i am going to repost what i wrote on the other forums in response to this article:
dr. foonat's class on interpreting meta-analyses and evaluating confounds in scientific research is now in session. play close attention.
meta-analysis is a method of evaluating trends in effect sizes across studies. because individual studies are biased in many different ways (measurement error, sampling error, etc.) meta-analysis essentially "averages out" all of the deficiencies in a group of individual studies to provide an estimate of association between variables in the population. therefore, a meta-analysis is only as good as the studies that go into one. if you have a bunch of really terrible studies, no amount of statistical wizardry will allow you to generate an accurate estimate of population-level effects.
if you look at table 1, you will see that all of the studies came from rural areas of China, Mongolia, and Iran. the article specifically states that in these places, water is naturally contaminated with high levels of fluoride (much higher than the amounts that are placed in the drinking water in the US). it is likely that very high levels of fluoride do cause problems, but the levels in the water in the US don't even come close.
further, forgetting about fluoride for a second, IQs in rural areas are generally lower than in non-rural areas. this is due to many factors. for example, socio-economic status, early childhood education, stimulation, nutrition, and healthcare are generally lower/lacking in rural areas (which also have the high fluoride) and these variables are all directly associated with lower IQ. so, this meta-analysis is unable to disentangle what the primary studies found. are the lower IQs because of high fluoride? are they because of the shittier conditions people live in in high fluoride areas? either way, the implications of this study for us here drinking water that has a tiny amount of flouride in it are quite limited