Miércoles 30 de Mayo de 2007, Ip nº 193

Why home doesn't matter
Por Judith Rich Harris

The BBC series "Child of Our Time" assumes that studying children with their parents will help us understand how their personalities develop. But this is a mistake: parents influence their children mainly by passing on their genes. The biggest environmental influences on personality are those that occur outside the home.

During much of the 20th century, it was considered impolite and unscientific to say that genes play any role in determining people's personalities, talents or intelligence. But we're in the 21st century now, the era of the genome. So when Robert Winston informs us, at the opening of each episode of the BBC1 documentary series Child of Our Time, that we're going to "find out what makes us who we are," we know he's going to say that people are the way they are partly for genetic reasons. (In case you've missed it, Child of Our Time is a project tracking the lives of 25 children for their first 20 years, returning to them each year to assess their progress).

Child of Our Time is itself a sign of scientific progress because of its enlightened approach to the genome. Nevertheless, the series is scientifically misleading. Simply depicting the lives of 25 children, or sprinkling little "experiments" here and there throughout the programmes, sheds no light on the nature vs nurture question. Psychologists studied child development in this way for the better part of a century and learned remarkably little. Observing children at home or in school, individually or in groups, is not the way to answer the question of why they turn out the way they do.

The problem is that most children are reared by their biological parents, so the parents provide both the genes and the home environment. The effects of the genes are therefore impossible to separate from the effects of the environment. Young Johnny has a strong drive to succeed? Well, that's not surprising—so does his father! But does Johnny's drive to succeed come from lessons learned from his father, from genes inherited from his father, or from some combination of the two? Most 20th-century developmental psychologists assumed it was mainly Johnny's experiences at home that made him who he is today. Even those who admitted that genes may play a role continued to feel that the child's environment—by which they meant the home environment—was of greater importance.

It wasn't until the 1970s that behavioural geneticists worked out productive techniques for answering questions about nature vs nurture. One method involved looking at adopted children, whose genes were provided by one set of parents and whose environment was provided by a different set. Another method involved finding identical twins separated at birth: same genes, different environments. A third involved comparisons between identical twins and fraternal twins reared in the same family (identical twins have the same genes; fraternal twins are genetically as different as ordinary siblings). Other research designs made use of the genetic differences between ordinary siblings, half-siblings, and step or adoptive siblings raised in the same family.

None of these methods is perfect, but they each have different flaws. It is therefore noteworthy that they all produced essentially the same results. Two results, actually—one surprising, the other not.

The unsurprising result was that genes matter. Since the 1970s, behavioural geneticists have measured many different human characteristics in many ways. They've looked at personality traits such as extroversion, conscientiousness and aggressiveness. They've looked at mental disorders, intelligence and aspects of people's life histories (such as careers). In virtually every case, the results were the same. About half the variation in the measured characteristic—the differences from one person to another—could be attributed to differences in their genes.

The surprising result had to do with the environment. Since genetic effects account for only about half of the differences among us, the other half has to be the result of environmental effects, right? Well, that was the assumption. But researchers still haven't been able to pin down which aspects of the environment are important. All they've been able to determine is which aspects of the environment are not important. The aspects of the environment that don't seem to matter are all those that are shared by all the children who grow up in a given family—which includes most of the things the word "home" makes you think of. Whether the home is headed by one parent or two, whether the parents are happily married or constantly rowing, whether they believe in pushing their children to succeed or leaving them to find their own way in life, whether the home is filled with books or sports equipment, whether it is orderly or messy, a city flat or a farmhouse—the research shows, counterintuitively, that none of these things makes much difference. The child who grows up in the orderly, well-run home is, on average, no more conscientious as an adult than the one who grows up in the messy one. Or rather, he or she will be more conscientious only to the extent that this characteristic is inherited.

These surprising results were what led me to propose, in The Nurture Assumption (1998) and again in my new book No Two Alike, that the important aspects of the child's environment are outside the home. This made no sense to most parents, and it will make no sense to most viewers of Child of Our Time, because anyone can see that children's experiences at home do have visible effects on them. The child whose mother is a poor and inconsistent disciplinarian is disobedient. The child whose father is about to undergo major surgery is anxious and clingy.

