What does an IQ test measure?
The easy answer would seem to be intelligence but there is a large body of research that would argue this is not the case. Or, at least, the intelligence that is being measured is a very specific subset of all the things that could be measured as intelligence.
IQ tests are intrinsically biased towards a particular type of person. They are also constructed with a certain ethnic group or social-economic structure in mind. Indeed, the origins of the tests date back to the early eugenics movements of the 20th century and were used to exclude people of color, different religions, and women from certain professions.
When someone argues that IQ tests "show" a specific result, it is always a good idea to take their assertions with a grain of salt. There are too many variables and it is important to understand all of the confounding influences.
That said, there has been a lot of research done on IQ tests since their early inception and, while they may not be the only or even best measure of intelligence, they do provide some insight. Particularly if they are analyzed longitudinally as an IQ test repeatedly delivered to the same group over a period of years or decades may provide a meaningful comparison.
In the case of the North American population, despite everything that you might have heard about our kids being not very bright, IQ tests suggest that the average intelligence is increasing in absolute terms. To understand this, we need to consider how a test is validated.
Essentially, each test is standardized by using a sample of test-takers and their average score is set to 100 with a standard deviation of 15 or 16 points. This means that the average person taking the test will have an IQ of 100 and with a standard deviation of 15 points two-thirds of the population will be between 115 and 85.
But as each test is internally standardized, each test will result in an average result of 100. However, when older tests are used on a new generation of test-takers, they routinely score much better than average.
This increase in IQ has been observed in communities as different as Des Moines, Iowa, and Dumfries, Scotland. Over a 100 year period, IQ seems to linearly increase about 3 points per decade. The good news is that kids are getting smarter.
The bad news is that they really do appear to be smarter than us. This phenomenon has even been given a name - the Flynn Effect.
Similar results have been seen in various countries around the world. Anecdotally, any science teacher will tell you that we now teach concepts in Grade 10 science that won Nobel Prizes 100 years ago.
Where does this put the argument about fluoridation of the water supply leading to decreases in IQ?
If you are using IQ tests as a measure of the average intelligence, then there is a direct correlation between the increasing use of fluoride to treat water and the average intelligence of a citizen of the United States. Fluoride has been added to water since the 1950s and IQs have increased every decade.
Would I claim that the increase in IQ is a result of fluoride? Of course not.
But it certainly would seem that fluoridation is not decreasing intelligence as some people would claim.
What about the studies done in China? Most of these studies appear to generate some differences but the question is the statistical significance of these differences.
Consider one study that is often cited. It considered both the effect of arsenic and fluoride in Shanyi county, Shanxi, China. The results would seem to suggest a statistical difference between various groups of students with students exposed to high fluoride exhibiting a decreased IQ.
In this case, the researchers endeavoured to account for confounding factors and used a specifically designed IQ test. The results were 105 plus or minus 15 points in the control group compared to 101 plus or minus 16 points in the high fluoride group. It is really hard, given that the sample sizes were relatively small, to argue that this difference is statistically significant.
But in any case, the control group was exposed to fluoride levels comparable to those used in Prince George at 0.5 ppm with a standard deviation of 0.2 ppm. The high fluoride group was exposed to fluoride at 8.3 ppm with a standard deviation of 1.7 ppm. That is more than 10 times the fluoride levels in our drinking water.
Indeed, if anything, the research by Chinese scientists on the effects of fluoride on IQ categorically demonstrates that fluoridation levels consistent with the optimal levels used in our water supply have no impact on a child's intelligence. Our water is their control sample.
