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The Science of Siblings

Updated: Apr 27, 2022

The topic of siblings comes up with my patients on occasion, and I was reminded of this article that I read back in 2006. At that time, my brother was getting married, and I was "designated" best man. "You get to be my best man by default, since you are my brother," is what my big bro said to me at the time.

I've had my share of ups and downs with my brother over the years, and now at our ages, we are more like peers rather than having the "older brother and younger brother" relationship. 16 years have passed since his marriage, and a lot has changed in our lives, but what does remain constant is our bond, our silly humor, and the shared life experiences of loss, defeat, success, and promotion.

At my brother's wedding, I had to deliver the best man speech. Looking back, the speech was way too long (and so was the 13 minute video/picture montage I created). What I do remember was specifically quoting a paragraph from a Time magazine article, which still resonates, today:

"From the time they are born, our brothers and sisters are our collaborators and co-conspirators, our role models and cautionary tales. They are our scolds, protectors, goads, tormentors, playmates, counselors, sources of envy, objects of pride. They teach us how to resolve conflicts and how not to; how to conduct friendships and when to walk away from them. Sisters teach brothers about the mysteries of girls; brothers teach sisters about the puzzle of boys. Our spouses arrive comparatively late in our lives; our parents eventually leave us. Our siblings may be the only people we'll ever know who truly qualify as partners for life. "Siblings," says family sociologist Katherine Conger of the University of California, Davis, "are with us for the whole journey.""

I am grateful for the relationship I've sustained with my brother. I am proud of our friendship and togetherness. While everything is not always ideal, I am happy to think that we have an 80% good relationship.

The New Science of Siblings

Your parents raised you. Your spouse lives with you. But it's your brothers and sisters who really shaped you. Surprising research reveals how.

by Jeffrey Kluger, Time Magazine, July 10, 2006

There are a lot of ways to study a painting, and one of the best is to get to know the painter. The splash or splatter of color makes a lot more sense when you understand the rage or whimsy or heart behind it. The songwriter, similarly, can lay bare the song, the poet the poem, the builder the building.

So what explains the complex bit of artistry that is the human personality? We may not be born as tabulae rasae. Any parent can tell you that each child comes from the womb with an individual temperament that seems preloaded at the factory. But from the moment of birth, a lot of things set to work on that temperament--moderating it, challenging it, annealing it, wounding it. What we're left with after 10 or 20 or 50 years is quite different from what we started out with.

For a long time, researchers have tried to nail down just what shapes us--or what, at least, shapes us most. And over the years, they've had a lot of eureka moments. First it was our parents, particularly our mothers. Then it was our genes. Next it was our peers, who show up last but hold great sway. And all those ideas were good ones--but only as far as they went.

The fact is once investigators had strip-mined all the data from those theories, they still came away with as many questions as answers. Somewhere, there was a sort of temperamental dark matter exerting an invisible gravitational pull of its own. More and more, scientists are concluding that this unexplained force is our siblings.

From the time they are born, our brothers and sisters are our collaborators and co-conspirators, our role models and cautionary tales. They are our scolds, protectors, goads, tormentors, playmates, counselors, sources of envy, objects of pride. They teach us how to resolve conflicts and how not to; how to conduct friendships and when to walk away from them. Sisters teach brothers about the mysteries of girls; brothers teach sisters about the puzzle of boys. Our spouses arrive comparatively late in our lives; our parents eventually leave us. Our siblings may be the only people we'll ever know who truly qualify as partners for life. "Siblings," says family sociologist Katherine Conger of the University of California, Davis, "are with us for the whole journey."

Within the scientific community, siblings have not been wholly ignored, but research has been limited mostly to discussions of birth order. Older sibs were said to be strivers; younger ones rebels; middle kids the lost souls. The stereotypes were broad, if not entirely untrue, and there the discussion mostly ended.

But all that's changing. At research centers in the U.S., Canada, Europe and elsewhere, investigators are launching a wealth of new studies into the sibling dynamic, looking at ways brothers and sisters steer one another into--or away from--risky behavior; how they form a protective buffer against family upheaval; how they educate one another about the opposite sex; how all siblings compete for family recognition and come to terms--or blows--over such impossibly charged issues as parental favoritism.

