We like to think of ourselves as rational, but human beings are fundamentally irrational creatures – and nowhere is that more apparent than in the fug of groupthink we see around us, from the boardroom to social media. Of the various forms of collective irrationality, groupthink is particularly dangerous. It involves adherence to a faulty consensus, often has a binary moral dimension (one is seen as either virtuous or evil) and is sustained through fear to challenge. Counter-intuitively, the most intelligent and erudite amongst us are particularly susceptible, and when groupthink takes hold, vigorous efforts are made to shut down debate and to bully and punish transgressors. As a result, toleration, liberalism, history, reason and science are under threat. Mass groupthink amongst both the elite and the masses affects millions of people. It has led to financial mismanagement leading up to the 2008 crisis and beyond; poor decision-making at the onset of Covid-19; exaggerated, unchallenged claims which have motivated nonsensical policies; and distortions in academia and journalism. In this remarkable and prescient book, Dr Jerome Booth investigates why some of us have abandoned reason in favour of trite memes, intolerance and hatred. Have we all gone mad? Or can we identify the patterns and causes of what is happening and try to stop it?