…But economists Kasey Buckles and Daniel Hungerman at the University of Notre Dame may have uncovered an overlooked explanation for why season of birth matters.
Their discovery challenges the validity of past research and highlights how seemingly safe assumptions economists make may overlook key causes of curious effects. And they came across it by accident…
The two economists, whose offices are across from one another, were comparing notes one day and realized that they might have stumbled across an answer to the season-of-birth puzzle that previous research had overlooked.
A key assumption of much of that research is that the backgrounds of children born in the winter are the same as the backgrounds of children born at other times of the year. It must be something that happens to those winter-born children that accounts for their faring poorly.
In a celebrated 1991 paper, economists Joshua Angrist of the Massachusetts Institute of Technology and Alan Krueger of Princeton University argued that season-of-birth differences in how far children go in school is due to how school-attendance laws affect children born at different times of the year. Children born in the winter reach their 16th birthdays earlier in the year than other children, 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.
…Other researchers have suggested other reasons for season-of-birth differences. Maybe vitamin D was playing a role, for example, because children born in the winter were getting less sunshine in early life. Or maybe being put in the same school year with children who are mostly younger makes children born in the winter less socially mature. A study published in the medical journal Acta Pædriatica in April found that children born in the winter have higher birth-defect rates and suggested it was due to a higher concentration of pesticides in surface water in the spring and summer, when the children were conceived.
There may be validity to all of that research. But if there was any truth to the pattern that Ms. Buckles and Mr. Hungerman discovered, it would question the weightiness of other factors from past research. If winter babies were more likely to come from less-privileged families, it would be natural to expect them to do more poorly in life.
The two economists examined birth-certificate data from the Centers for Disease Control and Prevention for 52 million children born between 1989 and 2001, which represents virtually all of the births in the U.S. during those years. The same pattern kept turning up: The percentage of children born to unwed mothers, teenage mothers and mothers who hadn't completed high school kept peaking in January every year.
