Dutton argues that the drop has been happening because we've been WAY too successful in reducing infant & child mortality since Victorian times. That was nature's way of keeping the population of humans born with high mutation loads - think of the Tiny Tims in premodern families - from reaching sexual maturity, reproducing and propagating harmful mutations into future generations.

IQ is a function of the human organism's "system integrity," and that comes from having relatively healthy genes. The decline in IQ is happening everywhere now, thanks to the spread of modernity in lowering child mortality, even in countries which don't fluoridate their water supplies.