But wait. One of the subjects of Child of Our Time is a boy named James, whose mother is a poor disciplinarian. As we would expect, he is a problem child at home. At school, however, Winston informs us, "James is a very different boy." According to his teacher, James's behaviour in school is quite acceptable, "on a par with the rest of the class."

It's an error made by almost all developmental psychologists, so we shouldn't be too hard on Winston when he occasionally makes it too. The error is the assumption that what a child learns in his home environment is automatically carried along with him to other settings. This assumption is built into most theories of personality development. For example, there are researchers who believe that a child's attachment to his mother in infancy sets the pattern for all his later relationships. If his mother gave him all the love and attention he desired, he'll do well in life because he has learnt to trust people.

But babies don't work that way. A baby is wise enough to understand, almost from birth, that people differ. The fact that his mother treats him well doesn't lead him to expect that his sister or the babysitter will also do so. How other people will act towards him is something he will have to find out for himself, person by person. Researchers have discovered that the babies of mothers suffering from postnatal depression tend to act in a sombre, subdued fashion in the presence of their mothers. But around other familiar caregivers, these babies act quite normally—much more lively and cheerful. Just because Mummy is depressed doesn't mean everyone is depressed. Just because Mummy lets me get away with murder doesn't mean I can act that way in school.

The ability to behave differently in different social settings and with different social partners is a built-in survival mechanism. What works in one setting won't necessarily work in another. There is no better example than the child of immigrants who learns to speak one language at home and a different language outside the home. Her parents will always speak English with a foreign accent, but she will drop the accent when she is away from them. Her accent outside the home will be the same as that of the other children in her neighbourhood. Children seem to know instinctively that patterns of behaviour acquired at home must be cautiously tested, and perhaps modified or abandoned, when they start to have a life outside the home. The child quickly learns that crying brings one response from Mummy, but quite a different one from the other children at the daycare centre. The influence of peers doesn't begin in the teenage years: it can be seen as early as age three.

I said earlier that the aspects of the environment that do not play a role in shaping personality are all those that are shared by children who grow up in the same family. But many psychologists continued to believe in the potency of the home. They therefore looked for aspects of the home environment that are not shared by all the children in the family: for example, parental favouritism toward one child, or differences based on birth order. But these within-the-family factors also proved unimportant. Research showed that parents do indeed behave differently toward their children—they may give one child more affection or more criticism than another—but in doing so they are responding to differences among the children rather than causing them. Nor did birth order provide the hoped-for answer. It's true that two children of different ages growing up in the same family will have different experiences within that family. But despite this, firstborns and laterborns are generally indistinguishable in their responses to standard personality tests. (Little differences are occasionally found, but they don't hold up from one study to the next.) Contrary to what you may believe, firstborns do not perform better at school, nor are laterborns more likely to rebel by refusing to do their homework or by dropping out of school.

And yet many people are convinced that firstborns and laterborns have distinctly different personalities. Why does everyday observation accord so poorly with research? The answer is the survival mechanism I mentioned previously: the ability of humans to behave differently in different circumstances. Firstborns and laterborns do behave in characteristic ways when they're in the presence of their parents and siblings, but they drop these behaviours when they're away from their family. Like James in Child of our Time, they adapt their behaviour to their setting. The firstborn who dominates his younger siblings at home doesn't automatically assume that he will also be able to dominate his classmates. After all, even though he's the largest child at home, he may turn out to be the smallest one in the playground. Research confirms that firstborn children are, on average, no more dominant in the playground than are laterborns. Nor do laterborns go through life permanently cowed. A laterborn who is pushed around by his older brother at home is fully capable of stepping into a dominant role with his peers.

But some children are more aggressive, or more impulsive, or more conscientious than others wherever they go. Researchers have found that when children display the same characteristics in a variety of settings, it is usually because of genetic influences. A child who has inherited a predisposition to be aggressive or conscientious will display these traits both at home and at school. It is this carryover of inherited predispositions from one environment to another—along with the fact that these predispositions tend to show up early in life—that gives the false impression of a lasting effect of the home environment.

