the travails of winter babies AND a really weird correlation between birth month and income

Excerpts from a long article by Justin Lahart in the WSJ on some amazing research...

Children born in the winter months already have a few strikes against them. Study after study has shown that they test poorly, don't get as far in school, earn less, are less healthy, and don't live as long as children born at other times of year. Researchers have spent years documenting the effect and trying to understand it.

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.

In 2007, Mr. Hungerman was doing research on sibling behavior when he noticed that children in the same families tend to be born at the same time of year. Meanwhile, Ms. Buckles was examining the economic factors that lead to multiple births, and coming across what looked like a relationship between mothers' education levels and when children were born.

"I was just playing around with the data and getting an unexpected result," Ms. Buckles recalls of the tendency that less educated mothers were having children in winter....

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...the pattern that Ms. Buckles and Mr. Hungerman discovered, it would question the weightiness of other factors from past research....

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. Over the 13-year period, for example, 13.2% of January births were to teen mothers, compared with 12% in May -- a small but statistically significant difference...

He and Ms. Buckles estimate that family background accounts for up to 50% of the differences in education and earnings....

The question now is what drives women from different socioeconomic backgrounds to tend to have children at different times of the year....

Ms. Buckles and Mr. Hungerman aren't entirely sure yet. Perhaps it has to do with fluctuations in employment; married women tend to conceive when unemployment is higher, research has shown. They also speculate it might be due to cooler temperatures in springtime, which don't adversely affect the fertility of poor parents, who may not have air conditioning, like hot temperatures do. Or they wonder if there might even be a "prom" effect at work. January is, after all, about nine months after many of those soirees.