From that research, scientists are gaining intriguing insights into the people we become as adults. Does the manager who runs a congenial office call on the peacemaking skills learned in the family playroom? Does the student struggling with a professor who plays favorites summon up the coping skills acquired from dealing with a sister who was Daddy's girl? Do husbands and wives benefit from the intergender negotiations they waged when their most important partners were their sisters and brothers? All that is under investigation. "Siblings have just been off the radar screen until now," says Conger. But today serious work is revealing exactly how our brothers and sisters influence us.

•Why childhood fights between siblings can be good

THE FIRST THING THAT STRIKES contemporary researchers when they study siblings is the sheer quantity of time the kids spend in one another's presence and the power this has to teach them social skills. By the time children are 11, they devote about 33% of their free time to their siblings--more time than they spend with friends, parents, teachers or even by themselves--according to a well-regarded Penn State University study published in 1996. Later research, published last year, found that even adolescents, who have usually begun going their own way, devote at least 10 hours a week to activities with their siblings--a lot when you consider that with school, sports, dates and sleep, there aren't a whole lot of free hours left. In Mexican-American homes, where broods are generally bigger, the figure tops 17 hours.

"In general," says psychologist Daniel Shaw of the University of Pittsburgh, "parents serve the same big-picture role as doctors on grand rounds. Siblings are like the nurses on the ward. They're there every day." All that proximity breeds an awful lot of intimacy--and an awful lot of friction.

Laurie Kramer, professor of applied family studies at the University of Illinois at Urbana-Champaign, has found that, on average, sibs between 3 and 7 years old engage in some kind of conflict 3.5 times an hour. Kids in the 2-to-4 age group top out at 6.3--or more than one clash every 10 minutes, according to a Canadian study. "Getting along with a sister or brother," Kramer says dryly, "can be a frustrating experience."

But as much as all the fighting can set parents' hair on end, there's a lot of learning going on too, specifically about how conflicts, once begun, can be settled. Shaw and his colleagues conducted a years-long study in which they visited the homes of 90 2-year-old children who had at least one sibling, observing the target kids' innate temperaments and their parents' discipline styles. The researchers returned when the children were 5 and observed them again, this time in a structured play session with one close-in-age sib. The pairs were shown three toys but given only one to play with. They were told they could move onto the next one only when both agreed it was time to switch and further agreed which toy they wanted next.

That, as any parent knows, is a scenario trip-wired for fights--and that's what happened. The experimenters ranked the conflicts on a five-point scale, with one being a single cross word and five being a full-blown brawl. The next year, they went to the same children's schools to observe them at play and interview their teachers. Almost universally, the kids who practiced the best conflict-resolution skills at home carried those abilities into the classroom.

Certainly, there are other things that could account for what makes some kids battlers in school and others not. But the most powerful variables--parents and personality--were identified and their influence isolated during the course of the two-year-long observations. Socioeconomic status, an X factor that bedevils studies like this one, was controlled by selecting all the families from the same economic stratum. Distill those influences away and what is left is the interaction of the sibs. "Siblings have a socializing effect on one another," Shaw says. "When you tease out all the other variables, it's the play styles that make the difference. Unlike a relationship with friends, you're stuck with your sibs. You learn to negotiate things day to day."

It's that permanence, researchers believe, that makes siblings so valuable a rehearsal tool for later life. Adulthood, after all, is practically defined by peer relationships--the workplace, a marriage, the church building committee. As siblings, we may sulk and fume but by nighttime we still return to the same twin beds in the same shared room. Peace is made when one sib offers a toy or shares a thought or throws a pillow in a mock provocation that releases the lingering tension in a burst of roughhousing. Somewhere in there is the early training for the e-mail joke that breaks an office silence or the husband who signals that a fight is over by asking his wife what she thinks they should do about that fast-approaching vacation anyway. "Sibling relationships are where you learn all this," says developmental psychologist Susan McHale of Penn State University. "They are relationships between equals."

• How not being Mom's favorite can have its advantages

MULTICHILD HOUSEHOLDS CAN BE NOTHING short of palace courts, with alliances, feuds, grudges and loyalties, all changing day to day. Perhaps the touchiest problem in most such families is favoritism.

Parents feel a lot of guilt over the often evident if rarely admitted preference they harbor for one child over another--the sensitive mom who goes gooey over her son the poet, the hard-knocks dad who adores his tough-as-nails daughter. If favorites exist, however, it may be not the parents' fault, but evolution's.

