No Two Alike

by Judith Rich Harris

  To the extent that people’s social experiences are correlated with their genetic similarities or differences, the results of such experiences don’t contribute to the unexplained variance: they contribute to heritability. To the extent the unexplained variance in personality results from people’s social experiences, the experiences in question must be uncorrelated with their genes. I am looking, therefore, for experiences that are not the result of inherited characteristics. Variations in genes make people differ from one another but something else makes them differ still more, so that even two people who have identical genes differ noticeably in personality. Can the status system—a system that shapes personality on the basis of self-knowledge obtained through a complex kind of social feedback—supply the something else?

  Yes, I believe it can, for three reasons. The first has to do with the way another mental system works: the relationship system. These two systems collaborate in producing differences between identical twins. The relationship system, you’ll recall, enables us us to identify particular individuals and to distinguish one from another. It motivates us to find and remember differences even between two people who look very much alike (including pairs who aren’t twins, like the golfers Nick Price and Nick Faldo, whom I used to mix up when I first started watching golf tournaments). If you have a pair of identical twins among your close friends or family, I’ll bet you can tell them apart, though you may not be able to say exactly how you do it. Doesn’t matter how you do it: what matters is that once you’ve learned to distinguish them, your relationship system can set up separate pages for each of them in your mental lexicon of people-information, and you can begin storing data for each twin separately. Donald (not George) owes you a favor. Shawn (not Jonathan) bit you in the ear.

  The consequence is that when Donald tries to read what you’ve recorded about him on his page in your lexicon, he will see something different from what George sees. Shawn will see something different from what Jonathan sees. They don’t get the same social feedback because your relationship system has discriminated between them. The relationship system is far more interested in the differences between people—the things that make each one unique—than in their similarities.

  The second reason has to do with the nature of status. Within a group of boys, only one can be the toughest. Within a group of girls, only one can be the prettiest. If a group contains a pair of identical twins—even if they’re both very tough or very pretty—one will inevitably be second best and the other will rank higher in the attention structure. The twin who receives more gazes will tend to speak out more in the group and will consequently receive yet more gazes, so that what might have started out as a tiny difference will widen. Questions or comments from the other members of the group will tend to be addressed to the more outspoken twin, and he or she may become the spokesperson for the pair.

  I opened this book with a description of Ladan and Laleh, the conjoined twins who died during surgery to separate them. Ladan was the one who explained to journalists, “We are two completely separate individuals who are stuck to each other.” She was the more outspoken of the two—the one who was described by a close acquaintance as “very friendly.”49

  The third reason has to do with the nature of social feedback. The status system’s purpose is to enable children to find out about themselves, so that they can adjust their social behavior accordingly. The input collected by this system comes in the form of social cues, information provided by other people. No other kind of information could do the job required of it. But social cues can be ambiguous, and other people have their own agendas. The result is that this system could have effects that are completely unrelated to genes. Does everyone defer to you because you’re big and strong or because you have a big strong brother who is quick to come to your defense? Is everyone nice to you because you’re beautiful or because your father is the village headman? If the social cues are the same, the status system will treat these alternatives as equivalent.

  In these cases, the alternatives are in some sense equivalent. But social cues that you repeatedly receive from others in your group may be based on things that shouldn’t have any evolutionary significance at all, because they’re random. You tripped and fell at a dinner party and landed in the salmon mousse. You shot an arrow into the air and it chanced to hit a bird. You made an offhand remark that was interpreted as a prediction and the prediction happened to come true. Incidents like these can give an individual a reputation that can persist for years, spread and perpetuated through gossip.

  The incidents may be random but their consequences are not. The status system is designed not to be hijacked by random events and to look for consistencies and trends, but a random event can cause everyone in your group to think you’re wise or clumsy or funny or brave, and to go on thinking that for years. I acknowledged a long time ago (in chapter 2) the undeniable fact that a random event can change the course of a person’s life. I gave the example of how writing a classified ad for a dog set me on a path that led to my becoming a writer of books. Some random events are isolated occurrences—they happen once and then they are over—but others have repercussions that echo through the years. It is this second kind of random event—the kind that has long-term effects on a person’s experiences—that can produce long-term changes in personality. What shapes personality is not the event itself but the persisting experiences that follow from the event. The outcome, which may also appear to be random, is actually the result of an orderly process. Helping a friend write a classified ad for her dog was an unimportant event in my life, but the repercussions turned out to be important: my friend recorded “has a way with words” on the Judy Harris page of her people-information lexicon. A few months later, when she again needed help in writing something, she thought of me.50

  I conclude that the status system can indeed be the perpetrator we’ve been looking for. It is capable of producing personality differences that are unrelated to differences in genes. It is capable of producing personality differences between identical twins.

