Chapter 7

BROTHERS AND SISTERS are friends, confidants, life partners and more. But they are also, simply and fundamentally, males and females—and no matter the relationship, no matter the life history, no matter whether two people are strangers or kin, there will always be some measure of cross-gender complexity that goes with that difference.

From babyhood on, opposite-sex siblings approach each other with a sort of anthropological puzzlement. There is sexual curiosity, of course—a kind of my-parts-and-your-parts fascination that is mostly about learning basic anatomy but also about a kind of primal titillation. The titillation piece usually goes away quickly, snuffed out entirely by rules and taboos that are equal parts biology and social training. What remains, however, are the rich and often irreconcilable differences that always separate the sexes but that brothers and sisters can put to use in ways that are unique among other opposite-sex groups.

Siblings growing up in all-boy broods like mine may easily fall together into a band-of-brothers clan, approaching the world in the same way and with the same temperament. But there’s a gender pressure-cooking effect that goes on in those situations too, as male traits get learned and relearned, and too often overlearned. There is little live-in experience with the different skills needed to engage with the female half of the human species. All-girl broods are similarly deprived of the chance to practice the skills they’ll need to engage with the boys and men who will inevitably populate their worlds.

In some ways we all spend our entire lives turning the Rubik’s Cube of the opposite sex without ever getting all of the colors to line up the right way. But the more you learn to work the puzzle when you’re a child—the more you’re required to consider the behavioral, sexual and temperamental dimensions of the brother or sister who shares your childhood—the better you’ll do in the big, complicated, mixed-sex life outside the home.

College women like to say that they can tell if a college boy grew up with a sister. He may be as loud and loutish as any other dorm rat, but he also knows when it’s time to dial it down. You can’t call it manners, exactly—that’s going too far—but it’s at least an awareness that his idea of fun is not the same as the girls’ and that sometimes he ought to behave accordingly. College men similarly insist that they can tell if a girl grew up with a brother. There’s a toughness to her, a winking lack of seriousness that feels ever so slightly male. What human beings think we observe about one another and what scientists agree we’re observing are not always the same things. But in this case, our perceptions are spot-on.

Early in a child’s life, the gender of your siblings doesn’t make a whole lot of difference, mostly because kids of both sexes, whether in the home or out, tend to mix very freely. Girls and boys in day care, pre-K and kindergarten play together indiscriminately, surely noticing gender but not acting on the difference in any meaningful way. All that begins to change about the time kids turn 6 and enter first grade. At that point, an inexorable segregation by sex begins, some of which is societally imposed. Boys and girls who may have shared a common bathroom in the classroom when they were in kindergarten are now steered to separate accommodations just like adults. Organized playground games that once were co-ed will often be single-sex, with boys competing against boys and girls against girls. Brownies and Boy Scouts, girls’ summer camps and boys’ summer camps: all sort the sexes further. But the culture isn’t the only thing at work, and a lot of the drift into two separate worlds is initiated by the kids themselves. The play and behavioral styles of the two genders are simply too different—with girls more inclined to gather into playgroups of three or four and engage in cooperative games and fantasy play, and boys more inclined to move in large, highly physical packs—for the groups to mix easily.

For a boy with a sister at home or a girl with a brother, that opposite-sex sibling now takes on a lot more meaning than he or she used to: a live-in member of a different tribe that, outside the home, is increasingly closing its ranks to you. Such a situation presents a lot of opportunities for learning. “As segregated playgroups form,” says anthropologist Thomas Weisner of UCLA, “the sibling world may become the only world in which you have an opposite-sex peer who you can talk to and observe and learn from. Having regular, everyday contact like that has a powerful effect on you, one that’s very different from the effect a same-sex sibling has.”

Kimberly Updegraff of Arizona State has conducted some of the field’s best studies on the ways brothers and sisters interact, focusing principally on adolescence—a crucible for all things related to gender, sex and sexuality. Her landmark study was published in 2000, but its findings remain fresh and its insights keen today. Updegraff surveyed 197 families in central Pennsylvania, scattered among 18 different school districts. All of the families had a firstborn child in 8th, 9th or 10th grade, and at least one younger sibling. Updegraff also recruited 357 friends of the children in the target families, divided more or less evenly between the friends of the firstborns and the friends of the later-borns. In total, she had more than 700 subjects.

