Parents try everything to influence their children. But new research suggests brothers and sisters have their own profound impact.

When I was 14, I spent most of my free time doing two things that I loved: I ran a lot, and I played a lot of piano, even though by high school it was clear I’d never really excel at either. Once, as a friend and I did the customary prerace walk of a cross-country course, we were so involved in conversation that we realized we were late only when we heard, in the far distance, the bang of the starting gun. And when my piano teacher tried to explain what I was doing wrong, she sometimes imitated my playing in a way that made clear I was not destined for Juilliard.

That year, my freshman year of high school, my brother, who is six years older, came home from college for Thanksgiving break and informed me that he thought I should join the high school newspaper. There was no newspaper, I told him; at some point, it disbanded from lack of interest. I can still picture my brother in the doorway of my bedroom: I longed to return to whatever I was reading (“A Tree Grows in Brooklyn”? “The Clan of the Cave Bear”?), but he stood there for what felt like an eternity, lecturing me about the decline of democracy without a free press, and the apathy of my classmates; the high school had to have a newspaper, and if no one else was going to revive it, he insisted, then I must.

I can only imagine how I would have responded had my parents given the same lecture: probably not at all. Like most teenagers, I was somewhat developmentally programmed to reject whatever they felt most eager to impress on me. But also — they didn’t suggest it. They didn’t have particularly strong feelings about our high school newspaper or whether we should have one, and maybe they didn’t know me the way my brother knew me.

Parents, I sometimes think, forever see their children as fixed, essentially unchanged from who they were when they first entered the world — as, say, a fussy baby or overeager toddler. I was the youngest of three, passive, a watcher more than a doer, someone who had to learn to talk fast because if I didn’t, I’d never get a word in edgewise at dinner. Siblings see one another out in the wild, how they interact with other children; siblings are spies, forever sizing up the competition, sometimes threatened, but just as often proud.

I did not want to face another lecture when my brother next came home. I valued not just his opinion but also his high opinion of me — and he thought I was someone who could start a high school newspaper. And so, to my surprise, I did. It didn’t win awards or break any news (I seem to remember a lot of editorials about student apathy). But as soon as I sat down with the first assignments that came trickling in, I knew I was in the right place. When my piano teacher told me I needed to drop my other extracurricular activities and focus on piano or find another teacher (subtext for: What is the point of all this mediocrity, really?), I didn’t think twice — the newspaper was my priority. My brother had all but bullied me into finding a vocation in journalism: He knew my environment, he knew what high school was supposed to be like and he knew me.

When we think about the forces that shape us, we inevitably turn to parents. The parent-child relationship is the basis of probably half a millennium’s worth of psychoanalytic conversation and intellectual discourse; parenting books are perennial best sellers, with advice that fluctuates as often as the health advice on what to eat or drink and how much. Their whiplashing instructions don’t stop many parents from reading them, and who can blame those mothers and fathers: Children are baffling, variable, not that verbal — and parents also know that if they get it wrong, their kids will blame them for just about everything.

And yet researchers, after analyzing thousands of twin studies, have come to the conclusion that the shared environment — the environment that siblings have in common, which includes parents — seems to do precious little to make fraternal twins particularly alike in many ways. They can be exposed to the same rules of oboe practice, dinnertime rituals, punishments, family values and parental harmony or discord, and none of it really matters in many key regards — siblings’ personalities may very well end up as different as those of any two strangers on the street. No one would argue that parenting doesn’t matter; it’s just that the choices so many loving parents agonize over — whether to co-sleep or not, whether to enforce the rules rigidly or sometimes let them go — don’t matter nearly as much as we imagine they do. Nor does that mean that genes are all-powerful; it’s just that nurture comprises so much more than parenting — the environmental effects a child is exposed to are vast, and include (just to start) the media they consume and the friends and teachers in whose company they spend most of the day.

And then there are siblings. “I think the influence of siblings on each other is an area in psychology that has not nearly received the attention it deserves,” says Lisa Damour, a psychologist and author who writes about adolescence. “When we look at child development, our main frameworks have been around the influence of parents on children, and that’s the established tradition that we’ve had a hard time moving past.”

