wicked

The mean IQ level was more in the region with medium fluoride concentration in drinking water (56.68) compared to areas with low fluoride concentration (41.03) and high fluoride concentration (31.59).

So according to that study, having "medium levels" (1.2-2ppm) of fluoride is much better. I checked three random water reports in Florida which had 0.5, 0.7 and 0.9 ppm.

Which is why I think sensitivity is the wrong word.

"It depends on the day whether more pleasure would be a good or bad thing for me"

That's a more precise version of your statement, I think.