The family began as--and remains--a survival unit, with parents agreeing to care for the kids, the kids agreeing to carry on the genes and all of them doing what they can to make sure no one gets eaten by wolves. But the resources that make this possible are limited. "Economic means, types of jobs, even love and affection are in finite supply," says psychologist Mark Feinberg of Penn State. Parents, despite themselves, are programmed to notice the child who seems most worthy of the investment. While millenniums of socialization have helped us resist and even reverse this impulse, and we often pour much of a family's wealth and energy into the care of the disabled or difficult child, our primal programming still draws us to the pretty, gifted ones.

Conger devised a study to test how widespread favoritism is. She assembled a group of 384 adolescent sibling pairs and their parents, visiting them three times over three years and questioning them all about their relationships, their sense of well-being and more. To see how they interacted as a group, she videotaped them as they worked through sample conflicts. Overall, she concluded that 65% of mothers and 70% of fathers exhibited a preference for one child--in most cases, the older one. What's more, the kids know what's going on. "They all say, 'Well, it makes sense that they would treat us differently, because he's older or we're a boy and a girl,'" Conger reports.

At first, kids appear to adapt well to the disparity and often learn to game the system, flipping blatant favoritism back to their shared advantage. "They'll say to one another, 'Why don't you ask Mom if we can go to the mall because she never says no to you,'" says Conger. But at a deeper level, second-tier children may pay a price. "They tend to be sadder and have more self-esteem questions," Conger says. "They feel like they're not as worthy, and they're trying to figure out why."

Think you're not still living the same reality show? Think again. It's no accident that employees in the workplace instinctively know which person to send into the lion's den of the corner office with a risky proposal or a bit of bad news. And it's no coincidence that the sense of hurt feelings and adolescent envy you get when that same colleague emerges with the proposal approved and the boss's applause seems so familiar. But what you summon up with the feelings you first had long ago is the knowledge you gained then too--that the smartest strategy is not to compete for approval but to strike a partnership with the favorite and spin the situation to benefit yourself as well. This idea did not occur to you de novo. You may know it now, but you learned it then.

•Why your sibling is--or isn't--your best role model

IT'S NO SECRET THAT BROTHERS AND SISTERS emulate one another or that the learning flows both up and down the age ladder. Younger siblings mimic the skills and strengths of older ones. Older sibs are prodded to attempt something new because they don't want to be shown up by a younger one who has already tried it. More complex--and in many ways more important--are those situations in which siblings don't mirror one another but differentiate themselves--a phenomenon psychologists call de-identification.

Alejandra and Sofia Romero, 5-year-old fraternal twins growing up in New York City, entered the world at almost the same instant but have gone their own ways ever since--at least in terms of temperament. Alejandra has more of a tolerance--even a taste--for rules and regimens. Sofia observed this (and her parents observed her observing it) and then distinguished herself as the looser, less disciplined of the two. Sofia is also the more garrulous, and Alejandra eventually became the more taciturn. "Sofie served as their mouthpiece," says Lisa Dreyer, 39, the girls' mother, "and Alejandra was perfectly happy to let her do it."

De-identification helps kids stake out personality turf inside the home, but it has another, far more important function: pushing some sibs away from risky behavior. On the whole, siblings pass on dangerous habits to one another in a depressingly predictable way. A girl with an older, pregnant teenage sister is four to six times as likely to become a teen mom herself, says Patricia East, a developmental psychologist at the University of California, San Diego. The same pattern holds for substance abuse. According to a paper published in the Journal of Drug Issues earlier this year, younger siblings whose older sibs drink are twice as likely to pick up the habit too. When it comes to smoking, the risk increases fourfold.

But some kids break the mold--and for surprising reasons. East conducted a five-year study of 227 families and found that those girls who don't follow their older sisters into pregnancy may be drawn not so much to the wisdom of the choice as to the mere fact that it's a different one. One teen mom in a family is a drama; two teen moms has a been-there-done-that quality to it. "She purposely goes the other way," says East. "She decides her sister's role is teen mom and hers will be high achiever."

Younger sibs may avoid tobacco for much the same reason. Three years ago, Joseph Rodgers, a psychologist at the University of Oklahoma, published a study of more than 9,500 young smokers. He found that while older brothers and sisters often do introduce younger ones to the habit, the closer they are in age, the more likely the younger one is to resist. Apparently, their proximity in years has already made them too similar. One conspicuous way for a baby brother to set himself apart is to look at the older sibling's smoking habits and then do the opposite.

