Something can be robustly true and yet still have edge cases

From The Effects of Fluoride in Drinking Water by Linuz Aggeborn and Mattias Öhman.  Found via Marginal Revolution.  From the Abstract:

Water fluoridation is a common but debated public policy. In this paper, we use Swedish registry data to study the causal effects of fluoride in drinking water. We exploit exogenous variation in natural fluoride stemming from variation in geological characteristics at water sources to identify its effects. First, we reconfirm the long-established positive effect of fluoride on dental health. Second, we estimate a zero effect on cognitive ability in contrast to several recent debated epidemiological studies. Third, fluoride is furthermore found to increase labor income. This effect is foremost driven by individuals from a lower socioeconomic background.

To fluoridate or not to fluoridate? - That is the question.  At ten or twelve, this was perhaps one of the earlier scientific controversies of which I became aware,  Even then, though,  in the early seventies, in most accounts I read, those taking the anti-fluoridation position were generally characterized as conspiracy minded Luddites.  

I am open to all interpretations but have never had the threshold concern that fluoridation was sufficiently questionable to invest much time investigating it.  I was unaware that some recent studies had raised questions.  

On the other hand, much knowledge is handed down as settled when in fact it has simply not been rigorously examined.  Glad to know that the received wisdom on fluoride is affirmed.  

In the comments at the Marginal Revolution post, there is a reminder that there are always edge cases, even when the main hypothesis is robustly true.