  The unpredictable zigs and zags in development called developmental noise produce little physical differences between identical twins, including physical differences in their brains. Some of the behavioral differences between twins are the result of this developmental noise. The close associates of identical twins notice these differences and record them in their people-information lexicons. One of the reasons identical twins don’t receive identical social feedback is that they aren’t exactly alike, due to developmental noise.

  Among the people who notice and respond to the differences between identical twins are the twins’ parents. This has led some researchers to claim that they’ve found new evidence for parental influence on children’s personalities. The researchers found differences in parental treatment toward identical twins that were correlated with differences in the twins’ behavior, and they interpreted these correlations as evidence of a causal relationship, the parents’ behavior being the cause and the children’s the effect. The idea is that they’ve controlled for genetic effects on behavior by using identical twins. Therefore, the researchers reason, any differences between the twins must be due to environmental differences, and if they can demonstrate an environmental difference that involves parental treatment, that must be the environmental difference that matters. At the very least, it must be one of the environmental differences that matter.51

  Sorry, but it still won’t work. The researchers have controlled for genetic effects but they haven’t controlled for child-to-parent effects: the fact that parental treatment is in part a response to the child’s behavior. If twins behave differently, the parents might be responding to these differences, rather than causing them. The behavioral differences may actually be due to developmental noise or to environmental factors the researchers haven’t considered.

  There is simply too much other evidence that parental influence is ineffective in shaping children’s personalities. If the little differen
ces in the way parents treat identical twins are responsible for the behavioral differences between them, how come the much bigger differences in parental treatment experienced by ordinary siblings who differ in birth order don’t have measurable effects on their personalities? If, as one group of researchers claimed, greater maternal warmth expressed toward one twin can produce differences between the twins in their ability to pay attention in school,52 how come intervention programs that increase maternal warmth have no effects on children’s behavior in school?

  There is no question that greater maternal warmth can improve the mother-child relationship. The relationship system adjusts behavior toward a particular person in order to take account of what it has learned about that person. But the relationship system doesn’t make long-term modifications of behavior. A parent’s effects on a child’s behavior are context specific: specific to the relationship itself or to the setting in which the parent and the child have interacted. The context specificity of learned behavior is not apparent to the naked eye; it’s hard to see because children’s genes influence their behavior in every context. Their behavior in every context may also be influenced by biological factors that are not genetic—biological factors that are due to developmental noise, or to illnesses or injuries, and that may differ even for identical twins.

  Parents do have some power to produce long-term effects on their children’s behavior, but their power is indirect: it comes by way of the socialization system. It resides in their ability to determine where and by whom their children will be socialized. The parents decide which society to live in, which culture their children will grow up in, where they will go to school. To some extent, they can even determine which group or groups within the society their children will become members of.

  Parents do not, however, have the power to influence the personality development of their children via the status system. This system either discounts information obtained from close relatives or averages it together with so much other information that it doesn’t have much impact. The way you are regarded by the “generalized other” is unlikely to be the same as the way you are regarded by your mother. Mother doesn’t always know best.

  I have sketched out the plans for a system that is capable of producing what looks like randomness on the basis of systematic information collected over a long period of time. Now I am going to suggest that the apparent randomness it produces may not be an accident: it may be one of the things this system was designed to do, or it may be a side effect that proved to have unforeseen benefits. Evolution may have given us a mental organ that widens the behavioral and personality differences among us because variation is itself advantageous.53

  In a previous chapter I mentioned self-organized systems or groups. Here is a good definition of self-organization from a recent book on the topic:

  Self-organization is a process in which pattern at the global level of a system emerges solely from numerous interactions among the lower-level components of the system. Moreover, the rules specifying interactions among the system’s components are executed using only local information, without reference to the global pattern. In short, the pattern is an emergent property of the system, rather than a property imposed on the system by an external ordering influence.54

  The pattern is determined by the rules that specify how the components of the system interact. If the system is biological, the rules are tuned by natural selection. Through the trial and error of natural selection, rules evolve that produce global patterns that are beneficial to the system, or to its owner (if the system is an organ), or to its components (if the system is a group of animals).

  Many self-organized systems use rules involving positive feedback. Fish that swim in schools and birds that fly in flocks are attracted to others of their species; these animals follow a rule of the form “I go where you go” or “I do what you do.”

  But there are also systems that depend primarily on negative feedback rules: “I don’t do what you do” or “I do what you don’t do.” An example is the olfactory system, which enables mammals to detect thousands of different odors. The olfactory system is constructed on a “one neuron, one olfactory receptor” principle. Each neuron has a single olfactory receptor, which determines its response profile. The negative feedback rule responsible for this arrangement has the following effect: in a given neuron, the production of a protein by a particular olfactory gene (or part of a gene) ensures that no other olfactory genes will produce their proteins in that neuron.55 It’s like a worker telling another worker, “I can handle this job. You go somewhere else and do something else.”

