Intelligence and stupidity are tricky terms. I’m not going to define them rigorously so I'll just say, in the simplest sense, that intelligence is the ability to understand complicated ideas. And most people can do it. Some people are better than others sure, but very few people are utterly incapable of understanding something if a) they want to learn it and b) it’s explained well.
But having said that, have you ever seen a news story or overheard a conversation on a bus and thought to yourself “wow, how can people be that stupid?”
I agree. People can act pretty dumb sometimes, but I don’t think it’s necessarily a lack of mental ability. I think it’s a lack of awareness of cognitive biases.
Cognitive biases are patterns of thought which prevent us from drawing good conclusions about the world. Everyone is prone to them and Scientists try, as much as possible, to get round them. You can't remove them altogether because they're part of your brain, but you can reduce their effects and that's the whole point of critical thinking and Science: training your mind to think well.
Here are 20 of the most common cognitive biases.
I’m probably guilty of them all.
1) Confirmation Bias
2) Seeking Validation, not negation
3) Availability of Ubiquitous Information
4) The Dunning-Kruger Effect
5) Associating physical attraction with value
6) The Genetic Fallacy
7) Anchoring Bias
8) Ingroup Bias
9) The Taxicab Fallacy
10) Ignoring Statistics (or failing to understand them)
11) Choice Supportive Bias
13) Selective Perception
14) Survivorship Bias
15) Selection Bias
16) You notice the more interesting/exciting idea
17) Current Moment Bias
18) Trusting Authorities
20) The Confabulation Effect (False memories)
I love science, let me tell you why.