This was in a novel I read in the 1980s (I think).
In the novel, about a decade prior, aliens arrive at Earth and make first contact.
About 75% of humans are so stodgy and set in their ways that they become permanently insane. Apparently aliens are not the sort of thing that are real in their mind set. The mass insanity is not necessarily a bad thing. The insane are placed in newly constructed insane asylums and cared for. The 25% who are not insane inherit the wealth and worldly goods of the insane, resulting in a drastically increased standard of living, and a drastically reduced level of stodgy humans.
This is all just a back-drop to the main story, which as I recall was about some aliens contracting some human detectives to solve some sort of mystery.