I thought we were talking about adding small amounts of flouride to drinking water for beneficial reasons, but that article seems to be about studies in China where there are naturally occurring high fluoridation levels.
Low = good, high = bad. "Sola dosis facit venenum". The dose makes the poison.
Someone asked for sources in this thread. GP provided the NPR source for: "The US government just cut down the amount of fluorine in the water supply." That was heavily downvoted.
So I provided the source for GP's other statement, "And it is detrimental to brain development."
In contrast you've provided a generic source for an old adage. Anyhow the Harvard study mentions all the levels studied, some of which are comparable to US levels.
Low = good, high = bad. "Sola dosis facit venenum". The dose makes the poison.
http://en.wikipedia.org/wiki/The_dose_makes_the_poison