"Must be the third kid thing." That’s what one of the other moms said to me as we were both watching our youngest children play fourth grade basketball as if their lives depended on it. Like me, the other mother had two older kids who, when confronted with a ball they were supposed to steal from another child, could take it or leave it. But our youngest ones—our "thirds"—clearly had something to prove.

As it turned out our observations actually align with the data. In 2014, 257 girls who had trained for the U.S. women’s soccer team in their age group took a survey. Close to three quarters were younger siblings. In her new book, The Family Dynamic: A Journey into the Mystery of Sibling Success, Susan Dominus recounts how researchers have interpreted this finding. First, the younger siblings were forced to play up a level if they wanted to hold their own with their older siblings. Second, their parents might have known better how to navigate the system if the older kids already went through it. Third, the younger child is likelier to start the game at a younger age. And finally, if, as studies consistently hold, the oldest child is more likely to stand out academically, then "younger siblings focus more on sports because that is the niche available to them—the domain where they can stand out."

There is no shortage of books on how to be a better parent. Whether it’s gentle parenting or tough parenting, parenting like they do in France or like they do in China, the authors often struggle with the age-old question of how much of the results are the product of nurture and how much are nature. Can we all have kids who are as smart and successful as those of Tiger Mom Amy Chua if we follow her advice? Or does the fact that she and her husband are both Yale Law professors mean their genes might have had something to do with it?

Studying siblings, who share similar upbringings and also some genetic material, may provide insight into what parents are doing right and wrong. But sibling differences are also large. Whereas one’s socioeconomic status is generally a good predictor of one’s educational attainment, one researcher notes that in explaining economic inequality in America, "sibling differences represent about three-quarters of all the differences between individuals." Parents may have different resources for their oldest and youngest siblings, depending on their economic situation at the time or which sibling they think holds the greatest promise.

But siblings are also their own x-factor. An older sibling’s academic success may encourage a younger one. Especially in families where attending college is not a given, an older sibling’s attendance of a selective school significantly influences the likelihood a younger one will go to the same school or a similar one.

There is also the competition among siblings. Award-winning novelist Lauren Groff tells Dominus that "eighty percent" of what drove her to push herself was teasing from her older brother Adam, now a doctor and serial entrepreneur. And their younger sister Sarah, a triathlete who competed in the 2016 Olympics, was inspired to become a faster swimmer by Lauren’s talent when they were younger.

The Family Dynamic tells the story of a number of sibling groups like the Groffs, brothers and sisters, she says, who are mostly or entirely high achievers. From the Brontë sisters to the Wojcickis—one of whom founded 23andMe, one who was the CEO of YouTube, and another who was a renowned anthropologist—Dominus wonders about the environment in which these children were raised and the parenting they experienced.

While competition was a common experience among the brothers and sisters, so was support. Indeed, parents in some cases tried to tamp down the competition. Wayne Bryan, father to the extremely successful tennis players Mike and Bob Bryan, wouldn’t let his children compete against each other in tournaments if both boys made it to the final round. He thought there was already enough natural competition between them.

For some—like the Holifields, whose three siblings went on to illustrious careers in law and medicine—it’s clear their parents had high expectations for them. Growing up black in the segregated South, they were never allowed to enter an establishment like a movie theater that wouldn’t treat them as it treated whites. They were among the first to integrate into the schools around them. But Dominus also credits their environment, growing up in an area near Florida A&M, which had a significant black middle class, allowing them access to more resources and more people who could help them than they might otherwise have.

More than one of the families experienced the death of another child, and the expectations of parents for the children who followed were changed. The Holifields made sure their children were excellent swimmers because an older child had drowned. The Murguías, who went on to be federal judges and work in the White House, had an older sibling with significant developmental delays. "All the siblings felt responsible for Martha, their big sister, and in some ways she was the family member who most shaped her siblings. … [Their mother’s] worry about Martha was never-ending. The children grew up knowing it, maybe faster because of it. They tried not to add to their parents’ worries."

Many of the sibling groups Dominus chronicles bonded together during times of hardship. In the case of the Chens—who attended Ivy League schools and have found successful careers in medicine and Silicon Valley—the oldest, Elizabeth, can still remember her mother fleeing Chinese authorities who were going to force her to abort her third child. She remembers her parents leaving her and her siblings for years while they went to start their lives in America. And she remembers how they started a Chinese restaurant in Appalachia, with the kids doing their homework every day there while they helped to clean and cook, each sibling encouraging the ones who came after him or her.

We are often quick to credit the immigrant parents with their children’s success, but Dominus notes that while parents "unleash the arrow of ambition … siblings are often the ones who guide the direction of the arrow into the future."

Indeed, it is not just the striving of parents that makes for children’s success into adulthood. It may also be their inability to micromanage their children’s lives. The Colfaxes raised their children "on the land" in the 1970s because they couldn’t earn a living as academics. They had a "screen-free household," but they also had no electricity or telephone. The kids built gardens and treehouses and raised goats. And so it was no surprise they turned into very capable adults.

"Children," writes Dominus, "are capable of so much more than we think—and they thrive when they are made to feel that their contributions are vital." Being part of a sibling group—particularly one with at least three or four children if the stories in this book are representative—can further that sense of efficacy. In an era where more people are having fewer children, though, it seems that many of those children will be raised without this sense.

The Family Dynamic: A Journey into the Mystery of Sibling Success
by Susan Dominus
Crown, 368 pp., $30

Naomi Schaefer Riley, a senior fellow at the American Enterprise Institute and the Independent Women's Forum, is the author of No Way to Treat a Child: How the Foster Care System, Family Courts, and Racial Activists Are Wrecking Young Lives.