The Sibling Effect

Think it was hard growing up in your house? It could have been worse; you could have been an egret. Egrets are not the most intelligent of nature’s creatures. Even grading on the generous curve of the bird world, they finish near the back of the classroom. Just how far back? “Assume the IQ of sewage,” says Douglas Mock, professor of zoology at the University of Oklahoma. “That’s pretty much what you’re working with when it comes to egrets.”

But that’s not to say that the species is entirely lacking in guile—particularly when it comes to the mother of an egret brood. Like all animals of breeding age, egret moms spend much of their time and most of their energy involved in a sort of reproductive marketing game. They’ve got a biological product to sell and distribute, and that product is their genes. Mating dances and courtship rituals are all about finding the fittest male to buy what’s being offered. Once the female succeeds—once the nest is built and the mating is done and the eggs are laid—she still has to protect her investment, and that requires a lot of strategizing.

There is no fixed number of eggs in an egret clutch, but four is a pretty common count. Even the mother can’t tell much about the chick inside each egg simply by looking at the shell. If one contains a frail baby that’s not likely to survive the season and another contains a bruiser that will grow fast and well and leave plenty of the family’s genes to later generations, she has no way of telling them apart. But she doesn’t have to intuit which chick will be the biggest and fittest; she can actually control it.

When a nest is full of eggs, a mother egret staggers the way she incubates them. All of the eggs get enough warmth and enough turning to keep them alive, but the mom will favor one slightly, giving it a little more brooding and care. This ensures that it hatches a day before the rest. She then lavishes similar attention on the next egg, and the next and the next, causing all of the birds to pop out sequentially—each about twenty-four hours after the one before it. That may be a sensible strategy for an overworked mother, sparing her the chore of having to keep four chicks fed on her very first day of parenthood, but it can make things awfully nasty in the nest.

Hatching before the other chicks means that the first one out—Mock calls it the A Chick—gets a head start on drying off, opening its eyes, and finding its footing, not to mention a full day of uncontested food. The moment the weaker B Chick hatches, the A Chick exploits its advantage and begins pecking at the newcomer—not enough to kill it, but enough to establish who’s in charge. When the C Chick comes along the following day, the B Chick administers a similarly harsh hello.

The mom knows this will happen—to the extent that an egret knows anything at all—and she’s pleased by it, too. In the wild, no year is just like the year before it in terms of food supply, weather, and the abundance of predators. A mother thus can’t handicap in advance if she’s producing too many chicks for what will turn out to be a lean and dangerous season, or too few for what will turn out to be a fat and secure one. A modern human mother in the same situation would probably play it safe and keep her output relatively low no matter what, since it is far better to have only three children when you would have liked four or five than to have four or five and watch one or more of them languish and die. To egrets, however, chicks are interchangeable—one healthy hatchling is as good as another. So the mother errs on the side of excess, laying more eggs than she can be sure will survive, and letting the peckhappy babies sort things out themselves.

“She overproduces a little bit,” says Mock. “That way there is a natural mechanism in place for the weakest bird to be dropped by the wayside if it turns out that it’s a typical year with a typical budget.” Mock, who wrote his seminal egret paper in 1991, and later authored the book More Than Kin and Less Than Kind: The Evolution of Family Conflict in 2004, dedicated his book to his three older brothers. He likes to say that one day he is going to print up a T-shirt that reads “D Chick Survivor.”

The animal kingdom is stuffed with similar examples of siblings ensuring their own survival at the expense of their litter mates, nest mates, and brood mates—perhaps none more savagely than the shark. In keeping with its rep for bloodlust, one shark species—the sand tiger shark—begins its murderous ways even before it’s born. After mating, a mother shark releases a clutch of fertilized eggs into her womb, where they hatch and produce a cluster of fetal young. No sooner have those incubating babies begun to develop mouths and teeth than they start to use them, thrashing and biting and systematically killing one another off, with the victors eating the remains of the vanquished. Only one baby survives this orgy of siblicide, and when that winner is at last determined, it settles back to continue its gestation in peace. The mother keeps her sole offspring fed throughout its remaining time in the womb by releasing ten thousand or so more eggs—ten thousand or so potential brothers and sisters—which the baby devours, surviving entirely on this cannibalistic diet of roe until at last it is born and can turn its carnivorous attentions to the outside world.

