A winter birthday is a notoriously bad omen. On average, those born between fall and spring make less money, get sick more often and do more poorly in sports and school, according to studies dating back to the early 1900s. Researchers have wrestled with reasons for this inequity, and in the last couple of decades, some well-meaning bureaucrats have even tried to counteract it, staggering start dates for kindergarten pupils in hopes that it would give the November and December kids a much-needed boost. The experiments typically assume that the age gulf portends failure. Younger kids feel out of their depth from the time they set foot in a classroom or a gymnasium, they speculate, and never quite catch up.
But what if they were looking through the wrong end of the telescope? What if being born late in the year was an effect—and not simply a cause—of socio-economic disadvantage?
A couple of economists at the University of Notre Dame raise just these questions in a recently published study, arguing the issue isn’t so much in what season children are born, as why they are born at a given time, and to whom. Using a combination of U.S. census data and information from birth certificates, Kasey Buckles and Daniel Hungerman reveal that fall or winter children are more likely to be born to teenage mothers, unmarried women or women who never finished high school. By statistical standards, the differences are striking: a kid born in January is 10 per cent more likely to have a mom without a high school diploma than one born in May; he has equally higher chances of having a teenage mom.
More intriguing still (and potentially more explosive) are the explanations. Buckles and Hungerman cite previous studies noting that women prefer to give birth during the spring or early summer. They also note that women who are educated, married, wealthy and past their teen years are more apt to plan their pregnancies and time their births. The result, they reason, is a disproportionately high number of babies born to young, single, poor and uneducated women during the winter. Mothers are only part of the picture. Sperm counts and motility go down during the heat of summer, the researchers note, so the male partners of those women are less likely to produce children who are born in the spring. “They’re more likely to work outdoors, or they may not have air conditioning,” Buckles explains in an interview from her office in South Bend, Ind. “In general, they’re just more exposed to the elements.”
The findings seem sure to reverberate through social science circles, not least due to their classist overtones. So-called “relative age” research was already enjoying top-of-the-agenda treatment after the Canadian-born author Malcolm Gladwell featured it in Outliers, a bestselling book exploring the roots of success. Relative-age theory holds that we plant the seeds of inequity in school and sports by lumping kids born late in the year together with those born months earlier: the older children enjoy an early advantage, Gladwell argues, because they are bigger, stronger and more intellectually developed; blessed with the confidence that comes with early success, they are more likely to thrive later in life. Buckles and Hungerman, however, suggest success may be partly predetermined, and they offer persuasive evidence linking winter birth with low-rent parentage, and spring birth to—well—poshness.
It is a classic nature-nurture debate, but not one Gladwell is interested in having. “I hate the ‘nature’ argument,” he recently told ESPN. “The only thing we can do something about is the nurture part, and that’s why we ought to spend so much more time talking about it.” Buckles, however, doesn’t think the two theories are incompatible. Starting school or hockey six or eight months younger than many of one’s classmates may indeed set a kid back, she says. And it’s not like she equates low socio-economic status with low intelligence.
Still, the results of the new research reveal a fatal blind spot in efforts to reverse the relative-age effect—in particular attempts to fiddle kindergarten start dates to help younger children. Many of these experiments are based on studies that indicate children born just before the enrolment cut-off date (typically Jan. 1) fare more poorly than those born just after it. Blaming the relative-age effect for the difference may be overly simplistic, says Buckles, because “those children may be different in other ways.” Which is not exactly good news to the babies of November and December. You can no more choose your family, after all, than you can decide the day you are born.