Evolutionary perspective suggests the following propositions may be true or may serve as plausible working principles until we understand the brain better.
1. Stupidity is partly genetic and partly acquired.
2. The genetic portion of stupidity is programmed into all of us and consists of “typical mammalian behavior.” That is, a great deal of the human nervous system is on autopilot, like the closely related chimpanzee nervous system and the more distantly related cow nervous system. The programs of territoriality, pack hierarchy, etc., are evolutionarily stable strategies and hence work mechanically, without conscious thought. These evolutionary relative successes became genetic programs because they work well enough for the ordinary mammal in ordinary mammalian affairs.
They only become stupidities in human beings, where the higher cortical centers have been developed as a monitoring system to feed back more sophisticated survival techniques and correct these stereotyped programs with more flexible ones.
In short, to the extent that a human follows the genetic primate-pack patterns, without feedback from the cortex, that human is still acting like an ape, and hasn’t acquired facility in using the New Brain.