Anyone raising more than one child, Damour says, or who has a sibling, intuitively knows that sibling relationships play a powerful role in affecting who we become. “If parents are the fixed stars in the child’s universe, the vaguely understood, distant but constant celestial spheres, siblings are the dazzling, sometimes scorching comets nearby,” wrote Alison Gopnik, a developmental psychologist, in a review of a book about siblings in 2011.

A body of research has been growing in recent years that adds clarity and depth to our understanding of how significant the impact of siblings can be.

Researchers have studied how siblings influence one another’s choices and life trajectories through competition; they have uncovered deeper knowledge of so-called spillover effects, the ripple effects of how one sibling’s experiences affect another’s; and they have brought rigor to bear on popular ideas about birth order. New tools of genetic research may challenge or sharpen previously held conceptions about siblings. The data set of families whose stories might shed light on some of these interactions is limitless, but one place to look is at siblings who have long fascinated me: those in families in which a surprising number of brothers and sisters have found their way to the top of the ladder of success.

Some of the dynamics described in the research, in those families, might be borne out in the extreme. Siblings, at their best, can urge one another on; competing and collaborating — whether intentionally or not — they help chart the course of one another’s lives.

Psychologists have long believed that siblings tend to find ways to differentiate themselves from one another, sharpening some edges, softening others, forcing one another into roles that can coexist within the space of their family. Certainly, the siblings in one extraordinary family I met, the Groffs, seem to bear that out with unusual clarity, pushing one another into distinct domains in which they all succeeded at exceptionally high levels. Sarah True, 43, is an Olympic triathlete turned world-class Ironman champion; her older sister, Lauren Groff, 46, is a celebrated novelist and a three-time finalist for the National Book Award; and Adam Groff, the oldest, 48, simultaneously acquired an M.D. and an M.B.A. at the University of Pennsylvania, then went on to become a serial entrepreneur in the health care field, and is an attending physician at Dartmouth Hitchcock Medical Center.

All three of the Groff siblings speak, with some reverence, of the industriousness and achievements of their parents — Jerry Groff, a doctor, and Jeannine Groff, who worked for many years as a science teacher. Each came from a hardscrabble background and attended college with work-study funding, eventually building together a life of comfort and opportunity for their children. But Sarah and Lauren would say that their siblings influenced them at least as much as, if not more than, their parents did.

Lauren Groff, 15 months younger than her brother, Adam, recalls feeling compelled, constantly, to assert herself in the face of what she perceived as his dominance. “I was an immense introvert with an older brother who wouldn’t let me speak,” she once told a reporter from The Guardian, “so I was the biggest reader.” She generally retreated into her room, into the books that kept her company there and provided emotional refuge. “Without it” — by “it” she means Adam’s standard-issue older-brother teasing — “I never would have read six billion books,” she told me. “The mere fact of his being older and smarter shaped me in particular, so as a young woman I was just trying to keep up and trying to show him that not only was I a worthwhile human being, but I was equal to him.”

Lauren is now a best-selling author who has published seven works of fiction, someone who serves on judging panels for top literary prizes; but she still believes that the emotional response to her perception of Adam’s superiority, and his self-certainty, lingered on from childhood. Eighty percent of what drove her to push herself in life for many years, Lauren estimates, was Adam.

Her younger sister, Sarah, in turn, felt the pressure of having not one but two high-performing siblings. She felt an uneasy admiration for her sister and brother, both of whom excelled at school in ways she feared she might not be able to match.

It is an uncomfortable fact of sibling research that — on average — birth order within a family does tend to predict which of the siblings will perform best at school. A robust body of research shows the same consistent (if relatively small) effect: The oldest sibling in a family tends to get the strongest grades. Some research has found that parents enforce rules with oldest siblings more stringently, but there are also benefits to the firstborn that start as early as pregnancy: Mothers tend to take better care of their prenatal health with their first babies, and parents lavish more attention on that first child during those developmentally crucial first months. Researchers analyzed databases comprising some 5,000 American children and found that the oldest children in the family performed best on cognitive tests as early as their first birthday, compared with their younger siblings when they reached the same age — and that those oldest children also went on to win better grades in school. Parents may love their children equally; but there’s no easy way of getting around the reality that they have more time to spend enriching the environment of one child than they do of two or more.

