Hugo Mercier's 'Not Born Yesterday' mounts a spirited defense of human rationality against the widespread belief that people are easily manipulated and prone to believe whatever they're told. Mercier, a cognitive scientist at the French National Centre for Scientific Research, argues that evolution equipped humans with sophisticated mechanisms for evaluating the credibility of information and its sources. Far from being gullible, people are actually quite skilled at rejecting claims that don't serve their interests or come from untrustworthy sources. The problem, Mercier contends, is not that people believe too much but that they use their critical faculties selectively, accepting information that confirms their existing views while scrutinizing contradictory evidence. This explains apparent irrationality in areas like political beliefs and conspiracy theories: people aren't being fooled so much as choosing which authorities to trust based on group identity. Mercier draws on extensive research in psychology and anthropology, examining phenomena from witch hunts to modern misinformation campaigns to show that mass delusions are rarer and harder to create than commonly assumed. The book challenges both pessimistic views of human nature and optimistic proposals for fighting misinformation through fact-checking alone. Readers will gain a more nuanced understanding of how belief formation actually works and why changing minds is so difficult.