Most criminals are kind of stupid though - at least the ones who commit crimes that don’t involve some form of financial gain. Note: this is the belief among criminals themselves.

I mean the convergence of beliefs across a bunch of religions and moral philosophies is probably pretty good evidence. They were not exactly running randomized controlled trials; but they were smart people who thought a lot about this stuff. I’m not sure that the methods of modern behavioral science are so much more advanced :)

Otherwise this is just like one of those “obvious”, self evident things (i.e. obvious to some not to others). I do concede that there are a subset of jerks who have Dunning Kruger syndrome as well as a small number of true sociopaths out there.

Criminals who get caught are often stupid. We know nothing about the ones that don't get caught. The ones that don't get caught might actually make up the majority of criminals.