Whether Sarah’s siblings surpassed her academically, or she simply perceived that they did, Sarah chose to do what many younger siblings do: She sought out a niche outside the realm of academics. Sarah’s first interest in swimming competitively was inspired by her love of the sport, she believes, but also by her desire to surpass Lauren in some domain. “I didn’t feel like I was trying to measure up to my siblings to please my parents,” Sarah says. “It was for me — it was omnipresent. I was never going to be good enough, and that was a failure of something about me — because we all had the same upbringing. I don’t think that I would be a professional athlete if it weren’t for my siblings. I wouldn’t have tried to carve out my own little world the way that I have.”

Some research has found that younger siblings are overrepresented in athletic pursuits. Matt Robinson, head of the sport-management program at the University of Delaware, worked with April Heinrichs, who at the time was the director of the U.S. Soccer Federation’s Women’s Youth National Team Program (and a former national player herself), to conduct a survey of around 250 female soccer players, each of whom trained for the U.S. women’s national team in her age category (ranging from under 14 to 23). The results, published in Soccer Journal in 2014, found that close to three-quarters of females playing at that elite level were younger siblings.

The researchers theorized that these girls and women pushed themselves so they could hold their own when they kicked a ball with their older siblings. The constant informal competition may have improved their game, and it was also possible that their parents could better navigate their way through choosing teams, training and coaching, having been through it with an older child. Younger siblings are also likely to start playing sports earlier. It might not dawn on a parent to hand a 2-year-old a soccer ball, but if her 5-year-old sister happens to be playing with one, that toddler might be drawn to it.

Jonah Berger, a professor of marketing at the Wharton School, in his book, “Invisible Influence,” explores the phenomenon of younger siblings in sports. He noted one study of some 250 athletes, all of whom came from families with about two children. The study found that younger siblings are overrepresented at the elite level even if they don’t have older siblings who played the same sport — which, he argues, suggests that younger siblings are seeking to differentiate themselves from their academically stronger older siblings, rather than just benefiting from their earlier expertise.

For Sarah True, the idea that swimming might be a way of finding her own area of excellence seems to have solidified by the time she was 14, when she decided to swim the length of Otsego Lake, a nine-mile body of water right outside her family’s home in Cooperstown, N.Y. When she told her parents her intention, her father had his doubts. “Honey, that’s a long way,” he told her. But Sarah was somehow confident she could do it. “I probably undertrained,” Sarah says now. “I knew other people had done it. I could swim.”

Everyone in the family had some concerns that she might not be able to pull off this swim, typically a five-hour proposition for amateur swimmers. “This is going to be a disaster,” her father told his brother, who was visiting.

The day of the swim, Sarah’s brother, Adam, and her father rowed alongside her as she made her attempt to cross the lake. Three hours 49 minutes later, Sarah stood, shaking a little, on a dock on the far shore. Adam’s hands were raw with blisters. Sarah had completed all nine miles and, to the astonishment of everyone in her family, had also broken the town record for all swimmers of any age. (Her record still stands.)

“Just the mere fact of swimming the lake, for my sister — it was a way of distinguishing herself,” Lauren says. “She was a third child — it was psychically huge in her life. It turned her into who she is, and not just as an athlete. It showed her who she was.”

How siblings differentiate is often a function of their family’s resources. Providing one child flute lessons and another tennis coaching, having the time to shuttle children around — those are luxuries middle- and upper-class parents can offer. Annette Lareau, a professor of sociology at the University of Pennsylvania, explores those kinds of class differences in child rearing in her landmark work, “Unequal Childhoods.” Siblings in working-class and poorer homes partake in fewer extracurriculars, she found. That means, in turn, that the siblings often spend more time together, increasing the likelihood that they’ll influence one another in myriad ways.