Each of the children in the sample group was interviewed one-on-one for up to three hours, and all of them answered a survey about their own personalities and the personalities of their friends in the study. The survey included such questions as “How much do you share your feelings and inner secrets with your best friend? How much do you go to your best friend for advice and support?” The subjects were also asked to agree or disagree on a five-point scale with such statements as “I make my friends do what I want to do.” All of those questions were clearly designed to get at the traits that are traditionally associated with either masculine or feminine behavior, and all were intended to provide Updegraff with some insight into how much the mere fact of having a sib of the opposite sex caused you to acquire some of their gender-based traits yourself.

From babyhood on, opposite-sex siblings approach each other with a sort of anthropological puzzlement.

Early on, gender doesn’t mean much among siblings. That starts to change about the time that children enter the first grade, as sex segregation begins.

“Younger brothers with older sisters,” says William Ickes, “were an instant hit with the women they were paired with.”

As Updegraff anticipated, the results of the study did show that the traits rub off—but only to a degree. Girls growing up with a brother scored higher in both the surveys and the interviews on measures of assertiveness, showing a markedly greater tendency to try to exert control over their friends and playgroups. Not only that, the girls also tended to choose female friends who were unusually low in the same traits themselves, as if they were trying to boost their dominance quotient even higher by ensuring that the people around them would willingly submit.

Among boys growing up with sisters, particularly older sisters, the results were very different. Rather than emulating their big sisters, the boys tended to de-identify with them, essentially doubling down on stereotypically male dominance traits, at least outside the home. Curiously, the boys also picked friends who had the same hypermale traits themselves.

A lot of caveats have to be applied to Updegraff’s findings, the first being that it is never wise to assign traits such as dominance and cooperativeness to opposite sides of the gender divide as if males and females alike don’t exhibit both traits. They do, of course—even if they do so to different degrees—and sometimes a cooperative boy or a domineering girl would be that way regardless of the sex of the siblings living in the home. What’s more, in any brother-sister relationship, age gap is always a factor. Generally, the bigger the difference in ages, the less involved brothers and sisters are with each other, and the less they are influenced by the other in the formation of their personalities. From one to three years is the optimal gap for sibling influence; much beyond that, and an older sister or brother becomes a little less like a sib and a little more like an aunt or uncle, which dilutes their temperament-shaping power.

Whatever influence brothers and sisters do have over one another hardly stops at adolescence, instead continuing as young teens mature and the sex-segregated playgroups of elementary and middle school reintegrate into mixed-sex social and dating circles. It’s then that having an opposite-sex sibling pays particular dividends, as William Ickes, a psychologist at the University of Texas at Arlington, proved. In 2005 Ickes designed a study in which he recruited a total of 40 male and female undergrads, each of whom had an opposite-sex sibling. He divided them into male-female pairs and set them chatting as if they were on a date, while video cameras rolled. He also had the subjects fill out a questionnaire afterward, describing the encounter. Since all of the people participating in the study had opposite-sex siblings, Ickes wasn’t looking for how that fact alone might affect their interactions with the opposite sex; he assumed it would, as studies like Updegraff ’s had already shown. What he was looking for instead was whether it made a difference if that opposite-sex sib was older or younger. As it turned out, it did make a difference—a big one.

The males in the study who had older sisters scored significantly higher than those with younger sisters in terms of how much and how openly they talked during the sessions with the females. They also tended to ask more questions, a rare and highly prized trait on the dating market, at least judging by reports from frustrated women dating incurious men. Those males were rewarded in the study with more gazes from the women as well as more verbal reinforcers. Afterward the women also gave the men higher likability scores.

“Younger brothers with older sisters were an instant hit with the women they were paired with,” says Ickes. “It seemed that they had learned what a woman thinks and what her expectations are.” Among younger women with older brothers, the results weren’t as clear, but the differences still existed, with little sisters of big brothers more inclined than the other women to initiate conversation with men, to smile at them and to elicit smiles in return.

“Even though the effects were weaker for women,” Ickes says, “both sexes proved to be better at breaking the ice with an opposite-sex stranger when they’ve had practice in the home. It’s one more way our siblings have a long-term influence on our behavior.”

Family sexual issues are never easy to sort out, but the hardest of all may arise on those occasions when an erotic spark gets struck between siblings themselves. Just how often sibling incest occurs is a hard thing to determine. One study of undergraduates at six New England colleges found that 15% of females and 10% of males reported having had some kind of sexual experience with an opposite-sex sib at some point in their lives. Another study put the figure at 16% for females, but that study asked the girls only if they’d been subjected to forced or coerced sex. The consensual kind was not addressed.