Domestic pigs, a decidedly more peaceable species, also can be rough on their same-age litter mates. Piglets are born with a set of eight irregular teeth that grow in a suspiciously lateral direction, jutting outside of the jawline. The babies’ first job—and pretty much their only job throughout infancy—is to secure a steady milk supply from Mom. All of the siblings quickly learn that the best nursing spots are generally in the middle of the mother’s abdomen, with the ones closer to her front and hind end producing a comparative trickle of milk. In the scrum that forms for the choicest places at the maternal table, the bristle of lateral teeth can start and settle a whole lot of fights. Indeed, within days of birth, it’s evident which piglets have the strongest teeth, since those are also the ones that fatten up quickest. When farmers intervene and remove the milk teeth immediately after birth, all of the piglets grow to more or less the same size. The piglets themselves are happy not to use these natural weapons if they don’t have to, and when the mother gives birth to a small litter and there’s plenty of milk to go around, the babies do a lot less fencing and jabbing.

The first scientist to study this phenomenon, David Fraser of the Animal Research Center in Ottawa, Canada, was so struck by what he observed that in his published paper—a sober work that appeared in the sober journal Behavioral Ecology and Sociobiology in 1990—he took the unheard-of liberty of writing the opening summary in verse. The journal took the equally unheard-of step of publishing it:

A piglet’s most precious possession
Is the teat that he fattens his flesh on.
We studied pig sisters and brothers,
When some had their teeth, but not others.
We found that when siblings aren’t many,
The weapons help little if any.
But when there are many per litter,
The teeth help their owners grow fitter.

If pigs usually strike us as harmless animals, penguins can seem downright comical, and yet they, too, may take a pitiless approach to selecting the fittest babies for survival. Biologist Colleen St. Clair of the University of Alberta, one of Mock’s former grad students, studied crested penguins in Tasmania and observed a surprising maternal pattern. The penguin mother generally lays just two eggs, spaced a few days apart, and in almost all cases, the chick inside one will outweigh the chick in the other by as much as 80 percent—though it’s never certain whether the first or second egg will contain the bigger sib. Whichever one it is, the larger chick typically outcompetes the smaller one, limiting the food it gets so severely that the smaller one eventually dies. On occasion, the mother is able to expedite things, stopping the competition before it even begins.

Black eagles have a similar one-baby policy, with the A Chick always outweighing the B Chick, but they settle the matter more messily than the penguins do: When the little sib is born, the big chick rips it to bits. “The mother,” Mock says, “just stands around yawning. The function of the second egg is insurance. If the first chick is healthy, the policy is canceled.”

Humans, we like to think, approach things differently. We love our babies. We dote on them. We produce a lot of them because we find rapture in raising them, and far from abiding or encouraging warfare among our children, we spend most of our time settling fights and brokering peace. No egret or shark or nursing sow could say the same. All of that is undeniably true. But it’s also true that, like the beasts, we commoditize our kids. And like the beasts, we’ve found a lot of ways—including stockpiling offspring—to ensure that our genes make it to the next generation and beyond.

As recently as the beginning of the twentieth century, a child born in the United States had no more than a 50 percent chance of making it to adulthood. Parents who bred and then lost a child would face the same break-even odds of keeping or losing the next one and the next. The only way to get ahead of the reaper was to produce so many babies that the probabilities would eventually tip your way. If you owned a farm or a business that required a lot of unpaid hands to run, you’d need more offspring still. If, in the course of producing your brood, you had too many girls—who are fine for certain kinds of work, but not others—you’d need to keep going till you manufactured more boys.

The sensible answer for a species like ours would be to breed in litters—cranking out an entire mixed-sex workforce in a single go—and if an economist had had a hand in designing human beings, that’s surely how things would work. But Homo sapiens has a big brain, and big brains mean big heads, and those take up a lot of space in the womb. One baby at a time thus became our general reproductive rule. What’s more, in order for the sole offspring—and that big head—to make it through the birth canal, gestation must be relatively short, meaning that we give birth to small, not wholly developed babies who go through a long period of dependency before they’re even able to eat solid food or toddle, much less pick up a hoe and work the fields.

