Are Boys Getting Dumber While Girls Get Smarter?

A new study has been published with some startling news: pregnant mothers who drink fluoridated water have babies with lower IQs. More accurately, they have baby boys with lower IQs. But if they have a girl, their baby will have a higher IQ. Here’s the basic table:

A 1 milligram increase in maternal urinary fluoride levels was associated with a decrease of 4.49 IQ points in boys and an increase of 2.40 IQ points in girls. This seems implausibly large, doesn’t it? But I can think of at least one crude way to check it. In 1950 virtually no water in the US was fluoridated. Today, fluoridated water reaches more than 70 percent of the population. At a minimum, this suggests that over the last 70 years the IQ levels of boys and girls (a) should have gone down, and (b) should have diverged by about 7 points. Has this happened?

In a word, no. Overall IQ scores in the United States have increased since 1950 and that steady upward trend has continued at least through 2014. And I’m unable to find any evidence that the mean scores of boys and girls has differed by more than one point or so during this entire period. Obviously lots of other things have been going on at the same time, so this doesn’t prove anything. But it sure puts a high burden on anyone claiming an IQ effect that affects boys and girls differentially by 7 points.

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