None of those numbers are anything like ironclad, and truly definitive ones may never be forthcoming. With the secrecy and social opprobrium that surrounds incest, the act is uniquely dependent on self-reporting—and in some cases it’s not even all that easy to define. When does anatomical exploration cross the line into something more? When are kids considered beyond the age of innocent curiosity and old enough to be aware of what they’re up to? Actual intercourse between a sexually mature brother and sister is hard to misinterpret, but there are a lot of developmental and erotic steps before that point. Siblings in early adolescence, for example, may be engaging in some kind of familiar, naive roughhousing or other physical play, only to discover that their newly maturing bodies are reacting very differently from how they once did. Is that incest or something less? In about a third of all cases, an incestuous episode occurs only once and is never repeated, and such self-limiting behavior is particularly likely if kids simply stumble into things this way.

However much incest does take place, in some respects it’s a wonder it doesn’t occur more, at least when you consider what our species looks for in a mate. Your sibs are young, available and often attractive, and you have compatible—indeed, identical— backgrounds. Find a gem like that at a college mixer or on a dating website, and you could be set for life. “Imagine someone who shares similar thoughts, hobbies and ways of interpreting the world and is physically attractive too,” says psychologist Debra Lieberman of the University of Miami. “Sounds like the perfect mate, except it’s your sibling.”

Across human cultures, the mere fact of family members living in the same home can be all it takes to snuff sexuality.

Such an automatic reaction is the behavior our primitive wiring sets up in us—and that’s the wiring that completely runs the show in other species, in which mating between brothers and sisters takes place all the time. Yet the overwhelming majority of the time, humans are decidedly not aroused at the sight of a young, sexy sib. Often we barely notice, and to the extent that we do notice, we may recoil at the thought.

There are a number of reasons for our species’ nearly universal incest taboo, the most commonly cited being the avoidance of birth defects and inherited diseases. Families can carry a genetic predisposition for a disease for generations, but since the trait is a recessive one, the disease may never be expressed. If a brother and sister both carry the gene, however, pairing up can be disastrous. The risk of some kind of birth defect in cases like these is as high as 50%, according to some scientists, but a lot depends on the particular defect or disease and the particular family in question. Whatever the exact number, it’s likely to be a whole lot lower when two unrelated people mate, a fact that our ancient ancestors recognized early on.

The stay-away training starts when brothers and sisters are young, with segregated baths and bedrooms, and sometimes a severe scolding if they’re caught touching or exploring each other in forbidden ways. Parents send both overt and implicit signals about what the limits are, but all of them convey the idea that sexualizing a sib is not just wrong; hitting or teasing a brother or sister is wrong too, after all, but siblings do it all the time. Sex play, it’s made clear, is bad in a deep and even unclean way.

The brain—and specifically, the olfactory system—may work to keep sibs apart too, thanks to the major histocompatibility complex (MHC), the region of the genome that produces proteins that allow humans to detect viable mates by smell. The less similar the MHC of two people is, the likelier they are to produce healthy children; the more similar it is, the likelier it is that there could be genetic complications. Brothers and sisters may love one another deeply and powerfully, but their too-similar MHC should keep things from going further.

Other cues work a keep-your-distance power of their own. Across human cultures, the mere fact of family members living in the same home can be all it takes to snuff sexuality. The longer sibs live together, the more indelible that mark becomes—and that may be true even if kids are not related but simply raised as if they were.

It takes a lot to overcome the visceral, deeply felt distaste for incest, yet some siblings still do. Not everyone is aware of or fears the risk of recessive diseases. Not all scientists yet agree on the role of MHC, and it’s possible the scent proteins aren’t as powerful as they seem. And while cultural norms forbidding incest are universal, they’re not insurmountable, particularly in situations of extreme familial dysfunction. Children who experience shared trauma and who are abused or isolated from other kids may find comfort only in one another, and that can develop into shared sexuality. Sometimes alcohol or drugs can play a role, as can a simple absence of other sexual options.

The misfortune, abuse and deprivation behind so many cases of incest rarely win victims much sympathy. Incest is illegal in most of the world, and in some places it can be severely punished.

But in most cases, the power of cultural learning—even in the absence of any innate biological aversion—overcomes everything else. Grow up in a home in which pork is forbidden, and you’re likely to feel physically ill if you eat what you think is a beef baloney sandwich and later learn that it was ham. Drink a glass of ice water in a restaurant and you’ll think nothing of it, unless someone then tells you the waiter secretly spat in it, even if no such thing happened. The same is true, in an exponentially deeper way, when you’re steeped in a culture that treats the brother-sister bond as a deep and meaningful thing—but a thing that for the good of the home, the self and the species as a whole must remain forever asexual. Learned early, it’s a rule that remains imprinted for life.

