A Subtle Shift Shaking Up Sibling Relationships

Growing up with a sibling who is much younger than you are can be a profoundly humbling experience. In casual conversation, you might suddenly find yourself fumbling to parse Gen Z terminology or pretending to know the identities of the alleged celebrities they keep name-dropping. You don’t even duke it out in the same way. Whereas siblings close in age might skirmish over whose turn it is to pick the night’s TV show, these debates take on a different contour when one is in middle school and the other is in college. You probably can’t convince a sixth grader to watch The Bear with you, so you may have to settle for Dash & Lily every time.

Siblings with a several-year age gap were once considered exceptional, but they are quietly becoming more common. From 1967 to 2017, the average time between sibling births increased by about three-quarters of a year, according to data from a study published in 2020. Siblings are now, on average, 4.2 years apart. The tit-for-tat arguments—over, say, who gets to shower first—that have been associated with the sibling relationship for decades are not going away completely. Yet these larger age gaps have opened the door for a new kind of dynamic—one premised more on mentorship than on a battle for limited attention or resources. Squabbles for parental attention are giving way, at least in some families, to a sense that there is enough to go around for everyone.

This new norm of spaced-out siblings seems to be a by-product of the changing American family. The reasons are difficult to parse, but “we know that partner switching explains some of it,” Christine Schwartz, a sociology professor at the University of Wisconsin at Madison who co-authored that 2020 study on the phenomenon, told me. Today, among parents with at least two kids, about 20.6 percent have had those children with different people. According to Schwartz’s study, having children with multiple partners adds an additional 1.6 years to the space between sibling births. And parents without a college degree are far more likely to have children with more than one partner: The increase in age gaps was about a full year for mothers without a high-school degree, compared with half a year for mothers with a college degree.

But the rise of multi-partner fertility is not the only driver of increased time between children. Another is the availability of contraceptives since the 1960s, which has helped women control the timing of their children. Parents might also be waiting longer between kids just because it’s so expensive to have one in the United States.

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Yet another reason could be the modern fertility rate. The average childbearing person in the U.S. had 2.6 children in 1967, at the start of Schwartz’s study; today, that number has dropped to 1.7 children. There is simply more flexibility to space out kids within a limited childbearing window when parents are trying to have fewer of them in the first place.

Regardless, these birth-spacing decisions are starting to shake up one of the most important relationships of our lives. Studies measuring the effects of a large age gap attribute changes in test scores and personality, among other things, to the time between siblings’ birth. If there is one common thread tying them together, it is resources. The further apart in age siblings are, the more attention—and, potentially, financial investment—each of them may receive from their parents.

Those extra resources can translate into early-in-life academic success. A 2012 study found that test scores improved for the older sibling the larger the gap until the next child. (There was no effect, positive or negative, for the younger sibling.) The time the older child spends on their own is key, Kasey Buckles, a professor at the University of Notre Dame who co-authored the study, told me. And the most extreme cases—siblings with many years between them—are likely seeing the strongest effects, as well as propelling the average population-level increase in age gaps, Brian Powell, a sociologist at Indiana University at Bloomington who researches siblings, told me. (The evidence seems to bear this latter point out: According to Schwartz, 22 percent of mothers now space out their kids by six or more years.)

A big sibling age gap has implications beyond academics. Children with many years between them are less likely to have intense rivalries and more likely to listen to each other: A classic study from 1973 found that younger siblings are more likely to take the advice of a sibling who is four years older than one who is just two years older. More recent research has even tied a larger age gap to an increased ability to diffuse conflict.

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That doesn’t mean the growing divergence in sibling births is all positive. Kids from different parents might have to contend with other issues—joint custody, the challenges of divorce—independent of any age differences. Also, sibling arguments can teach important lessons about cooperation and splitting up resources. A recent paper found that children who had a sibling more than three years older or younger were less likely to share a cupcake with a close friend than children who had one closer in age. “Having access to resources can play in different ways,” Powell said. “You can have greater self-esteem, but it also could mean that you don’t learn how to share with others.”

For Powell, it seems pretty clear that this new, age-spaced reality is benefiting older siblings, at least for those with the largest gaps between them and their younger siblings. They are getting more attention for longer during their core developmental years. But he said that the research is still mixed on the extent to which younger siblings are seeing a benefit. Sure, younger siblings might receive more wisdom and encounter less conflict from their elder siblings, but how much that translates into a measurably better life is less clear.

The average sibling age gap probably won’t grow forever: The limited biological window in which people can have kids puts a cap on how far apart in age a set of children can be. Plus, more spacing between births means more time at home raising young kids and, potentially, greater interruptions to the career of one or both parents. For parents without a high-school diploma, the average sibling age gap has widened so dramatically that these parents are actually spending more years of their life raising young kids than they would have in 1967.

This is the irony of sibling age gaps. More time between births might benefit kids, but it doesn’t necessarily translate into a lighter load for parents. The struggle for resources—parental leave, child care, education—remains, even if it ends up stretched across a longer period. Plus, any time-saving that could come from combining parenting duties, such as dropping off both kids at soccer practice at the same time, is lost when those kids are in very different life stages. Older children may be able to lend a helping hand, but that responsibility can be its own burden. Now that potential parents have more control over when they want to start a family, their decisions about birth spacing remain a quandary of resource allocation: Devoting more time for each kid may mean, in the end, more strain on the adults.