Their collective IQs may not exceed 80.
I remember reading somewhere that an IQ of 80 is the "center of the sweet spot for criminal behavior."
While I can't find the exact quote using Google, I did find this, on Quora. I won't use the person's name, but it's easy to find if you want:
"Texas gives every new prisoner an IQ test, and publishes the average of everyone in the system in a report each year. Typically the average score runs around 91 - 93."
Smarter, and the person doesn't need to be a criminal to survive in the modern world.
Dumber, and the person is too stupid to survive even as a criminal much past teenage years, and gets taken out of the game early.