Emma Zang, a professor of sociology at Yale University, was part of a cohort of researchers who realized the importance of this insight. She thought that the possibility of this influence might be used to create policies or interventions that could maximize opportunity for low-income children. Zang wondered: If you could improve an older child’s academic experience, would that benefit spill over to younger siblings too?

To figure that out, Zang looked at school-start data from North Carolina for thousands of students who entered school from 1988 to 2003. A large body of research suggests that students who happen to be relatively old for their grade tend to do better in school. Zang wanted to know if the younger siblings of those students would benefit from their older siblings’ advantages, and she found that the answer was yes: The younger siblings of kids who were among the oldest for their grade performed better academically, landing higher test scores than the younger siblings of children who entered school on the young side — and that was true regardless of whether those younger siblings were old or young for their grade.

Joshua Goodman, an associate professor of education and economics at Boston University, found a similarly striking effect at the college level. Goodman looked at a data set of students whose SAT scores were right on the margin of a cutoff point established for admission to what he called “target colleges.” The candidates were essentially equivalent, with scores that differed by no more than 10 SAT points, a function of one student’s getting maybe just one more question right — a difference so slight that it might be left to chance; but on average, those right above the threshold gained admittance, and the ones right below didn’t. Goodman found that the younger siblings of those who were admitted were significantly more likely to end up at an equally selective college than those whose older siblings missed out by just a few points.

The younger siblings who ended up at selective colleges may have had their expectations raised; they could see a path forward; they could benefit from what their older siblings did.

Michelle Obama’s experience in college can be seen as a reflection of Goodman’s findings, though she applied decades before he undertook his research. Obama’s parents raised her in a working-class neighborhood on the South Side of Chicago. Her older brother, Craig, was a strong student, but Ivy League schools were not on their parents’ radar. Craig, however, also had the benefit of being a star athlete, which is why he was recruited to play basketball at Princeton University. As Obama writes in her book “Becoming,” seeing where her brother ended up expanded her own sense of possibility. “No one in my immediate family had much in the way of direct experience with college, so there was little, anyway, to debate or explore,” Obama wrote about a visit to her brother at college. “As had always been the case, I figured that whatever Craig liked, I would like, too, and that whatever he could accomplish, I could as well. And with that, Princeton became my top choice for school.” A guidance counselor told her, she recalls, she was “not Princeton material”; that did not dissuade Obama. She writes of her own faith in herself; but it’s very likely that she knew her brother well enough to assess his talents relative to her own. She knew that if he was Princeton material, she surely was as well.

Zang’s and Goodman’s findings suggest that effective interventions aimed at one child in a lower-income family might have positive knock-on effects for their siblings too, which means that successful interventions could have more impact than previously realized: Improve the older sibling’s experience, and it could have ripple effects that change the trajectory of the entire family.

Zang’s research found that nearly a third of siblings’ academic similarity can be attributed to the spillover effect (as opposed to their shared environment or their overlapping genetics). But the spillover effect can work negatively as well, especially in disadvantaged families. A child growing up in a disadvantaged home is more likely to suffer academically because of various disruptions; but that child’s academics will additionally suffer from whatever traumatic exposures have hurt his sibling’s success at school, Zang theorizes. Because test scores are reliable predictors of income later in life, sibling influences in these families can translate to lower lifetime earnings.

Zang and Goodman both found the spillover effect to be strongest in less-advantaged families, highlighting the need for researchers to appreciate that sibling influence functions differently across class lines. A study published in 2022 in Frontiers in Psychology, for example, complicated the often-replicated finding that oldest siblings are the most academically high-achieving in their families. The oldest siblings in high-risk families and in families in which the parents are not native English speakers do not, in fact, score higher on cognitive tests when they reach age 2 or show more school readiness at age 4. In those families, there is no birth-order effect, or the younger children score higher, probably because they benefit from their older siblings’ fluency and the experience their parents gain over time in interacting with preschools and schools.

