For Will Storr it started with the question 'why don't facts work?' The result is The Heretics, his entertaining and insightful investigation into why people hold irrational beliefs, often in the face of overwhelming evidence. 'You go into a project like this saying "I'm going to find answers",' he says, 'but you don't really believe it.'

Over the course of his research, he met a colourful array of psychics, psychologists, scholars, healers, sceptics and scientists. He experienced past-life regression, extreme Buddhist meditation and, in an unsettling chapter that will be a focus of this event, joined a tour group of neo-Nazis led by controversial historian David Irving. 'The holiday was extremely tense! It was exhausting pretending to be a racist for a week. The timing was especially awkward, as I came back the day before my wedding. The fiancée wasn't pleased.'

Storr discovered that even the most ‘rational’ thinker is not immune to the trickery of their own brain. 'The main surprise for me was that the atheist-sceptical community are so often on such shaky ground. They are just as vulnerable to the processes of bias and assumption as the rest of us.'

Edinburgh International Book Festival, Charlotte Square Gardens, 0845 373 5888, 13 Aug, 2pm, £7 (£5).