You can see it: the more comfortable a society, the more luxurious, affluent a culture, the more people go mad. The more poor a society – starving, hungry – the less people go mad. Madness needs, in the first place, a mind. But a hungry person has no nourishment for the mind. He is undernourished: his mind is not in a situation to go nuts. For that the mind needs more energy than ordinarily is involved in life.
Madness is a rich man's disease. The poor man cannot afford it.