Millions of Americans believe in conspiracy theories — including plenty of people who you might expect would be smart enough to know better.

Despite mountains of scientific evidence to the contrary, at least 20% of Americans still believe in a link between vaccines and autism, and at least 37% think global warming is a hoax and believe astrology works. And still believe conspiracy theories like, the moon landings were faked, the government had advance warning of the 9/11 attacks

Intelligence doesn't seem to matter. Kary Mullis who won the Nobel Prize in chemistry for inventing PCR believes in astrolog.
See Dancing Naked in the Mind Field by Kary Mullis

Now, a study published online in the journal Personality and Individual Differences provides new insights into why so many of us believe in things that just aren’t true: In some cases, we simply want to believe.

Psychologists Thomas Ståhl of the University of Illinois at Chicago and Jan-Willem van Proojien of the Vrije Universiteit Amsterdam measured the subjects in a study on the so-called Importance of Rationality Scale (IRS) and the Morality of Rationality Scale (MRS).
They also measured their overall cognitive ability. To determine this, the people answered a number of questions that measured their numeracy — or basic mathematical skills — and their language abilities.

What’s most troubling — and a little mystifying — is the fact is that so many people in the studies score high on all of the rational and intellectual metrics and yet nonetheless subscribe to disproven theories. That’s the case in the real world too, where highly educated people traffic in conspiratorial nonsense that you’d think they’d reject. In these cases, the study concluded, the reason may simply be that they’re invested—emotionally, ideologically—in believing the conspiracies, and they use their considerable cognitive skills to persuade themselves that what’s untrue is actually true.
Source: Why Smart People Still Believe Conspiracy Theories | Time, Nov 14, 2017

Other reasons I've heard

  • People want fit in with their peer group.
  • People associate a belief with something else. E.g. If you're against big government, you may associate climate change with more government spending on remedies.

last updated 20 Nov 2017