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.