62

  If that describes you, aren’t you glad to know that it’s not your housemates’ fault that you ended up with the job of dishwasher? Nor, for that matter, is it your fault. It’s an emergent property of the system. Blame the system.

  Geneticists used to think that much of the variation within species, as well as the differences between species, was due to differences in genes. Now it appears that much of the variation, both within and between species, is due instead to differences in gene expression. When a gene is expressed, or turned on, it means that it is used as a template to produce a particular protein. Genes are turned on and off during the course of the lifespan; they are expressed in some cells and not in others. A given gene, present in both of two individuals, may be expressed in one and not in the other.63

  The expression of genes can be influenced by environmental factors—in other words, by experience. Researchers studying honeybees have found differences in gene expression between “nurse” bees, which take care of the eggs and larvae, and bees that go out of the nest to forage for nectar and pollen. Worker bees usually start out as nurses and then, when they get older, switch to foraging. This change in jobs, researchers have found, is accompanied by changes in the expression of certain genes in the bee’s tiny brain.

  Are the changes in gene expression the cause or the result of the job change? The answer, it turns out, is both. If all the older bees in a hive are removed, the young bees that remain will divide up the duties: some will become foragers and the rest will remain nurses. The same differences in gene expression between the brains of nurses and foragers were found even when they were all the same age. Switching to foraging at a younger-than-usual age caused the foraging genes in their brains to turn on.64

  What this means is that the dishwashing specialist and the nondishwasher may be expressing different genes in their brains, even if they were born with exactly the same genes.

  Division of labor is an emergent property of human groups; no doubt it always has been. As the British evolutionary biologist Matt Ridley has suggested, it was one of the things that made our species a blockbuster success.

  In human society, the advantages of society are those provided by the division of labour. Because each person is a specialist of some sort—usually from an early enough age to have become good at their chosen trade while still mentally malleable—the sums of all our efforts are greater than they would be if each of us had to be a jack of all trades.65

  Does that remind you of something? That’s right, the mind. Or, for that matter, the body. Or, for that matter, the car. They’re all modular—made up of an assortment of devices that were each designed to do a specific job. Ridley likens the division of labor in human societies to the organs of the body. “The division of labour is what makes a body worth inventing,” he says. “Each organ, each muscle, each tooth, each nerve and each bone plays its separate part in the whole enterprise.”66

  Ridley doesn’t claim credit for the idea that a society is greater than the sum of its parts because each part can specialize in something: he credits the Scottish philosopher and economist Adam Smith, who said much the same thing back in 1776. But Adam Smith, writing a century before Darwin, didn’t know about evolution through natural selection. Or did he? He certainly seems to have had some of the relevant ideas. He attributed division of labor not to the individual’s desire to benefit his society but to his desire to benefit himself:

  It is not from the benevolence of the butcher, the brewer, or the baker, that we expect our dinner, but from their regard to their own interest. We address ourselves, not to their humanity but to their self-love, and never talk to them of our own necessities but of their advantages.67

  Smith also suggested how the division of labor might come about through self-interest: in “a tribe of hunters,” one who proves adept at making arrows can do better by trading his arrows for meat than by going out to hunt for it himself.68

  Humans are motivated by self-interest because, unlike ants, they live in groups that do not consist entirely of close relatives. That’s one of the things that made our species such a blockbuster success—our ability to form big, inclusive groups that aren’t necessarily based on kinship. But the consequence is the dilemma I mentioned when I was talking about ants: whether to try to maximize one’s success at the expense of one’s groupmates or to put one’s efforts into supporting and defending the group. It is a true dilemma for humans (though it isn’t for ants), and different humans solve it in different ways. Some are willing to fight and die to defend their group. Then there’s Woody Allen, who doesn’t even want to work for his group, much less die for it. “It’s not that I’m afraid to die,” he explained. “I just don’t want to be there when it happens.”69

  When people work or die for their group, the motivation to do so has been provided by the socialization system—an ancient system that dates from the time when all the members of the group were close relatives. The motives provided by the status system, in contrast, are entirely those of self-interest. It is this self-interest that produces divergence in human groups. The status system doesn’t give a damn about the group’s welfare; it asks, “How can I best compete?” To find the best answer requires self-knowledge, so the status system searches for self-knowledge in the social cues provided by others. It then uses this information to plot a long-term strategy that will involve direct competition only in those arenas of endeavor in which the individual has a hope of succeeding and, if possible, avoid competition in other arenas. The result is that individuals seek unoccupied niches; they specialize in different things. I cannot better Adam Smith’s description of the long-term outcome of this process:

