As if wintertime weren't bleak enough, the future for those born during this season just got darker. Studies have shown that babies with birthdays in December, January or February have poorer health, attain fewer years of education, receive lower wages and die at an earlier age than those with birthdays in the spring, summer or fall. As Kasey Buckles and Daniel M. Hungerman, professors of economics at the University of Notre Dame, point out in "Season of Birth and Later Outcomes: Old Questions, New Answers," a National Bureau of Economics Research paper published in 2008, there exists a solid body of research that has shown season of birth is associated with a host of undesirable outcomes: autism, schizophrenia, dyslexia, shyness, suicide risk and severe menopause. (Currently doing the rounds at various conferences, the paper received a smattering of attention earlier this year and was discussed this week in an article in the Wall Street Journal.) Children who arrived in the winter are, perhaps not surprisingly, even said to be "less lucky" than their peers. Since luck was self-reported, however, it may rather be that winter babies have gloomier outlooks. Maybe all those years of indoor birthday celebrations -- no pool parties, relay races or scavenger hunts -- begin to take a toll.
For years, researchers have attempted to explain the link between season of birth and poorer life outcomes and have propounded all manner of theories, from vitamin D deficiency to greater exposure to colds and flus, to, most notably, the way in which compulsory schooling laws affect dropout rates. "Children born in the winter reach their sixteenth birthdays earlier in the year than other children," writes Justin Lahart in the WSJ, in a tidy summary of the well-known 1991 findings of economist Joshua Angrist and Alan Kreuger, "which means they can legally drop out of school sooner in the school year -- which some do, leading to lower education levels in the group." (Lower education leads to lower wages leads to lower socioeconomic status leads to poorer health -- you get the daisy chain.) The common thread in these prior explanations is the notion that, as Buckles and Hungerman put it, "social and natural phenomena intervene after conception or birth to create differences in outcomes."
Yet as it turns out, babies born during the winter tend to differ from babies born during the rest of the year "at conception" -- or at least according to Buckles and Hungerman. Examining data drawn from both the U.S. Census and the Centers for Disease Control, which logs information from all live birth certificates in the United States in any given year, the researchers found that winter babies are "disproportionately" born to mothers of lower socioeconomic status -- "younger, unmarried, nonwhite, and less educated" -- women more likely to be teenagers or to not have attained a high school degree. The numbers are significant: In January, for instance, the fraction of children born to women who have not graduated high school is 10 percent higher than in May. "Thus the well-known relationship between season of birth and later outcomes is largely driven by fertility patterns across socioeconomic groups," the authors write. Their research may explain why babies born during this time in the year often fare differently in life (and why some winter babies, if their mothers were educated or married, might fare just fine). It also puts the lie to the widespread notion that birth season is distributed randomly across the population. Still, Buckles and Hungerman caution that their work should be considered "a complement, not a substitute" to previous explanations, such as the effects of school-attendance laws or natal exposure to flu.
The question that arises, of course, is why would poorer, less educated women tend to give birth during the winter? One possibility, the authors posit, is summer heat at the time of conception, which can lower sperm count and motility. "Low SES individuals" -- that's socioeconomic status, folks -- "may be more exposed to temperature extremes." This is science-paper speak for "they tend not to have air conditioning." Sperm counts aside, anyone who has ever endured August without air conditioning can tell you that being sticky, sweaty and irritable doesn't make you want to get it on. Another possibility is that "high-SES women" are, for reasons that go unstated in the paper -- more education? greater economic resources? better access to birth control? -- more successful at planning the timing of their conception and birth. In interviews, Buckles has also suggested "the prom factor" as another likely influence. Count back nine months.
But this week, as the WSJ article floated through the blogosphere, commentators were generally disbelieving and dashed off anecdotal evidence to refute the findings. "My grandmother was born in winter and she missed her 99th birthday by about a week," wrote "madison10" on Free Republic. "[I] can blow that theory all the [way to] Hades and back," she continued. "I was born in December, was in Honor classes in school, and a member of Mensa." Another, "ModelBreaker," noted, "I think I read somewhere that the most common birth month for U.S. Presidents is November" and suggested Valentine's Day as a likely culprit for November births. But the prolific "Madison10," of the Honors classes and Mensa membership, also proffered an astrological alternative: "Well, Pisces, Capricorns, and Aquarians are at a disadvantage because of the sign they were born under, it's that simple." But those of us who are winter babies (I was born in February) can take heart: It's not.