We’ve written before about the historical and social aspects of conspiracy theories, but wanted to learn more about the psychology of people who believe, for instance, that the Boston Marathon bombing was a government “false flag” operation. Psychological forces like motivated reasoning have long been associated with conspiracy thinking, but scientists are learning more every year. For instance, a British study published last year found that people who believe one conspiracy theory are prone to believe many, even ones that are completely contradictory.

Professor Stephan Lewandowsky, a cognitive scientist at the University of Western Australia, published a paper late last month in the journal Psychological Science that has received widespread praise for looking at the thinking behind conspiracy theories about science and climate change. We asked him to explain the psychology of conspiracy theories. This conversation has been edited for brevity and clarity.

First of all, why do people believe conspiracy theories?

There are number of factors, but probably one of the most important ones in this instance is that, paradoxically, it gives people a sense of control. People hate randomness, they dread the sort of random occurrences that can destroy their lives, so as a mechanism against that dread, it turns out that it’s much easier to believe in a conspiracy. Then you have someone to blame, it’s not just randomness.

What are the psychological forces at play in conspiracy thinking?

Basically what’s happening in any conspiracy theory is that people have a need or a motivation to believe in this theory, and it’s psychologically different from evidence-based thinking. A conspiracy theory is immune to evidence, and that can pretty well serve as the definition of one. If you reject evidence, or reinterpret the evidence to be confirmation of your theory, or you ignore mountains of evidence to focus on just one thing, you’re probably a conspiracy theorist. We call that a self-sealing nature of reasoning.