I saw compelling signs of positive sibling spillover effects in a family of four siblings, the Chens, who grew up in Bristol, a small city in Virginia. The first three Chen children emigrated from China in 1994 along with their parents, neither of whom attended college or spoke English fluently. Elizabeth, the oldest, would eventually become a doctor, working closely with many Chinese immigrants (to protect her privacy, she asked us to use her American name). Yi, now the chief executive of an A.I. business, was a member of the starting team of five at Toast, the restaurant-software platform that had, in 2021, the biggest tech public offering in Boston’s history; Gang now works with a leading A.I. language-instruction business; and Devon, who was born in the United States, is a software developer with Amazon.

Elizabeth, when she arrived in the United States at age 10, spoke little English; as a result, her mother decided to put her in a class two grades below that of her age. Teachers always liked her, she told me, which she thought, even as a young girl, was a function of positive stereotyping: They expected her to be a good student because she was Asian. But maybe she simply had the benefits of being a couple of years older than everyone else in third grade: She may have been more organized, had more self-control. She might have become a teacher’s pet for that reason; school became a warm environment where she could excel. Her expertise became school; she passed it down to her two younger brothers, who, in turn, also excelled.

Because their parents, who owned and ran a Chinese restaurant in town, worked long hours, the three older children leaned heavily on one another for support. A cousin who lived with them for several years said that by the time he arrived, the oldest three siblings, then in high school, seemed to him to be one another’s primary motivators. “The way those three pushed each other was the key to their success,” he observes, with the siblings’ interactions amplifying whatever talents they had.

Their mother insisted that they each take up at least one instrument; but the siblings were the ones who helped one another grow musically. When their cousin pictures one Chen child playing piano, a sibling is often on the bench as well, refining the younger sibling’s technique; they leaned over homework together, the older teaching the younger. Elizabeth coached Yi about which A.P. courses to take and, when the time came, looked over his college applications; they both did the same for their younger brother Gang when the time came. Years later, they’d all chip in, as a team, to help their youngest brother apply to college. All four children were valedictorians in their respective grades (although Devon’s margin was uncomfortably small for Elizabeth. “He did all right,” she says, sighing). “We each wanted the younger one to do better than we did,” Yi explains.

Elizabeth guided her siblings in other ways — she encouraged her brother Yi, one year Gang’s senior, to throw his energy into wrestling, believing it was the rare sport in which someone Yi’s size (he was slight compared with his peers) would not be disadvantaged. Yi was a fierce competitor, and consistently ranked second statewide in his weight class his senior year. It was only natural that Gang would take up wrestling, too, given how close he was to Yi — at which point Yi devoted himself to training Gang, pinning him, talking him through maneuvers, pushing him to the point of exhaustion. “I wanted Gang to be better than me,” he said. “That was my goal.” The brothers believe that Gang could have gone further than Yi, competitively — except that Gang was also the most accomplished musician of the three of them, a saxophone player who landed the first-chair tenor spot in the statewide band, and could therefore not devote himself fully to the sport.

When Gang was a senior in high school and his sister was a pre-med student at Vanderbilt University, he applied to Yale. The day acceptances went out, he was competing in a wrestling tournament and away from his computer. From a lab where she was working, Elizabeth tried to log into Yale’s admissions portal on his behalf, but he could not remember his PIN and kept giving her the wrong ones. “You’re such an idiot, you don’t deserve to get in,” Elizabeth, agitated, yelled at him. She was nervous, not just because she wanted the best for him but also because she knew he wrote his college essay about her — about the way that she first translated America for him, told him what to wear to school, when to make sure he had money from home for lunch or for a school trip. She worried that “if he didn’t get in, it was because I wasn’t good enough,” Elizabeth says.

Finally, Gang remembered his PIN and gave it to Elizabeth. Later, she and a manager at Elizabeth’s lab who was there at the time would laugh about it: One minute she was screaming at her brother about what an idiot he was, and the next she was silent, tears running down her face, as she stared at the word scrolling across the screen: Congratulations.