The burden of rearing such helpless offspring falls overwhelmingly on the mother, and for her it’s a fantastically expensive proposition, in more ways than merely the hours and energy devoted to the job. The number of calories a new mother expends producing milk after she gives birth is actually greater on a month-to-month basis than the amount she spent growing the baby during the pregnancy itself. It takes more than 100,000 calories to feed a newborn for just the first four or five months of its life, which is nearly three times the 36,000 calories a woman of average weight has on hand at any one time in terms of body fat. There’s a reason new mothers are advised to indulge themselves in milk shakes and other high-calorie treats when they’re nursing, since so much of what they consume hangs around in their bodies just long enough to be reprocessed into nutrients for someone else. Of course, in the preindustrial era, milk shakes were unheard of, and humans were often just one failed harvest away from starvation. The same conditions prevail in much of the developing world today.

And that’s only if all the kids are healthy. But what if they aren’t? What if, as with the chicks, there’s a frail or sickly baby in the family? Should the mother continue to pour resources into what’s likely to be a failed reproductive enterprise, or should she let the weak newborn die and concentrate on its siblings? For our species, the answer is usually easy: save the baby. Indeed, far from giving up on a sick or special-needs child, parents often double-down on their investment in its care, devoting time and money to keeping it alive that may exceed by a large measure what they spend on their healthier offspring. That drives nature nuts. There is no rational reason any species should behave this way, but reason isn’t at work here, and we take justifiable pride in the way our compassion can trump our survival drives. In other, subtler ways, however, we’re a lot more calculating and a lot less compassionate than we know.

In 1999, anthropologist Edward Hagen of the University of California, Santa Barbara, conducted a controversial study arguing the provocative premise that postpartum depression (PPD)—which strikes about 10 percent of new mothers—may be less an affliction than a sort of cunning adaptation. PPD goes beyond simple melancholy, which most new mothers may experience, and instead becomes a deep, disabling gloom, one often accompanied by loss of interest in the newborn and even thoughts of harming it. In the case of a mother with a nonviable child, this is just what evolution would want, since the condition would paralyze the mother into discontinuing feeding and other care—in effect, cutting the baby’s lifeline and cutting her own genetic losses in the process. That unlovely idea caused the paper to generate a lot of heat and meet a lot of resistance at the time it was published, and Hagen has not pursued the topic since. Still, he amassed a lot of data that make at least a credible case for his premise—in part simply by ruling out most other possible explanations for PPD.

For one thing, hormones, which are one of the first places people look in trying to explain various kinds of depression, appear to play at best a secondary role in the postpartum variety. Levels of progesterone, estrogen, prolactin, and, significantly, cortisol—a stress hormone—often remain within the same range for depressed and nondepressed mothers.

Similarly, socioeconomic factors such as occupational status, education level, the number of other children in the home, and even the mother’s marital status also do not closely correlate to maternal PPD. Yet if anything ought to send a new mom into a funk, you’d think being out of work, undereducated, or unwed would do it.

The two big things that do correlate with postpartum depression are the amount of child-care assistance the mother feels she is getting from the father, and the health and viability of the infant. When either one is lacking, PPD is statistically likelier to appear—and the baby is statistically likelier to get less care. “Mothers with PPD mother less,” Hagen wrote. Their depression informs them that “they have suffered a reproductive cost and that successfully motivates them to reduce this cost.”

Such withholding of resources doesn’t always mean a mother is actually giving up on the baby. In the case of a nonsupportive husband, she may simply be going on what Hagen called a “labor strike,” forcing Dad to step up his game or risk losing his own genetic investment in the child. In the case of the nonviable baby, Hagen does believe that evolution may be trying to drive the mother to let go. The voices of the mothers who were quoted in his paper certainly suggest that.

“I would be going along and being okay,” said one woman, “and then I would get up to that changing table and in a matter of seconds my mind would have started with, ‘Oh, the baby is going to fall off the table. I don’t care if she falls off the table.’ Why did I think that I don’t care? Of course I care.” Of course she does—but evolution doesn’t.

With such behavior baked into our genes, the sibling brood ought to be nothing at all like the close and nurturing thing it usually becomes, but more like a team of rivals, a childhood-long battle, with life itself often the thing being fought over. This is especially so, since parental selectivity continues well after babies are out of infancy.

