Via Eugenia Cheng's ICFP 2018 keynote address, I learned of Carlo Cipolla's laws of stupidity (you can read his original essay here). I think Cipolla developed an insightful definition of stupidity: A stupid person is a person who causes losses to another person or to a group of persons while himself deriving no gain and even possibly incurring losses.
I appreciate this definition because most of the stupid things I've done and the stupid things I've witnessed fall into this mutually destructive category.
Because of this, Cipolla argued there are stupid people everywhere! It doesn't matter how many Ph.D.s you have, if you behave in a mutually destructive way, you are stupid.