When families try to make sense of their own internal workings, they often turn to theories about birth order — homegrown conceptions or schema that have been widely popularized — to explain why the various siblings act the way they do. In 1996, Frank J. Sulloway’s book on the topic, “Born to Rebel,” quickly became a runaway best seller, praised by intellectuals like E.O. Wilson, who called it “one of the most authoritative and important treatises in the history of the social sciences.”

Sulloway argued that oldest children, who spend the most time alone with their parents and tend to identify with them, become conscientious and are inclined to reinforce the status quo; younger children, by contrast, are more likely to rebel and innovate. Sulloway drew on historical data to make the case that younger children were significantly overrepresented in uprisings like the French Revolution, and were responsible for a disproportionately high number of the scientific discoveries that required the greatest breaks with traditional thinking.

Many of Sulloway’s findings have since been widely challenged. Sulloway argued, for example, that birth order was a better predictor of social attitudes — like upholding conventional values — than gender, race, or social class; and yet an analysis of a survey of 1,945 adults published in American Sociological Review in 1999 found the opposite to be true. And other research considered the gold standard since then has found that when it comes to the “big five” personality traits — conscientiousness, agreeability, openness, neuroticism and extroversion — birth order doesn’t seem to have an impact.

One problem with a lot of birth-order research is what is known as confirmation bias: Subjects in studies who were asked about their siblings may well have had preconceptions about siblings and birth order that they were mapping on to their own family. Parents, too, are subject to those kinds of preconceived notions; from there, expectations can be set that can have long-term consequences. Consider the brothers Emanuel, who grew up in Chicago, the children of a mother who devoted herself to civil rights activism when she wasn’t trying to keep her three rambunctious boys in check and a father who was a social-justice-minded physician. Ezekiel Emanuel is now a prominent bioethicist at the University of Pennsylvania; Rahm Emanuel served as ambassador to Japan and was chief of staff to President Barack Obama; and Ari Emanuel is one of the country’s most powerful sports-and-entertainment executives.

Ezekiel’s path seemed to unfold in keeping with Sulloway’s ideas: He was the oldest child, the conscientious one who would follow his father into the field of medicine. He was not yet 6, he writes in his book “Brothers Emanuel,” when his parents started suggesting that he ought to go into medicine. “I was the firstborn child of an immigrant who himself was a doctor,” he wrote. “Plus I was a goody-goody and got good grades in all my school subjects, and I especially liked science, where I could literally poke and probe nature.” He continues: “It seemed almost predetermined that I should be a doctor. That I was going in the medical direction relieved Rahm and Ari of any career pressure.”

His brothers have said that of the three sons, Ezekiel was the smartest. And maybe he was the most well suited to his father’s profession; or maybe his parents projected that idea onto him, one that he and his siblings adopted. It’s incontrovertibly true that he was the oldest, and therefore statistically more likely to excel; but all research on birth order deals in averages. It’s predictive rather than conclusive. Was he inherently better suited than his brothers to become a doctor? Either way, his brothers’ careers clearly did not suffer from their parents’ focus on Ezekiel’s future profession.

But parents’ subjective beliefs about their children’s relative intelligence or qualities — whatever the cause of that assessment — can sometimes serve their children poorly, especially because they’re not always right. In 2015, Susan McHale, now an emeritus professor of human development and family studies at Penn State, was an author of a study in The Journal of Family Psychology that found that even when two siblings’ grades were essentially equivalent, parents often had beliefs that one student was more academically talented than the other; and that belief seemed to predict even better grades down the road for the student possibly misperceived to be more academic. The students thought to be more academic also expressed more interest in academic extracurricular activities than the other sibling, who, McHale theorized, might well have been a good student but seemed less likely to perceive that to be true. “When one parent thought one kid was smarter than the other, that kid got progressively better over time,” she said. “Little differences had increasingly big implications over time, by virtue of the social comparison.”