  I used the word worker on purpose, because the most impressive example of a self-organized biological system is a colony of ants. Look at the ant, the Bible advises us, but phrases the advice in a needlessly insulting way: “Go to the ant, thou sluggard; consider her ways.”56 If you have gotten this far, thou reader, you are no sluggard and have no reason to feel inferior to the ant. On the other hand, the ant does what she does with a lot less equipment than you have at your disposal. Her brain is no bigger than a grain of salt.

  And yet ants vie with humans in their domination of the earth. Though they are very small, they are so numerous and so widespread that the biomass of ants is approximately equal to the biomass of humans. The secret of the ants’ success is their ability to form complex social groups called colonies. What the Bible really should have advised us to do is to look at the ant colony, not at the individual ant. As the myrmecologists Bert Hölldobler and E. O. Wilson observed, “One ant alone is a disappointment; it is really no ant at all.”57

  In the movie Antz, Woody Allen played a worker ant who wasn’t happy with his lot.

  It’s this whole gung-ho, superorganism thing that—that, you know, I can’t get. I try, but I don’t get it. I mean, what is it? I’m supposed to do everything for the colony? And—and what about my needs? What about me?

  There, in a nutshell, is the evolutionary dilemma of the group-adapted animal: Should I try to maximize my success at the expense of my groupmates, or should I put my efforts into supporting and defending the group? My goal is to put as many of my genes as possible into the next generation, but if the group ceases to exist, my offspring will have no chance of surviving.

  It’s a real dilemma for a human but not for an ant, due to the peculiarities of reproduction in the order Hymenoptera (the ants, bees, and wasps). In these insects, sex is determined in a way that seems bizarre to us mammals: fertilized eggs develop into females, unfertilized eggs into males. Males consequently have only one set of chromosomes; females have two. The result, in an ant colony founded by a single queen who mated with a single male, is that the queen’s daughters are more closely related to each other than are full siblings in a species such as ours. Two female workers in an ant nest (all the workers are females) share three-quarters of their genes. In terms of genetic similarity, they are midway between human siblings and identical twins.58

  This means that a female ant shares more genes with her sisters than she would with her own offspring, if she had any. In order to maximize the number of her genes in future generations, therefore, what an ant should do is help her mother produce more sisters. And that is what she does. The social system of the ant colony—an egg-laying queen supported by workers that do not reproduce—has been so successful that it has evolved independently a dozen times in the order Hymenoptera.59

  The ant’s tiny brain gives her instructions that cause her to spend her life working to maintain, defend, and if possible expand the colony. An individual ant—the one Hölldobler and Wilson described as a disappointment—will unhesitatingly give up her life to benefit her sisters. Workers whose job it is to go out of the nest and forage for food have a death rate of 6 percent per hour.60

  What has made the social insects so successful is the division of labor. All ants practice division of reproductive labor: only the queen reproduces. Most ants also divide up other kinds of labor. There are lots of
jobs to do in an ant nest—digging tunnels, clearing away debris, caring for eggs and larvae, defending the nest against intruders, foraging for food. In some species, ants take on inside jobs (such as caring for larvae) when they are young and then graduate to outside jobs as they get older. In other species, an individual ant may spend her entire life working at a single job. In either case, the choice of the job is determined not by the ant’s genes but by environmental factors, because the ants in a nest are basically all alike genetically. Here’s the puzzle of the ants’ nest: they have almost identical genes and yet they do different jobs!

  As the Bible pointed out, ants do their jobs with “no guide, overseer, or ruler” their queen is neither a despot nor an organizer but merely an egg-laying machine. So how do the different jobs get distributed? How does the colony manage to function in such a way that all the important jobs get done? The answer: through positive and negative feedback. Ants respond to signals from other ants and from the environment. If ants foraging outside the nest come across a rich source of food—what we beleaguered humans call a “picnic”—they recruit other ants to come and help them carry the booty home. That’s positive feedback. If the nest has been damaged by a heavy rain, the ants give up foraging for a while and concentrate on nest repair. That job works on negative feedback, like a thermostat: once the nest has been repaired, the stimulus for repairing it disappears and the ants can do something else.61

  What got me interested in the parallels between ant societies and human societies was a recent article in the journal Science titled “Social Insect Networks.” Its author, the biologist Jennifer Fewell, uses the word “networks” to describe the simple interactions among the members of an insect colony that have global effects on the colony. Self-organization in insect colonies, Fewell explains, can produce emergent properties that make the whole greater than the sum of its parts—properties such as mass actions, social hierarchies, and the division of labor. Division of labor—Fewell calls it “divergence”—is produced by negative feedback in which the performance of a task by one individual reduces the chances that another individual will do it, because it no longer needs to be done. But to produce stable division of labor, positive feedback is needed as well: