No Two Alike Page 13

  But perhaps Cheever was overstating the case, as was his wont. Some people believe that parents continue to favor their firstborn even after they have other children. Those who hold this belief point to the custom of primogeniture: when the crown or the farm must be handed down to a single offspring, the honor almost invariably goes to the firstborn son. Data on infanticide and homicide also support the view that parents favor firstborns—or, more accurately, favor older children over younger ones. In traditional societies in which infanticide was practiced, a common reason for deciding to abandon a new baby was that the previous child had not yet been weaned. When it came down to a choice between saving the newborn and saving the two-year-old, the two-year-old would almost invariably be chosen. In contemporary societies, homicide rates are highest for infants and decline with the child’s age.14

  Evolutionary psychologists explain that there is a good evolutionary reason for parents to favor older children over younger ones when faced with life-or-death decisions. Under ancestral conditions, when at least half of all newborns never lived long enough to have children of their own, the older ones were a better bet. They had survived the most hazardous stage of life and were closer to the time when they could pay back their parents’ investment by providing them with grandchildren.15

  But infanticide statistics may be a poor indication of how parents behave after they have committed themselves to rearing the newborn. Infants are at risk precisely because they require so much care; nevertheless, most parents are able and willing to provide that care. Robert Trivers, one of the founders of evolutionary psychology, pointed out that parents should actually be more concerned about the well-being of their youngest and neediest offspring, because that is where their investment can make the most difference. That is where a failure to pay sufficient attention can do the most harm.

  The offspring is typically more helpless and vulnerable the younger it is, so parents will have been more strongly selected [by natural selection] to respond positively to signals emitted by the offspring the younger the offspring is.16

  Trivers’ statement is consistent with child-care practices all over the world. In hunter-gatherer and tribal societies, babies and toddlers are cosseted—carried around and nursed on demand—but paradise ends when a younger sibling is born. The older child is sent out to join the local play group and thereafter receives a minimum of attention from his mother. Her attention is now focused on the new baby.17

  Favoritism toward the younger child is less blatant in industrialized societies but still clearly visible. In a recent study of 3,762 families with two or more children from four to eleven years old, researchers found that the youngest children received the most attention and affection, the oldest ones the least. When parents have more than one child to take care of, they pay more attention and give more affection to the younger one.18 A surprisingly large number of parents are even willing to admit that they love the younger one best. In two separate studies, British and American parents of two small children were asked whether they felt more affection for one than the other. More than half admitted that they did. The overwhelming majority of these parents—87 percent of the mothers and 85 percent of the fathers in the American study—said they favored the younger child.19

  These are big differences in parental affection. They don’t occur in every family—there must be many in which the older child happens to be more lovable than the younger one—but on the whole parents love their younger children best, at least while they’re still small and living at home.

  So firstborns experience not only the “turning point” of dethronement: it goes on for years. The one Mom loves best is their little brother or sister. And this is by no means the only difference between the family experiences of firstborns and laterborns. Parents give more responsibility to firstborns. Their expectations for them are higher; they tend to be stricter with them.20 But there are compensations. Firstborns are bigger and stronger than their siblings and they know more. They can push around their little brothers and sisters. The dominance hierarchy—also known as “pecking order”—is seen in many social species, from chickens to humans, and in almost every case the dominant one is the biggest and strongest. An individual low in the hierarchy has to learn to avoid evoking the anger of the higher-ups. The home life of a laterborn isn’t all milk and honey.

  Plomin and Daniels’ question—why are siblings so different?—reached the ears of investigators outside of behavioral genetics and developmental psychology. One ear it reached belongs to a historian of science named Frank Sulloway. Sulloway has wide-set blue eyes (or so they appear in photos) and an old-fashioned, pencil-thin mustache. Since receiving a Ph.D. from Harvard in the history of science, he has followed an unusual career path that enabled him to remain in the academic world without becoming a member of a university faculty. Instead, he has made a career for himself as a visiting scholar, giving an occasional lecture and supporting himself primarily by writing books (he has written two, both quite successful) and receiving research grants.

  In his 1996 book Born to Rebel, Sulloway proposed that sibling differences are due to birth order and presented a well-worked-out theory of how birth order shapes personality. I’m going to examine Sulloway’s theory in detail, partly because it’s the best one around (most of the books you see on birth order are complete balderdash), partly because it makes use of concepts derived from evolutionary biology and evolutionary psychology.

  According to Sulloway, the source of nongenetic sibling differences in personality is competition for family resources. “Disputes over these resources, especially over parental affection, create rivalries.” Firstborns, being bigger and stronger, have an edge in this competition: “They arrive first within the family and employ their superior size and strength to defend their special status.” Thus, he says, “Firstborns tend to be dominant, aggressive, ambitious, jealous, and conservative.” Laterborns develop their own counterstrategies to the firstborn’s attempt to dominate: “Although laterborns often manifest a distinct inclination to rebel, they also work hard to improve their lot through good-natured sociability and cooperation.”21 In short, firstborns compete by being nasty, laterborns by being nice. At this point a disclosure is in order: I am a firstborn. Yes, Sulloway is a laterborn.

  Sulloway’s explanation for why competition leads to personality differences is couched in Darwinian terms. “Siblings become different,” he wrote, “for the same reason that species do over time: divergence minimizes competition for scarce resources.” Although full siblings are close relatives—as similar genetically as parents are to their children—that doesn’t mean they will necessarily behave benevolently toward each other. “Because siblings share, on average, only one-half of their genes, altruism among siblings—while considerable—has its limits. Siblings will tend to disagree about the allocation of shared resources.” Each sibling wants the lion’s share of the goodies for himself; his brother or sister can have whatever is left.22 Every parent of two or more children will sigh and nod.

  Though Sulloway’s description of sibling relationships sounds convincing, there are some theoretical problems with his explanation of the personality differences between siblings. The first is that siblings don’t “become different” from each other as a result of their interactions with each other or with their parents. The notion that behavioral genetic data provide evidence for sibling differentiation or sibling contrast effects is an error. Sulloway is not the only one who has been misled; the misunderstanding is a common one. Let me see if I can clear this up.

  The primary source of confusion is what the behavioral geneticists have said about the unexplained variation in personality. They attributed it to the “nonshared environment,” which is often defined as environmental influences that “make siblings different.”23 So it sounds as though something in the home environment was actually creating or widening personality differences between siblings, causing them to become more dissimilar than they would have been if the
y were reared in separate homes. But the behavioral genetic evidence doesn’t show that growing up in the same home makes siblings different—only that it fails to make them more alike. Prevailing theories of development, such as the theory that children are socialized by their parents to express emotions or to bottle them up, had led researchers to expect that being reared by the same parents would make siblings more alike. The question “Why are siblings so different?” is the mournful cry of unfulfilled expectations. What the behavioral geneticists really wanted to know was: Why aren’t siblings more alike?

  It isn’t growing up together that makes them different. If sibling differences were due, say, to the siblings’ own efforts to differentiate themselves from one another, then siblings reared in the same home should actually be less alike than those reared in separate homes. The evidence doesn’t support this prediction; in fact, pairs reared together are neither more alike nor less alike than those reared apart. Adoptive siblings are not less alike than adoptees reared in different families. Identical twins separated at birth are as similar (and as dissimilar) as those who grow up together.24 So defining the nonshared environment as “environmental influences that make siblings different” is misleading. A more accurate definition is “environmental influences that are uncorrelated between siblings.” The difference-making effects on a pair of siblings are the same as the difference-making effects on a pair of unrelated people who never set eyes on each other.

  Another source of confusion is an often-cited article titled “Growing Up and Growing Apart.”25 According to its authors, identical and fraternal twins growing up together become less alike during childhood and adolescence. That certainly sounds like sibling differentiation—it was probably this report that led Sulloway to tell a journalist in 1996 that “the longer [siblings] live together, the more different they become”26—but again, the evidence has been misinterpreted. What really happens is that sharing a home produces some transient similarities in the early years, especially in IQ. These shared environment effects gradually fade as the twins get older, eventually falling to zero or near zero. But the similarities attributed to sharing a home do not fall below zero and turn into differences. Growing up together may temporarily make twins or siblings a little more alike, but it doesn’t make them different.

  What about contrast effects? Behavioral geneticists do use that term, but what they mean by it is the tendency for parents to see their children as more different than they really are. Contrast effects are in the eyes of the beholder, not in the children themselves. Parents who see their firstborn as “difficult” tend to see their secondborn as “easy,” and vice versa. Both children may actually be more difficult than average, or both may be easier, but what the parents notice is that one is more so than the other. Sibling differences tend to be exaggerated in parents’ reports.27

  The idea of sibling differentiation is sometimes invoked to explain away the failure to find effects of the shared home environment. The reasoning goes like this: Maybe being raised by the same parents does make siblings more alike, but the sibling relationship itself, or some other aspect of the home environment, has the opposite effect—it makes siblings more different.28 Maybe the two effects cancel each other out, so that the bottom line, the net result, is zero!

  Good try, but there’s no evidence for either of these effects. In any event, postulating effects that increase the differences between siblings doesn’t help to account for the unexplained variation in personality if, in order to postulate such effects, it is also necessary to postulate countervailing effects that make siblings more similar. If the two hypothesized effects cancel each other out, then nothing has been gained, because a cancelled-out effect can’t explain why siblings differ.

