Does Bran Make the Man? What Statistics Really Tell Us

Can eating breakfast cereal determine the sex of your baby?

A debate over that question in a British scientific journal shows why some observational studies should be taken with a big shaker of salt.

The original study, "You Are What Your Mother Eats," in the journal Proceedings of the Royal Society B, made headlines around the world last April. Researchers at Exeter and Oxford universities asked 740 pregnant women to record what they ate during pregnancy and just before. Not surprisingly, their diets during pregnancy had no correlation with their babies' gender.

Michael Sloan

But 56% of women who consumed the most calories before conception gave birth to boys, compared with 45% of those who consumed the least. Of 132 individual foods tracked, breakfast cereal was the most significantly linked with baby boys.

How could that be? The authors said animal studies also found male offspring are more common in times of plenty; they speculated that higher glucose levels in mothers may favor the survival of male embryos, which are slightly heavier than females.

Baloney, said some U.S. statisticians, who suspected the finding was simply a false association that can occur by chance in a large set of data.

"Think of it this way: The probability of getting all spades in a given bridge hand is infinitesimally small, but in all the bridge games all over the world, somebody might," says Stan Young, assistant director of the National Institute of Statistical Sciences in Research Triangle Park, N.C. He obtained the study data, re-analyzed it and wrote a commentary in the journal's current issue saying the cereal finding was pure chance.

The study's authors wrote a rebuttal disputing Dr. Young's analysis and standing by their findings.

Behind the cereal squabble lies a deep divide between statisticians and epidemiologists about the nature of chance in observational studies in which researchers track peoples' habits and look for associations with their health but don't intervene at all.