Diet.
Really?
The per capita consumption of sugar has gone down, to be sure. But it has been replaced by other sweeteners. And per capita consumption of items like soft drinks and candy has actually increased, if I'm not mistaken.
Would there be any difference between cane (or beet) sugar and, say, high-fructose corn syrup in terms of their contribution to the incidence of caries?
Purportedly, children consume more calories today than they did 30-40 years ago. So, they're getting their -oses somewhere...