  The difference of natural talents in different men is, in reality, much less than we are aware of; and the very different genius which appears to distinguish men of different professions, when grown up to maturity, is not upon many occasions so much the cause, as the effect of the division of labour. The difference between the most dissimilar characters, between a philosopher and a common street porter, for example, seems to arise not so much from nature, as from habit, custom, and education. When they came into the world, and for the first six or eight years of their existence, they were perhaps, very much alike, and neither their parents nor playfellows could perceive any remarkable difference. About that age, or soon after, they come to be employed in very different occupations. The difference of talents comes then to be taken notice of, and widens by degrees.70

  Accidentally or on purpose (so to speak), evolution provided a primate with a mental organ that has the effect of increasing behavioral diversity. The result was a species, Homo sapiens, that is well suited, not just to living in groups, but to living in large groups. The most obvious advantage this gives us is that large groups are capable of wiping out smaller ones. That’s another thing we share with ants.

  In some circumstances and for some behaviors, it is advantageous to minimize behavioral differences within a group. That’s what socialization does. For example, communication within the group works best if everyone speaks the same language and has the same accent. The better communication benefits both the individual and the group.r />
  In other circumstances or for other behaviors, it is advantageous for individuals to maximize the behavioral differences between them. Differences in personality, when they aren’t genetic, are at first the result of behaving differently; later they become the cause of behaving differently—of following divergent paths in adult life. The reason that the differences persist may be that different genes have been turned on, or cranked up, in the brain.

  As a consequence of the way the status system operates, even identical twins will develop different personalities. It doesn’t matter whether they grow up in the same home or in different homes, whether they belong to the same group or different groups. In either case their close associates are going to see them as distinct individuals. Once their associates see them as distinct individuals, it doesn’t even matter that they are twins. A person is seen as an individual whether he or she is one of a pair of twins, a set of siblings, or a group of friends. We see people as unique individuals because the relationship system goes to pains to distinguish them. 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.

  The collaboration between the relationship system and the status system doesn’t just make twins different: it makes everyone different. And it doesn’t make twins differ only from each other: it makes each twin differ from everyone else—everyone in their group, everyone of their age and sex. The others with whom they will have to compete in adulthood.

  10

  Denouement

  AT THE END of a classic British mystery novel there is usually a scene in which the detective assembles all the characters in the library of a stately mansion and ties up the loose ends, explaining how the perpetrator did it and why the clues make perfect sense when put together the right way. In the American mystery novels featuring the private investigator Nero Wolfe, this scene takes place in Wolfe’s spacious office on the ground floor of a three-story brownstone in Manhattan. Nero Wolfe seldom leaves his house, so he arranges for all the suspects, plus a police officer or two, to come to his office. Then he asks some questions, makes some deductions—that is, abductions—and pretty soon everything is all wrapped up. The perpetrator picks from the standard menu of emotions—denial, anger, grief, resignation—and may try to make a break for it, but is quickly collared by the cops and marched off in handcuffs.1

  I’m writing this in my office, considerably smaller and more cluttered than Nero Wolfe’s, on the ground floor of a two-story house in suburban New Jersey, and wondering whether the cops brought enough handcuffs, because there are three perpetrators in this case. Three mental organs or systems. All three affect social behavior; all three collect information that comes from other people. Though only one, the status system, can actually be held responsible for producing personality differences between identical twins, it couldn’t have done the deed without the help of the relationship system. Together, the three systems left such a welter of confusing clues that it was impossible to solve the mystery without figuring out the modus operandi of all of them.

  The three systems work in different ways (see the table on the next two pages for a summary). They collect different kinds of information from the environment and process the data differently. They provide different motivations and different emotions. Disagreements sometimes arise. Occasions in which two mental mechanisms issue contradictory commands—“the human heart in conflict with itself,” as the novelist William Faulkner put it—are a familiar part of human life.2

  Relationship System

  Socialization System

  Status System

  Goal

  To establish and maintain favorable relationships.

