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Is the variation in winter babies due to vitamin D – 2009

New Light on the Plight of Winter Babies – WSJ 2009

Researchers Stumble Upon Alternative Explanation for the Lifelong Challenges Faced by Children Born in Colder Months
By JUSTIN LAHART WSJ http://online.wsj.com/article/SB125356566517528879.html

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.

It's said that "every baby born in a modern hospital in the world is looked at first through the eyes of Dr. Virginia Apgar." In a 1964 video, Dr. Apgar assists a nurse through checking a newborn's reflexes.

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.
Related Reading

* Kasey Buckles, Daniel M. Hungerman: Season of Birth and Later Outcomes: Old Questions, New Answers; (2008)
* Joshua Angrist and Alan Krueger: Does Compulsory School Attendance Affect Schooling and Earnings? (1990)

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.

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. 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, they say.

"Honestly, when we first saw these patterns, we were so stunned we wondered if we made some mistake," says Mr. Hungerman. "We weren't even excited, we were like, 'Is that right?' "

He and Ms. Buckles estimate that family background accounts for up to 50% of the differences in education and earnings. That suggests to them that the compulsory-schooling effect Mr. Angrist and Mr. Krueger described could still be there, but that it can't be used to measure how schooling affects later earnings because it still mixes the effects of privilege and education instead of isolating them.

Mr. Angrist, who has reviewed their research, disagrees. "The bottom line is a slight change in the estimate," he says. "It hardly overturns our finding." (Mr. Krueger, now an assistant Treasury secretary, deferred to Mr. Angrist for comment).

The new research has hit the conference circuit. University of Texas economist Daniel Hamermesh, who selected for a conference discussion the working paper that resulted from Ms. Buckles and Mr. Hungerman's research, says that it makes an important point about the assumptions that economists rely on in natural experiments. "I love the paper," he says. "It means you have to think about things more than you want to think."

Janet Currie, a Columbia University economist who also selected the paper for a conference, says that what strikes her is how the pattern Ms. Buckles and Mr. Hungerman found shows up even in simple charts. "You can take a look at those graphs and see the clear pattern and that it's remarkably stable over time," she says. "It speaks for itself — you don't have to put a lot of interpretation on it."

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.

See also VitaminDWiki

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