Both across history and across cultures, for example, girl children have traditionally ranked lower in value than boys, and have been treated accordingly. While such a sexual caste system is now seen as morally indefensible in much of the world, natural selection does help explain how it got started. In general, girls don’t hunt, they don’t go to war, and while they do breed, it’s in a comparatively limited way, with nine months of gestation followed by a long period of nursing, during which ovulation usually stops. Throughout human history, only the most prolific mothers have turned out nine or ten children in a lifetime—and most give birth to far fewer.

A male, on the other hand—particularly a young male—is nothing short of a reproductive machine, with every mating theoretically able to produce a child, and many matings with many partners possible in a single day. The eighteenth-century Moroccan ruler Moulay Ismaïl is said to have fathered 888 children with his 500 concubines. Genghis Khan, the Mongol emperor who ruled much of Asia during the twelfth and thirteenth centuries, makes Ismaïl look practically barren. A 2003 analysis of the Y chromosome of 2,123 men now living across the former Mongol empire suggested that there are 16 million males living today whose line stretches back to the great conqueror—or 1 out of every 200 males now on the planet. Since all of the other men living in Genghis’s time are thought to have about 20 male descendants currently at large in the world, that means Genghis outperformed them by a factor of 800,000. At some level, all parents are hardwired to want a child like Genghis who will mass-produce the family genes so successfully, and no female could ever qualify for the job. “In terms of genetic payoff,” says Catherine Salmon, professor of psychology at the University of Redlands in California, “a son is going to be a much better bet than a daughter.”

Human culture has only widened that value gap. The custom of a dowry, essentially a bribe to a groom to take a marriageable daughter off her birth family’s hands, imposes a burden on parents every time a girl is born. Somewhere, fifteen or twenty years down the line, they’re going to have to come up with a cow or a horse or a sum of cash to pay off the eventual husband, lest the daughter remain unwed and dependent. A newborn boy, on the other hand, will not only not use up the family’s wealth but generate some of his own. While the general death of the dowry along with a host of other cultural factors have eliminated much of this primal bias in the West, in parts of the East it’s only grown stronger.

When an overpopulated China introduced its one-child-per-family policy in 1979, the national gender balance between boys and girls was roughly fifty-fifty. By 2000, there were about 124 births of boys being recorded in China for every 100 girls, and young males under the age of twenty exceeded young females by a whopping 32 million. The missing girls were simply abandoned, given up for overseas adoption, or, with the advent of modern pregnancy screening techniques, aborted. Neighboring India, even without a one-child policy, was little better. “In 1980, a paper in the journal Science looked at amniocentesis rates in India in tandem with abortion rates,” says Mock. “It was two hundred to one against daughters.”

Gender and health are not the only variables that can raise or lower the perceived value of one sibling compared to another. Age plays a powerful role, too. Firstborn favoritism is a very real thing, and family psychologists have offered a lot of theories to explain it—most having to do with the sheer novelty of becoming a parent for the first time, and the inevitable habituation that comes from repeating the experience again and again. That is surely part of it, but biological economics explains the rest.

Every calorie, dollar, and hour parents spend to feed, raise, and care for a child is a calorie, dollar, and hour the parents are never getting back. Corporations refer to such investments as sunk costs—revenue devoted to product development that will pay off only if the product succeeds. Sunk costs are a powerful motivation for those companies to keep their attention focused on the line of merchandise furthest along in design and development. Sports teams, whose product line is human athletes, do the same thing: A pitcher who’s spent three or four years in the minor leagues and is almost ready for the majors will be a lot less likely to be traded than a recently drafted rookie. It’s that way with families, too.

“There’s a kind of resource capital that parents pour into firstborns,” says Ben Dattner, a business consultant and organizational psychologist at New York University. “They build up a sort of equity in them. If a child has to be sacrificed—due to poverty or famine, say—it’s very rare that any society encourages taking resources from the firstborn. It’s almost always the second-born who is sacrificed.”

Children, at least very young children, ought to be oblivious to the biological bookkeeping that drives parents to behave the way they do. The parents, after all, are often wholly unaware of it themselves. But if parents are genetically programmed to pick and choose among their kids, the kids come equally programmed to fight to be the chosen ones. In that sense, they compete no less vigorously than animal siblings—and in many ways compete much harder. Animals, after all, have little more than instinct fueling their competitiveness; humans have emotion—jealousy, resentment, outrage—and that can turbocharge even the mildest rivalry.