  To be a member of a group.

  To be better than one’s rivals.

  Motivations provided

  To acquire knowledge about other people. To share that knowledge with others.

  To affiliate with a group. To be accepted by its members. To conform to the group’s norms. To defend the group.

  To compete. To improve one’s status. To acquire self-knowledge by comparing oneself with others.

  Emotions

  Love, hatred, dependency, trust, aggressiveness, lust, jealousy, etc.

  Hostility toward other groups, group pride or patriotism, unhappiness at being excluded.

  Ambition, envy, triumph, conceit. Embarrassment, anger, or unhappiness at losing status.

  Typical behaviors

  Infant attachment behaviors, making friends, dominance contests, courtship, trading favors, gossiping.

  Adopting the behaviors, language, accent, dress, and attitudes of one’s groupmates. Fighting for one’s group.

  Matching oneself against one’s peers. Competing in contests one might win and avoiding contests one is likely to lose.

  Typical errors

  The fundamental attribution error. Believing and passing on unreliable gossip.

  Underestimating within-group variation. Automatically regarding one’s group as good and other groups as bad.

  Overestimating oneself and underestimating one’s rivals.

  Important components

  A mental lexicon of people-information. A face-recognition module. A mindreading mechanism. A relationship sociometer.

  A categorization module. A calculator of central tendencies. A group-acceptance sociometer.

  A mindreading mechanism. An eye-gaze detector. A sociometer that gives detailed, multidimensional information about status.

  Data collected

  Information about specific people, based on personal experience or hearsay.

  Information about the members of social categories.

  Information about how one compares with others in one’s social category.

  How data are processed

  Information about each individual is stored separately.

  Information about the members of social categories is averaged to form prototypes.

  Information obtained from different individuals is combined. Eye-gazes are counted. Hierarchies are assessed.

  Level of consciousness

  Fully available to consciousness.

  Most of the work done by this system is not available to the conscious mind.

  Though some work (e.g., eye-gaze counting) may be done unconsciously, most is available to consciousness.

  Developmental timetable

  Ready to go at birth and remains active throughout the lifespan.

  Working by age three. Socialization appears to be largely complete by adolescence. Affiliation with new groups is possible throughout the lifespan, but fervor may decline in the later years.

  Competitiveness is evident in three-year-olds, but other components of this system develop slowly. Changes in strategy are common during adolescence and still possible in adulthood.

  My goal was to explain the variation in personality—the big and little differences among individuals—that cannot be attributed to variations in their genes. The behavioral geneticists had shown that this variation was not due to differences among the homes in which the individuals grew up, because the amount of variation between two people was about the same, whether they grew up in the same home or in different homes. I looked at one plausible explanation after another, including gene-environment interactions and environmental differences within the family, but they all proved to be red herrings. So I had to come up with a theory that hadn’t been considered previously.

  Or hadn’t been considered lately. Here is a statement made in 1799 by the American political philosopher Thomas Jefferson:

  I consider man as formed for society, and endowed by nature with those dispositions which fit him for society.3

  Jefferson got it right. Humans are formed for society—they are a group-adapted species with a highly active social life—and they are endowed by nature with abilities and motivations that fit them for that life. Isn’t it interesting that Jefferson said �
�endowed by nature” sixty years before Darwin explained how nature did it?

  I came across that quote only yesterday, so I can’t claim it was Thomas Jefferson who inspired me to think up a theory in which humans are endowed by nature with a number of dispositions that fit them for society. My inspiration came from more recent sources, especially Steven Pinker’s work on language and Simon Baron-Cohen’s work on the mindreading mechanism. These theorists first showed that using language and reading other people’s minds are special abilities that have mental systems of their own. Then they took the systems apart and figured out how they work, using evidence of many different kinds.4

  The disposition to acquire a language is one of the things that fit humans for society. So is the disposition to figure out what other people are thinking. Three other things that fit humans for society are their disposition to form and maintain personal relationships, their disposition to adjust their behavior to the norms and customs of their society, and their disposition to compete with the other members of their society and, if possible, outdo them in some way.

  Here’s how I think the perpetrators do their work. Here’s how babies develop, how they become socialized, and how each one acquires a unique personality.