The Gay Factor

How Siblings Can Influence Sexual Orientation

IF YOU GREW UP AS A YOUNGER BROTHER in a houseful of boys, you have probably lost count of the ways your big brothers shaped your life. But science shows that if you’re gay, they may have had a role in that too.

A full scientific understanding of what determines sexual orientation has been slow in coming, but one explanation is the size of the sibling brood. Biological anthropologist Paul Vasey of the University of Lethbridge in Alberta has been conducting studies in Samoa of a phenomenon he calls the maternal fecundity theory. As the term suggests, Vasey believes that the mere fact that a woman has a lot of children might make it somewhat likelier that at least one of her sons will be gay.

On its face, such an idea is self-evident, by dint of mere probability. But there’s more to it than arithmetic; there’s also genes.

Vasey chose Samoa for his work because the comparatively isolated island culture makes it possible to run experiments in which fewer confounding variables come into play. His studies involved comparing the fecundity of mothers of heterosexual boys with that of mothers of gay boys—or fa’afafine, a Samoan term that means “in the manner of a woman.” Vasey’s surveys did reveal that the average fa’afafine had more siblings than the average heterosexual boy, and not only did the mothers of the boys tend to produce more children, so did the sisters when they grew up.

The reason for that may lie in a neat bit of reproductive balancing. The mothers of gay boys, Vasey suggests, carry a gene that codes for androphilia—or a particularly robust attraction to men—which would help account for the mothers’ own prodigious reproduction. A boy who inherits the gene may be androphilic, too—in other words, gay—and be less likely to wind up reproducing at all. In terms of total baby count, this ought to be a wash, since the mother’s overproduction is offset by the fa’afafine’s nonproduction. That makes the gene useless in survival terms, or at least it would ordinarily. But the sisters inherit androphilic tendencies too and, with their own prolific breeding, push the family’s reproductive output into positive territory.

Whatever the exact process is at work, the result is more sex, more breeding and thus more offspring, which goes a long way toward explaining why a non-procreative trait such as homosexuality did not vanish in the early history of the human species. The female fecundity phenomenon is not unique to Samoans. Biologist Andrea Camperio Ciani has conducted related studies in Italy and found something similar there, and a human characteristic that has a toehold on the European continent is usually one that spreads readily around the world.

Intriguing as Vasey’s work is, it’s not the whole story. A far larger body of studies expands the role of the sibling brood further, showing that it takes more than just a lot of siblings in a family to increase the odds of producing one gay member: it takes a lot of older male siblings. The greater the number of boys, the higher the mathematical likelihood that one of the youngest would be gay, a finding that has been replicated over the decades in many studies. The number of older sisters does not make any difference.

In 2006 psychologist Anthony Bogaert of Brock University in Ontario sought to quantify more precisely how powerful the fraternal birth-order effect is, and in addition postulated a theory explaining what’s behind it. In a survey of 944 men—some of whom were raised with their biological brothers, some of whom were raised apart from them and some of whom had only stepbrothers—he found that the likelihood of homosexuality in the youngest boy increased by at least one third for every blood brother who preceded him. Bogaert began with an estimate of roughly 3% for the overall share of gay males in the general population—a figure that on its face seems low and is disputed by many psychologists, some of whom put it at 10% or above. But even with such a conservative starting point, the findings were dramatic: a single older brother, Bogaert concluded, boosted the probability of a gay younger brother to 4%; two older brothers took it to 5%; and three or more caused it to top out at about 6%—double the starting point.

The likeliest explanation, Bogaert and a growing body of other scientists think, has to do with the mother’s immune system. A woman’s body does not recognize either a boy fetus or a girl fetus as alien, but it does see boys as slightly more alien than girls. Some mothers’ systems may react by developing antibodies to certain sex-specific proteins a male fetus carries. Every subsequent boy baby will trigger a release of those antibodies in the womb, with the reaction getting stronger with each pregnancy. Ultimately, the antibodies may cross the placenta and have an impact on the baby’s developing brain—including the area that governs sexual attraction. (The study of homosexuality among siblings is a growing field, with far more work to date being devoted to boys than to girls.)

For families, questions about genes and hormones are largely academic matters when it comes to understanding the forces that shape a loved one. The larger issues are how all of the sibs can best absorb and embrace the news that a member of the brood is gay or lesbian.