Of all the kids in a growing family, it’s once again the oldest who stands out in terms of feeling most acutely the competition for resources. Firstborns have it awfully good in the early part of their lives—borne along by their parents’ doting care and liking that just fine. When a younger brother or sister comes along, all that changes. Dattner is a psychologist who has read deeply in the literature of sibling dynamics, but when he describes the trauma singletons experience when they’re suddenly presented with a baby sibling, he quotes an unlikely authority: Cindy Crawford, ex-supermodel and the mother of two.

“Crawford once observed that if you want to know what it’s like for a firstborn when a little sister or brother joins the family,” he says, “try to imagine a woman whose husband tells her, ‘Honey, I’ve got a new, better-looking wife coming along and you’ll be moving to a room down the hall. But don’t worry, I’ll love you both the same and there’s plenty of room for all of us.’”

Terrible as that sounds, it’s actually worse for the child than for the hypothetical wife. A displaced spouse at least has the power to up and leave. Single children who suddenly find themselves merely one of a pair are stuck with their new, reduced station. Psychologists call what the firstborn goes through in these situations “dethronement,” and it’s an experience that can leave scars for a lifetime. Author and family researcher Judith Rich Harris, who wrote The Nurture Assumption: Why Children Turn Out the Way They Do, likes to cite the case of Pulitzer Prize–winning novelist John Cheever, whose older brother Fred suffered from depression and alcoholism—conditions that Cheever believes had their roots in his own arrival in the family.

“He was happy, high-spirited, and adored,” Cheever wrote about his brother, “and when at the age of seven, he was told that he would have to share his universe, his forebodings would naturally have been bitter and deep.... His feeling for me was always violent and ambiguous—hatred and love—and beneath all of this must have been the feeling that I challenged him in some field where he excelled—in the affection of his parents.”

Cheever is likely wrong about the causes of his brother’s alcoholism and depression—two clinical conditions with a complex mix of genetic and environmental variables—but not about the profound resentment his own arrival probably caused his brother. What’s more, in the hothouse of a small family, Cheever himself probably felt that anger coming his way every day—and he returned it in his own fashion, with his fiction often featuring conflict between brothers, some of it violent. In a 1977 interview, the year after his older brother’s death, Cheever’s daughter conducted an interview with her father and asked him a very candid question: “Did you ever want to kill Fred?”

Cheever responded with equal directness: “Well, once I was planning to take him trout fishing up at Cranberry Lake, which is just miles away from everything in the wilderness, and I realized if I got him up there he would fall overboard, [and] I would beat him with an oar until he stayed. Of course, I was appalled by this.”

In most families, things never get this serious. When the firstborn is very young—say, three years old—the arrival of a baby sibling commonly leads first to the nuisance behavior known as regression, with the older child reverting to a period of infantile behavior. Toilet-trained kids suddenly return to bed-wetting; kids who have moved beyond the bottle or the sippy cup suddenly want to go back. In the child’s mind—at least the unconscious mind—this makes perfect sense. It’s all the diaper changings, feedings, and bathings the baby needs that are taking so much of Mom’s and Dad’s time. If you once again need the same care, you’ll once again get the same attention.

Behavior such as regression is unheard of in species that breed in litters or even in twos and threes, since the mother gives birth to a single, same-age group all at once and by the time she’s ready to do it again, the first brood is grown and gone. Among the few animals that breed single babies with long periods of dependency, however, selfinfantilizing by the older sibling is more common. Jane Goodall, who has spent decades in the wild with chimpanzees, has often observed regression behavior in the troops she studies as a female with a newborn finds herself having to push away a newly clingy firstborn. When regression fails, aggression follows, with firstborn chimps handling baby siblings in ways that could pass for rough play, except that the baby is clearly having no fun.

