Reasoning aside, we know that people often acquire their beliefs about the world for reasons that are more emotional and social than strictly cognitive. Wishful thinking, self-serving bias, in-group loyalty, and frank self-deception can lead to monsters departures from the norms of rationality. Most beliefs are evaluated against a background of other beliefs and often in the context of an ideology that a person shares with others. Consequently, people are rarely aa open to revising their views as reason would seem to dictate.

There are some things that we are just naturally bad at. And a mistake people tend to make across a wide range of reasoning tasks are not mere errors; they are systematic errors that are strongly associated both within and across tasks. As one might expect, many of these errors decrease as cognitive ability increases. We also know that training, using both examples and former rules, mitigate many of these problems and can improve a person’s thinking.

On this front, the internet has simultaneously enabled two opposing influences on belief: on the one hand, it has reduced intellectual isolation by making it more difficult for people to remain ignorant of the diversity of opinion on any given subject. But it has also allowed bad ideas to flourish – as anyone with a computer and too much time on his hands can broadcast his point of view and, often enough, find an audience. So while knowledge is increasingly open source, ignorance is, too.

Sam Harris