“Eating fast food may lead to lower student test scores in math, science and reading, a recent study of U.S. school children said,” reports Reuters.

A survey showed that fast-food consumption by 8,544 fifth-graders forecast lower academic achievement in eighth grade, according to the study published in Clinical Pediatrics.

“These results provide initial evidence that fast-food consumption is associated with deleterious academic outcomes among children,” the study by Ohio State University and University of Texas researchers said.

In terms of growth in achievement, the researchers found that eighth-graders who ate fast food daily were behind those who ate no fast food by four points in reading. They were behind by three points in math and four points in science.

According to the article, researchers are blaming the lack of nutrition in fast food:

The results may be caused by lower levels of nutrients in fast foods, especially iron. The high level of fat and sugar often found in fast-food meals also can affect attention and reaction times, the report said.

The link between fast-food eating and academic performance remained intact even when such variables as physical activity, television watching and socioeconomic status were included, it said.

The study was based on respondents to a 2004 food consumption questionnaire targeting over 11,000 5th graders.

The problem I see with the conclusions is that there may be some selection bias going on. The correlation between higher rates of fast food consumption and lower academic achievement may have nothing to do with nutrition and everything to do with indifferent parenting.

Regardless of socioeconomic status, parents who put less effort into what they feed their kids choosing fast food over healthier prepared meals might also be, it’s easy to believe, less likely to put effort into making sure the kids do their homework.

I see this same sort of selection bias on other areas as well. We’re often told of the correlation between college education and higher lifetime earnings by people who seek to excuse the exorbitant amounts of money taxpayers and students spend on higher education. But is it the degrees that increase earning power? Or are the sort of people who go to college going to be the sort of motivated, hard-working people who are going to be more successful in life whether they have a degree or not?

Which isn’t to say that degrees don’t matter at all, but only to suggest that these situations might be a bit more complicated than degree=more money and fast food=dumber kids.

But then, superficial and shallow conclusions are powerful justification for a lot of very bad public policy.