Insanity as a collective phenomenon.
My dad was born in 1920 in a peasant farming village in what was then Germany (now Poland). My mom was born in Ludwigsburg 
, Germany, in a small city 12 km from Stuttgart. 1933.
So they both grew up in Nazi Germany. They experienced a nation that had gone insane, then the consequences. From them, I got the conviction that nations just do sometimes go insane.
It would be better to say that *societies* or *cultures* go insane, but this formulation is punchier I think.
The reason it would be better is that it's often a subculture that goes insane and drags the larger society along with it. Examples of Social Insanity
The largely-US Satanic Panic of the 1980s is a good example of "drags the larger society along with it." It's notable that police and prosecutors quickly bought into it, the press reported it credulously, it gained a scientific veneer in the form of repressed memory , and so on. That it would require many, *many* thousands of infants being ritually slaughtered *without anyone reporting they were missing* was (ritually?) left unnoticed.