From the WSJ,
http://online.wsj.com/article/SB125356566517528879.html
New Light on the Plight of Winter Babies
Researchers Stumble Upon Alternative Explanation for the Lifelong Challenges Faced by Children Born in Colder Months
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.
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.
While it was well known that the educated earned more, they also tended to come from privileged backgrounds -- something that also affects later earnings. Up until then, no one knew how to separately judge the impact of education on higher earnings. Data already showed the researchers that winter babies tended to earn less. Once Mr. Krueger and Mr. Angrist knew how much winter births affected education level, they were able to estimate how education affects later earnings.
The statistical techniques Mr. Angrist and Mr. Kreuger developed helped set off a flurry of research that employs "natural experiments." In these, economists use some random or natural influence to approximate the controlled conditions of laboratory experiments -- the type of research that University of Chicago economist Steven Levitt popularized in the book "Freakonomics." Examples include using differences in the introduction of television in different parts of the U.S. to see how TV affects children's cognitive ability and using changes in daylight-saving time to see how setting clocks forward every spring affects energy use.
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.
Article continuied....