• How a sibling of the opposite sex can affect whom you marry

FAR SUBTLER--AND OFTEN FAR SWEETER--than the risk-taking modeling that occurs among all sibs is the gender modeling that plays out between opposite-sex ones. Brothers and sisters can be fierce de-identifiers. In a study of adolescent boys and girls in central Pennsylvania, the boys unsurprisingly scored higher in such traits as independence and competitiveness while girls did better in empathic characteristics like sensitivity and helpfulness. What was less expected is that when kids grow up with an opposite-sex sibling, such exposure doesn't temper gender-linked traits but accentuates them. Both boys and girls hew closer still to gender stereotype and even seek friends who conform to those norms. "It's known as niche picking," says Kimberly Updegraff, a professor of family and human development at Arizona State University and the person who conducted the study. "By having a sibling who is one way, you strive to be different."

But as kids get older, that distance from the other gender must, of necessity, close. Here kids with opposite-sex siblings have a marked advantage. Last year William Ickes, a psychologist at the University of Texas at Arlington, published a study in which he paired up male and female students--all of whom had grown up with an opposite-sex sibling--and set them to chatting with one another. Then he questioned the subjects about how the conversation went. In general, boys with older sisters or girls with older brothers were less fumbling at getting things going and kept the exchange flowing much more naturally.

"The guys who had older sisters had more involving interactions and were liked significantly more by their new female acquaintances," says Ickes. "Women with older brothers were more likely to strike up a conversation with the male stranger and to smile at him more than he smiled at her."

If siblings can indeed be as powerful an influence on one another as all the research suggests, are all siblings created at least potentially equal? What about half-sibs and stepsibs? Do they reap--and confer--the same benefits? Research findings are a bit scattered on this, if only because shared or reconstituted families can be so complicated. A dysfunctional home in which parents and siblings hunker behind barricades alongside the ones they're biologically closest to does not lend itself to good sibling ties. Well-blended families, on the other hand, may produce step- or half-siblings who are extraordinarily close. One of the best studies on this topic is being conducted in Britain with a large group of many different kinds of nontraditional families. In general, the researchers have found that the intensity of the relationships closely follows the degree of physical relatedness. No hard rules have emerged, but the more genes you share, the more deeply invested you tend to grow. "Biological siblings just get into it more," says Thomas O'Connor, an associate professor of psychiatry at the University of Rochester Medical Center. "They are warmer and also more conflicted."

• How those early bonds can grow stronger with age

ONE OF THE GREATEST GIFTS OF THE SIBLING tie is that while warmth grows over time, the conflicts often fade. After the shooting stops, even the fiercest sibling wars leave little lasting damage. Indeed, siblings who battled a lot as kids may become closer as adults--and more emotionally skilled too, often clearly recalling what their long-ago fights were about and the lessons they took from them. "I'm very sensitized to the fact that it's important to listen to others," a respondent wrote in a recent study conducted in Britain. "People get over their anger, and people who disagree are not terrible," wrote another. Even those with troubled or self-destructive siblings came away with something valuable: they learned patience, acceptance and cautionary lessons. "[You] cannot change others," wrote one. "[But] I wasn't going to be like that."

Full-blown childhood crises may forge even stronger lifelong links. The death of a parent blows some families to bits. But when older sibs step in to help raise younger ones, the dual role of contemporary and caretaker can lay the foundation for an indestructible closeness later on. Wayne Duvall, 48, a television and film actor in New York City and the youngest of three brothers, was just 13 when his father died. His older brothers, who had let him get away with all manner of mischief when both parents were in residence, intuitively knew that the family no longer had that luxury. "I vividly remember them leaning down to me and saying, 'The party's over,'" Duvall recalls. "My brothers are my best friends now, though they still consider me the little brother in every imaginable way."

Such powerful connections become even more important as the inevitable illnesses or widowhood of late life lead us to lean on the people we've known the longest. Even siblings who drift apart in their middle years tend to drift back together as they age. "The relationship is especially strong between sisters," who are more likely to be predeceased by their spouses than brothers are, says Judy Dunn, a developmental psychologist at London's Kings College. "When asked what contributes to the importance of the relationship now, they say it's the shared early childhood experiences, which cast a long shadow for all of us."

Of course, that shadow--like all shadows--is a thing created by light. Siblings, by any measure, are one of nature's better brainstorms, and all the new studies on how they make us who we are is one of science's. But the rest of us, outside the lab, see it in a more primal way. In a world that's too big, too scary and too often too lonely, we come to realize that there's nothing like having a band of brothers--and sisters--to venture out with you.

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