It is wrong, however, to overestimate the power of inherited predispositions. Identical twins provide a good illustration. Because identical twins have identical genes, and therefore should have exactly the same inherited predispositions, they are of tremendous interest to anyone who wonders about nature and nurture. The trouble is that most of the stories you hear about identical twins emphasise their eerie similarities. These accounts are designed to convince you of the power of genes, but this is the 21st century, and everyone admits that genes are powerful. What you need to hear now about identical twins is that they're not nearly as alike as you might think. Even when they are reared in the same home by the same parents, most identical twins differ noticeably in personality.

This is certainly true of Alex and Ivo, the pair of identical twin boys who are among the 25 children in Child of Our Time. As Winston shows us in the second episode of the new series, "Fitting In and Standing Out," Alex and Ivo are already, at age six, very different people. Alex is a macho little creature who roughhouses with his friends, all boys. When led into a room containing a doll, he throws it in the dustbin. Ivo, by contrast, is willing to play with girls. When given his turn in the room with the doll, he carefully puts a nappy on it. The twins' parents are baffled by these differences. "We don't treat them any differently," the father says. Perhaps the boys differ in their level of the masculinising hormone testosterone? But no; their testosterone levels are duly tested and found to be the same. The difference between them remains a mystery.

Why do identical twins reared in the same family differ in personality? Genes explain the similarities between them, but can't account for the differences. Some are probably the result of random biological processes in development—there are slight physical differences between the brains of identical twins, just as there are in their fingerprints. However, it is unlikely that these minor physical differences can be the whole story. Surely their experiences must play some role?

None of the existing theories of personality development offers adequate explanations for the personality differences between identical twins. Attachment theory, for example, stubs its toe on the question of why one identical twin but not the other would form a secure attachment to their mother. Nor can it explain why identical twins separated at birth and reared by two different mothers are as similar in personality as those reared by the same mother. As for birth order theory, it's a non-starter. Identical twins are well matched in size and strength and generally don't regard each other as rivals.

The explanation I came up with rests on a discovery made by neurobiologists and evolutionary psychologists. The human brain, it seems, is not a unitary organ: it is instead a toolbox of various devices, each designed by evolution to perform a specific function. Each "module," as these devices are called, works according to its own rules. Each responds to a specific type of information provided by the environment; each uses the information in a particular way.

This modularity of the mind can explain some of the mysteries of development. In No Two Alike, I propose that the human mind contains three different modules for collecting and responding to information from the social environment. I call them the socialisation system, the status system and the relationship system. These systems sometimes issue contradictory orders, as vividly illustrated by the Child of Our Time episode mentioned above, "Fitting In and Standing Out." The socialisation system makes us want to fit in—to conform to our peers. The status system makes us want to stand out—to be better than our peers. We can see these motivations in people of all ages.

The socialisation system and the status system are organs of the mind. Like other organs, they are found in all neurologically normal human beings. But, also like other organs, they vary somewhat from one individual to another. For some children, the socialisation system generally takes priority; for others, the status system often gains the upper hand. But these differences become visible only when the two goals—fitting in or standing out—conflict. At other times the two goals can peacefully coexist. The child in school sits quietly in her seat like her classmates: she conforms. At the same time, she may try to stand out by excelling in reading or maths.

The purpose of the socialisation system is to adapt the child to his or her culture. This involves acquiring the local language and accent, the appropriate behaviours and customs, and the prevailing attitudes and beliefs. Acquiring the appropriate behaviours is tricky, because people within a culture do not all behave alike. Around the world, males behave differently from females and children behave differently from adults. A child who imitated his same-sex parent and behaved like an adult (other than in the context of a game) would be seen as impertinent or weird. So the child's first job is to figure out what sort of person he is—child or adult, male or female, serf's son or princeling. Then he has to figure out how the other people in his social category behave and adjust his behaviour accordingly. This is why children acquire the accent of their peers rather than that of their parents. This is why (in the Child of Our Time episode on gender stereotyping) the kids growing up in modern households, with both parents sharing the childcare chores, nonetheless take a traditional view of gender roles: the woman belongs at home, the man at work. Such attitudes come from the children's culture, not from the home, but in most cases you can't see the difference because the home and the culture agree.