Dalton Conley, a sociologist by training who also has a Ph.D in biology, is interested in trying to untangle innate inclinations from environmental influences like those that can exist within a family. For example: Do parents make assumptions about their children’s talents, relative to one another, because the children are in fact innately gifted in those areas, or do parents push their children — or do the children push themselves — in one direction or another because of certain family dynamics?

Conley believes that advances in genetic analysis may make it possible to answer some of these questions. Over the last decade, scientists have analyzed tens of thousands of individuals’ genomes, creating a bank of genetic data that reveals genetic markers for various traits. For any given person, genetic analysis can now generate what are known as polygenic scores: numbers (still crude and somewhat controversial) that denote an individual’s genetic predispositions for given qualities (or illnesses), based on their combined genetic variants. Polygenic scores tell us to some limited extent how much someone’s genetic variants increase that person’s likelihood of obtaining a college degree, for example.

While this research is in its early stages, Conley believes that polygenic scores will eventually make it possible to tease out the ways that siblings’ effects on one another — as opposed to their genetic overlap — improve their opportunities or possibly hold them back. He is now undertaking research to try to determine, by looking at polygenic scores, whether parents sometimes end up assigning their children niches in ways that might even defy their natural leanings. Conley, the author of “The Social Genome,” a book that explores the interaction of nature and nurture, wondered, for example, about a student who had a strong aptitude for math, as indicated by a polygenic score — but was an even more outstanding athlete. If he had a brother who was not as strong a math student but was extremely, noticeably unathletic, would the family come to believe that the second brother was, in fact, the family’s math student — that he was even innately more gifted at math?

Conley believes that working with polygenic scores could unlock many of the mysteries and mechanisms of family systems that, to date, have largely been understood only at the level of theory. “What happens in families has been a black box for as long as human or social sciences existed,” Conley says. “I think we have an opportunity to really understand family dynamics much better now with these tools.” He believes that for humans, who are such fundamentally social creatures, a better understanding of the interplay of nature and nurture would be meaningful. “All these intuitions about siblings that we think are true — now we have a way to test them,” he says.

Parents who feel the pressure of making choices that maximize their children’s potential may also feel that it falls on them to shape their siblings’ interactions — how close they are; whether they collaborate or compete; whether, if they do compete, that competition is good-natured or unhealthy. But if parenting one child is a task that can feel like a never-ending, possibly unwinnable game of checkers, then trying to manage a sibling relationship would be more like playing chess, blindfolded — it’s so multifaceted and complicated, with so many random events bearing down on the children, which in turn affect how the siblings interact with one another. A parent would have to be nothing short of a savant to engineer the relationship for maximum peace and fulfillment.

Or maybe it would require nothing short of clairvoyance, because so much of what happens in our children’s lives — in our own lives — turns on quirks of fate. A decision hinges on a mood or on happenstance, and a life is changed, a personality tweaked as a result, yielding a newly altered person who will interact, on and on, with an endless succession of tiny formative moments.

About three years after Sarah, the eventual Olympian, left her mark on Otsego Lake, Lauren, who would become the novelist, decided she was going to attempt the same swim. When she was about a mile from finishing, her father, who was rowing alongside her, let her know that she was within reach of setting a record — she need only pick up the pace a little bit. “To be perfectly frank,” Lauren says, “I’m a very competitive person, so there was this war going on.” She thought about what it would feel like to break the record. Then she thought about her younger sister. Lauren decided to keep swimming at precisely the same pace. Sarah’s record would stand.

“Sarah was young,” she says. “It was the thing that she was really proud about.”

It’s hard to know what it would have meant to Sarah, as a teenager, had Lauren’s time beaten her own that day on the lake. Had Lauren made a different choice, would Sarah have gone on to pursue the triathlon at college, eventually competing in the Olympics as a triathlete — not once but twice — and tackling the 140.6-mile Ironman race, winning three of them, proving herself one of its greatest female competitors?

It’s easy to say now that it turned out for the best; all the Groffs would agree.

We can’t choose our families, but we can choose the stories we tell ourselves about them.

Article by Susan Dominus
Published May 19, 2025
Published by The New York Times