  I should mention that this discussion of sibling differentiation applies specifically to personality and to behaviors (such as aggressiveness) that reflect personality. The evidence I’ve looked at doesn’t rule out the possibility that differentiation or divergence might operate in other areas, such as choice of interests or careers. Twins or siblings who have similar personalities and aptitudes might choose to specialize in different things, like the identical pair described by twin researcher Nancy Segal: “Both twins are talented in performing arts, but Julie specialized in dance and Lisa specialized in drama.”29

  The second problem for Sulloway’s birth order theory is the fact that the behavioral genetic findings hold true for all kinds of sibling pairs. Whether the subjects are identical twins, fraternal twins, ordinary siblings, half-siblings, or adoptive or step-siblings, the conclusions are the same: all, or nearly all, of the nongenetic variance is of the nonshared kind. The unexplained variance is approximately the same—about 50 percent of the total variance—for all these sibling pairs.

  I described this counterintuitive finding in chapter 2 and said it was an important clue. Now you can see why. The consistency of the behavioral genetic results implies that the same processes are responsible for the differences between all sibling pairs. We need an explanation for sibling differences that will work as well for twins as for children born singly.

  Can a theory that attributes the personality differences between ordinary siblings to birth order explain the differences between twins? Don’t be too quick to say no. After all, twins have to emerge from the uterus in single file, and virtually all parents of twins know which one came out first. Moreover, as Sulloway pointed out, “Birth order can be seen as a proxy for differences in age, size, power, and status within the family. Common sense tells us that causation probably lies in these other variables, not in birth order per se.”30 Although identical twins do not differ in age, they may differ in power or status within the family.

  But twins are a problem for Sulloway for another reason. According to his theory, the engine that drives sibling divergence is competition. Siblings compete because their interests do not coincide. Their interests do not coincide because they share only 50 percent of their genes. But identical twins share 100 percent of their genes.

  Identical twinning is uncommon; no one believes that the human mind comes equipped with a set of instructions that tell you what to do if the person seated next to you at the dinner table happens to be your clone. But kin selection theory—one of the cornerstones of evolutionary psychology—leads us to expect that animals will respond to cues of biological relatedness by being nicer to relatives than to nonrelatives and nicer to close relatives than to distant ones.31 Evidence of the ability to make such distinctions has been found in many species, including our own. Though scientists haven’t yet figured out how they do it, even immature humans appear to have some way of estimating the proportion of genes they share with the people in their lives. In polygamous communities, children feel closer to their full siblings than to their half-siblings. Identical twins cooperate with each other more and compete less than do fraternal twins. When one twin dies, the other feels more grief if they were identical.32

  If competition were the source of sibling differences, kin selection theory would generate the following prediction: nongenetic differences should be large between adoptive siblings, moderate for fraternal twins and ordinary siblings, and small for identical twins. This prediction is at odds with the evidence. Though identical twins compete with each other less than other sibling pairs, the nongenetic differences between them are as wide.

  The third problem for Sulloway’s theory has to do with the proposition that siblings diverge “for the same reason that species do over time: divergence minimizes competition for scarce resources.”33 Is there any evidence that divergence is indeed a successful strategy for children—that behaving differently from their siblings will gain them more parental attention or family resources? None that I know of. In the absence of evidence, is there any theoretical reason to expect that diverging from one’s siblings will increase what evolutionary psychologists call “parental investment”?

  If anything, theoretical considerations would seem to generate the opposite predi
ction: that older children would profit from behaving more like their younger siblings. As I mentioned earlier, the evolutionary psychologist Robert Trivers said that parents should devote more attention to younger offspring than to older ones, because “the offspring is typically more helpless and vulnerable the younger it is.” He continued,

  This suggests that at any stage of development at which the offspring is in conflict with its parent [over the distribution of resources], it may be selected to revert to the gestures and actions of an earlier stage of development in order to induce the investment that would have then been forthcoming. In short, it may be selected to regress when under stress.34

  Regression, especially around the time of weaning or when a younger sibling is born, has been observed in nonhuman animals as well as in humans. But acting younger and needier than they really are doesn’t seem to be a strategy firstborns stick to. On the contrary, the terms often used by family members to describe firstborns—responsible, conscientious, serious, bossy—are usually associated with greater maturity.35 Sulloway interpreted this kind of behavior as a way of “currying parental favor.” Firstborns, he said, “are likely to seek parental favor by acting as surrogate parents toward their younger siblings.”36

  I have a different explanation for these observations. Firstborns behave in a more mature manner in the presence of their younger siblings, not to curry favor with their parents, but because this behavior is evoked by the presence of younger children. Whether or not they are close relatives, older children react in predictable ways to younger ones. In an overview of children’s behavior in societies around the world, the cross-cultural psychologist Carolyn Edwards noted that younger children evoke two responses from older ones: nurturance and dominance. Which response gets the upper hand depends in large part on the age of the younger child: with children under the age of two and a half, it’s mainly nurturance; above that age, dominance comes into play. As Edwards defines it, dominance includes not only efforts to control, but also efforts “to protect, socialize, or involve the child in socially useful activity.”37 In other words, to act as a surrogate parent.