[Real short post. Random. Complete speculation.]
Childhood lead exposure reduces one’s IQ, and also causes one to be more impulsive and aggressive.
I always assumed that the impulsiveness was due, basically, to your executive function machinery working less well. So you have less self control.
But maybe the reason for the IQ-impulsiveness connection, is that if you have a lower IQ, all of your subagents/ subprocesses are less smart. Because they’re worse at planning and modeling the world, the only way they know how to get their needs met are very direct, very simple, action-plans/ strategies. It’s not so much that you’re better at controlling your anger, as the part of you that would be angry is less so, because it has other ways of getting its needs met.