The fact that people may be clever enough doesn't in any way mean they can't make themseleves stupid, when they really want to.

It's as if we've got a little switch in our mind, and we can just turn it off, and large parts of our mind settle back and go to sleep.

Every now and again large number of people do that, and it can have dire results - and the dire results can make them turn the switch back on.

Of course, that only works if the dire results happen to the people with the switched off minds, rather than being exported to other parts of the world. But sooner or later it does seem to happen that the trouble comes home in a way that can turn that switch back on.