In general, the human race is a grave disappointment. Most of our failings (though perhaps not all) can be lumped under the heading “stupidity.” People are, by and large, stupid.
I’m not sure stupidity is an innate human characteristic. I think children learn it from their parents.
A strong case can be made that evolution favors quick decision-making based on fragmentary information. If a hungry predator (or a hostile human) is approaching, you need to make a very quick decision and take action. If you make the wrong decision, you’ll die. That’s not stupidity, it’s just the way things happen. Your fragmentary information (such as, how fast the point of the spear is approaching, and from which direction) may be inadequate, or it may be too complex for you to process it in time to react appropriately. Either way, you die.
But most of the actions we take are not in that category. More often, we have plenty of time to gather information and evaluate possible courses of action. And yet, people very consistently make stupid decisions, decisions that harm themselves and others.
I think this behavior is learned. I’m pretty sure kids watch their parents being stupid and conclude that the way their parents are doing it is the way you’re supposed to do it.
Plenty of adults operate, for instance, on the basis that they should never, ever admit that they’re wrong. That’s stupid.
Plenty of adults operate on the basis that once they have made a decision, they need gather no further data. Further data might force them to re-think their decision, and they feel anxious about doing so. That’s stupid.
And this stuff is learned behavior; it’s not innate.
Most people are very bad at evaluating the long-term, indirect consequences of their actions. Evolution did not equip us with a very good mechanism for doing this kind of thing, because in the environment where our ancestors lived, there were no long-term, indirect consequences! Or rather, there were — you could get killed, thus failing to have offspring. But if there were going to be long-term consequences, you’d usually get immediate feedback about them: Nog stuck me with a spear, and it hurt. I’d better not let him do that again. If there was no immediate feedback, our ancestors had no reason to contemplate the long-term consequences, so they didn’t.
We no longer have that luxury. But it’s known that our ability to evaluate long-term consequences is poor. So it’s even more urgent that we learn to do so. Failure to learn this fact is just blindingly stupid. But it’s also shatteringly common. People are stupid.