The mistake I made in my earlier book, The Nurture Assumption, was to attribute environmental effects on personality to socialisation. But socialisation makes children more similar in behaviour to their same-sex peers. Thus, though the socialisation system can explain some of the behavioural differences between cultures, it can't account for the non-genetic differences in personality between reared-together twins or siblings. As they are growing up, children become more like their peers in some ways, but less like them in other ways. The uniqueness of each child's personality is due to the status system.

The purpose of the status system is to enable children to compete successfully with their peers. In order to do this, they must acquire self-knowledge. Children have to discover how they compare with other children along a variety of dimensions. Am I tall or short, strong or weak, pretty or plain, smart or dull? Without answers to these questions, they would have no way of deciding whether to try to dominate others or yield without a fight, to make suggestions or follow the suggestions of others, to turn down potential mates in hopes of doing better or take whatever comes along. Based on their understanding of their own strengths and weaknesses, of the options offered by their environment, and of the particular set of other children with whom they must compete, children work out their own individual strategy of behaviour. "I'm not good in maths," James (the boy whose mother is a poor disciplinarian) admits. But he's popular with his peers. Every child has to find out what he is good at and place his bets on the things that are most likely to pay off. Even identical twins will find different niches to occupy.

As to Alex and Ivo, I can only speculate. Some pairs of identical twins differ more than others at birth due to the random biological processes I mentioned earlier. These differences can set them on diverging paths in life. The friends they seek out and the activities they pursue have effects that compound themselves over the years. Twins are seen as distinct individuals by the people who know them. They may take on different roles in their peer group, or one may have higher status than the other. The fact that members of their social world see them as different people causes the little differences between them to widen. For example, their friends may address questions more to one than the other, just at random at first, but eventually this first twin may become the "spokesperson" for the pair and develop a more outgoing personality.

Most of the work of the socialisation system goes on underground. We're not conscious of the things we do to become socialised (just try asking someone how she acquired her accent). Some of the work of the status system also goes on below the level of consciousness. In contrast, the workings of the third system, the relationship system, are fully conscious. The relationship system is also the earliest to develop—it's ready to go at birth.

The relationship system motivates us to form new relationships, to maintain existing ones if they're going well, and to find out as much as we can about new people we meet. The urge to learn new words and new facts gradually declines as we get older, but we never lose our curiosity about people. Gossip is a popular sport even in the old people's home. It is the relationship system that fuels our hunger for biographies and novels, and makes us want to look at photos of movie actors and sports stars. The appeal of a series like Child of Our Time, which focuses on specific children whom, little by little, we get to know, comes from the relationship system.

The relationship system is a splitter, not a lumper. Its job is to discriminate among people—to notice and remember what makes each individual different from all the rest, even if that individual happens to have an identical twin. It chooses our friends, selects our romantic partners, and tells us who our relatives are and how to behave with them. Knowing that someone is a close relative increases the chances that we will help them, but reduces the chances that we will have sex with them.

It is the relationship system, with its bulging storehouse of memories of Mum and Dad, that makes us believe that our parents played a central role in making us who we are. This system contributes more than its fair share to our conscious memories. The strong emotions associated with these memories make us feel that they must be important. But the truth is that people don't know why they turned out the way they did. Many of the important things that happen during development go on outside the purview of the conscious mind. Asking people why they are the way they are may produce interesting answers, but we should not place much weight on them.

Asking their parents is even less likely to be profitable, because parents see only one part of their children's lives. Though relationships with parents greatly affect the day-to-day happiness of children, just as marital relationships greatly affect the day-to-day happiness of adults, neither leaves deep marks on the personality. In the long run, it is what happens to them outside the parental home that makes children turn out the way they do. After all, outside the parental home is where they are destined to spend their adult lives.


  30/05/2007. Prospect Magazine.


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