There's the evidence of my own eyes as I look around at the behaviour of people and wonder how someone with a fully functioning brain could do what they do.
Stuff like piling as much alcohol, lard and pharmaceuticals into themselves that they can barely function.
Not scientific but worth noting.
The explosion of various types of autism could, I suppose, be related in some way with the human body fighting off the worst of the disease but not managing to conquer it completely.
That's my cheery thoughts.