Human children are no different, and how they will treat a baby sibling is no less age dependent. Small children think nothing of snatching a toy from a smaller toddler sib. Babies who have just learned to get up on their feet are often knocked squarely back off them by an older brother or sister. In the weekend war room of our family’s playroom, my brothers and I may have worked out a strategy to keep Bruce out of harm’s way, but that doesn’t mean we didn’t sometimes try to inflict that harm ourselves, as our mother discovered one day when Bruce was barely a toddler and my other brothers and I were two, three, and five. She passed by the living room to find Bruce playing absently on the floor while the rest of us stood menacingly on a chair above him, looking for all the world like we were going to pounce. She had no idea if that was our intention—and I certainly have no recollection of the moment—but she didn’t wait to find out, sweeping into the room, scooping up Bruce, and taking care that we never played with him unsupervised again until we were all a little older.

In our case, the firstborn of our brood, Steve, was actually a little bit past the age at which he should any longer have posed such a physical threat. Four years old is about the time that most children are not only less menacing to babies, but actually starting to become solicitous of them—helping to feed them, cooing at them, sometimes mimicking the baby talk that was directed at them not so long before. “Among huntergatherers,” says Salmon, “four was about the age at which a child was fully weaned and no longer needed the resource of Mom in order to eat. When you’re not competing with your baby sibling, you’re less likely to try to hurt it.”

Few modern mothers would nurse a baby anywhere near as long, and most four-year-olds have plenty of other sources of nutrition beyond breast milk. But the survival imperatives of the ancient world take a long time to catch up with the customs of the contemporary one, and firstborns will cling to Mom until their genes tell them it’s safe to let go.

When the age difference between siblings opens up beyond four years—the six-, seven-, and eight-year gaps that sometimes occur when parents either have an unplanned child or decide late in the reproductive game that they’d like to have just one more—older siblings adjust much more easily. They are still utterly dependent on their parents, but they have also begun to chafe at that dependence, pulling away or looking put out if Mom tries to kiss them in front of their friends or if Dad tries to hug them when he picks them up at school. What the new baby needs—the constant nuzzling attention of a hovering pair of parents—is precisely what bigger kids don’t want. “By seven years old,” says Salmon, “the separation process has begun.” That’s not always so easy for parents to experience.

When my younger daughter, Paloma, who is now seven, was in kindergarten, she would leap into my arms and hug me good-bye when I dropped her off or picked her up at school. By the time she reached first grade, she’d put such childish things behind her and would grant me only a demure peck as she was saying good-bye—taking care to do it out of sight of her classroom. When my wife dropped by the school early one afternoon on an errand and surprised Paloma in the cafeteria, she was greeted with similar coolness. Later that night while getting ready for bed, Paloma—exhibiting the predictable ambivalence of a child experimenting with independence and not knowing quite how she feels about it—burst into tears over what had happened and promised my wife she’d never turn down a hug in school again. That, of course, is a promise she won’t—and can’t—keep.

For Paloma, this growing-up process is tough enough, but for our older daughter, Elisa, now nine, it’s a little more complex. The very maturity that impels children to edge away from Mom and Dad comes with other familial burdens—specifically helping to care for the younger sibs. Alloparenting—the custom of aunts, uncles, grandparents, and other adults participating in the rearing of a child—is practiced only fitfully in the United States, and generally doesn’t go much beyond babysitting or looking after the kids while the parents take a rare weekend trip. Here as well as everywhere else in the world, however, big siblings, who are present all the time, are forever being drafted into helping to care for little ones—even if it’s merely in such small-bore matters as shoe tying, coat zipping, or assisting with homework. In Africa and other places with more traditional cultures, the responsibilities are much, much greater—and the relationships among siblings are much, much different.

Tom Weisner, a professor of psychiatry and anthropology at UCLA, has conducted extended studies of families in multiple cultures to observe parenting practices and, specifically, the role siblings play. One of his most in-depth projects was a long-term survey of forty-four nuclear families in Kenya, made up of roughly 400 people, or nine individuals per family. He found that not only do older brothers and sisters do more work to help raise little sibs than their counterparts do in the West, but that they actually do most of the work. “Siblings care directly for each other,” Weisner says. “Parents serve as the indirect managers of a socially distributed caretaking system.”

