News has come in from China that air pollution has a large and cumulatively damaging effect on intelligence, particularly on older people with less education. Perplexingly, the effect is on Vocabulary, but not Maths. Even more specifically, the verbal decrement hits men harder than women. What is going on? How could bad air have such a specific effect, and all this within a mere 3 years?
In terms of the wider debate about whether intelligence tests mean anything, does this prove that intelligence tests actually test intelligence, or does it mean that they only test intelligence when measuring pollution, but not when they test differences between one person and another, or one group and another? Perplexing stuff. Odd that some critics who doubt that people can be validly tested for intelligence have no doubt about intelligence when the effect of a pollutant is identified.
However, in many eyes this paper about a pollution effect on intelligence proves the case for bicycles and the banning of diesels, or cars generally.
This would not be the first time that researchers have found that pollution damages the mind.
Those researchers found that for each 5ug/dl exposure at age 11 overall IQ dropped by 1.61 points. The effect on perceptual reasoning was a bit higher, the effect on working memory a bit lower, and nothing much on verbal comprehension.
What have the new researchers found?
The impact of exposure to air pollution on cognitive performance
Xin Zhanga, Xi Chenb, and Xiaobo Zhang
The paper is detailed, and carefully argued. Population sampling is very good. They studied sulphur dioxide, nitrogen oxide and particulates (PM10).
A 1 SD increase in average API (exposure to pollution) over 3 years before the interview is associated with 1.132 points (0.108 SD) drop in verbal test scores.
Air pollution has a stronger effect on white matter (required more by verbal tests) than on gray matter (required more by math tests). Since men have a much smaller amount of white matter activated during intelligence tests, their cognitive performance, especially in the verbal domain, tends to be more affected by exposure to air pollution.
I am not sure why the response to overall pollution is so specific. It just seems odd. I am also not sure about the proffered explanation for the sex difference. It is based on a finding from Haier 2005 about white and grey matter activation in doing verbal and mathematical tasks. Anyway, they say it is a future research topic.
The scoring system is not the total number of items correct, but the highest rank of question achieved. This might introduce artefacts if the items are not perfectly ranked. It might also cause problems if the ranking of maths problems is easier to achieve than the rankings of word recognition. The test starts at different points according to the completed school stage of the participants. So, it is targeted at ability levels, and may be a little less reliable than a full test, but this is probably a minor issue. They control for family income and years of education, which will reduce individual variation in scores because such “corrections” assume that neither variables are caused by intelligence. They tested 25,486 respondents from the national sample, so perfectly good for this study. They omitted the 1.3% of subjects who had moved from one part of the country to the other (though with more time it would have been interesting to see if their results were affected by moving to more or less polluted places). They excluded those with very polluted occupations.
The population-weighted annual mean concentration of PM10 over 2014 in China is 112 μ g/m3, much higher than the National Ambient Air Quality Standards (NAAQS) published by the U.S. Environmental Protection Agency (EPA). This is a very important point in the supplementary materials, which in my view would have been better placed in the body of the main text. I could not find tables showing the effects for cities versus the countryside, which would have been informative.
It is good to see intelligence testing being used in this work. The results are interesting and quite alarming. However, it is hard to see how they could be having the particular reported effects on Vocabulary and not Maths, and for men more than women. If the very high level of pollution in China is having a massive impact then I would expect that no person or ability would remain unaffected. If pollution in 3 years is having such a big effect, even on Vocabulary alone, it is a puzzle that intellectual and scholastic achievement tests in China do not appear to reveal this effect, and regularly come well above Western achievements. The effect of pollution on intelligence is worth further investigation.