With regard to 'tighty-whities' I've just asked my husband this morning about this. (Since I haven't actually gone around Africa examining men's underpants) He tells me that most men there nowadays wear budgie-smuggler type underpants under their African clothes. So it would seem that's not a factor.

If I were a gambling lady, which I'm not, I'd put my money on alcohol consumption and obesity, plus sedentary lifestyle.