That’s partly a result of the economic system in parts of traditional Africa, with jobs often seasonal and requiring parents to travel in search of them. Rather than uproot the entire family, they leave the big kids in charge of the little ones while they’re away earning a living. Even when cultures live more fixed and settled lives, however, a tradition of sibling alloparenting remains. Weisner has found that to be particularly true in Polynesia and Hawaii, both lands with long agricultural histories predating industrialization, and both lands in which older brothers and sisters are still expected to do some heavy lifting to help rear younger ones. This not only eases the burdens on the parents, it also leads to stronger lifetime bonds among sibs—sometimes in ways Westerners would find inexplicable.

“In Polynesia,” Weisner says, “it would be an expected or valorized part of life for a married couple to live apart from one another and live with siblings. In the United States, we’ve de-emphasized the sibling relationship relative to other relationships, particularly the parent-child relationship and the romantic relationship. But what if your mythology, movies, bedtime stories, and commercials were about siblings and how they do things together and have fun?” Brothers and sisters raised in these cultures still fight and tease and fiercely compete, they just do so less—and alloparenting is part of the reason. “It isn’t a cure,” Weisner says, “it’s just another solution.”

President Barack Obama’s close, or at least cordial, ties with his sprawling web of half siblings may be less a result of alloparenting—he grew up without any siblings around at all—than of the multiple cultural legacies that helped shape him. Born in kin-conscious Hawaii, with a father, half siblings, and other assorted relatives in Kenya, he got a double dose of family-centric training. Obama did not even meet his half sister Auma until 1982, after their father died, but as he wrote in Dreams from My Father, the connection was immediate. “I knew at that moment, somehow, that I loved her. Even now I can’t explain it; I only know that the love was true, and still is, and I’m grateful for it.”

The president’s relationship with his older half brother, Malik, is similarly close. The two never met until 1985, when Barack was twenty-four and Malik was twenty-seven. Nonetheless, they stood as best men at each other’s weddings, and the little brother made a point of bringing his fiancée to Kenya to introduce her to his big brother before getting married—a gesture of familial respect not common among half sibs who did not share their lives at all when they were children. Like many Kenyans, Malik considers extended family members who do not live in the home country “culturally lost,” though close kinship bonds with those living overseas can help honor the ideal of the family all the same.

Of course, parents don’t have to live in Polynesia or space their kids more than four years apart to foster intimacy and reduce competitive tension in the home. The mere fact of having more than two children can actually contribute to less rather than more conflict—at least in that critical period when a new baby is getting integrated into the family. In the same way that parents habituate to the successive arrivals of newborns, becoming slightly less enthralled with—or at least slightly less consumed by—the birth of each, so do siblings themselves. In a sense, says Richard Zweigenhaft, a professor of psychology at Guilford College in Greensboro, North Carolina, that’s because each sibling is born into what is essentially a different family.

That, certainly, was how things were in my home. I never knew what it was like to be an only child, and so I had nothing to miss. I barely remember not having a little brother, either, since I was just sixteen months old when Garry was born. If the psychologists are right, however, at some level I did resent his arrival. I do remember Bruce’s babyhood—I was nearly two and a half when he was born—and what I recall of it I enjoyed, partly because by then I was surely used to the loss of my last-born status. What’s more, I was just old enough to find Bruce cute, to enjoy hearing him struggle to pronounce my name when he began to speak—though it seemed indistinguishable from the way he pronounced Steve’s name. That may have said something about the interchangeable roles his two biggest brothers played for him (Steve’s and my names sound nothing alike, after all). Garry, who was Bruce’s closest playmate and full-time roommate, he learned to address by name quickly and clearly. We may have all been part of the same brood, but we already had very different roles and histories in it.

Dattner takes a business consultant’s view of the same idea. If a family is a corporation and Mom and Dad are the co-presidents of it, they’re going to take one approach to running a small operation and a different one to running a larger one. And the employees will notice. “Each child takes up a proportionate share of the parents’ attention,” Dattner says. “It’s going to be harder to go from 100 percent to 50 percent, than it is to go from 50 percent to 33, or 33 to 25.” By the time kids get down to the single-digit percentages of truly large families, communal living and distracted parents are all they ever know.

In the course of this, the siblings are forced to take a corporate perspective of their own: No matter when they joined the payroll, they’re now part of a joint enterprise. The job of the entire team is to keep the operation running smoothly and peacefully. That, they will soon learn, is much harder to do than they could have possibly imagined.