My personal favourite from Festinger’s many brilliant examples of cognitive dissonance deals with people’s beliefs about the link between smoking and lung cancer. He was writing at the very birth of research on the causes of cancer. It was a unique window of time during which it was possible to test whether groups of smokers and non-smokers would accept or reject new information that had been uncovered about a link. Festinger saw the effects you’d expect from anyone suffering cognitive dissonance: heavy smokers – those who had the most to lose from the new research being right – were the most resistant to believing that a link had been proven; only 7 per cent accepted the validity of the new research. Twice as many moderate smokers accepted the link, at 16 per cent. Non-smokers were much more willing than smokers to believe the link had been proven, but as a mark of just how far social norms have swung since then, only 29 per cent of them believed the link had been proven, despite having nothing to lose.