Narrator: One day, a young university student is walking home when he’s approached by a frantic, middle-aged man. The man claims to be a doctor who has lost his wallet and desperately needs twenty euros for a cab to the hospital. He’s convincing, even offering a business card and promising his secretary will wire the money back. After some hesitation, the student hands over the cash. He later calls the number on the card, only to discover there is no doctor, no secretary, and no chance of getting his money back. That student was Hugo Mercier, and years later, he would write a book arguing the very opposite of what his experience suggests. In Not Born Yesterday: The Science of Who We Trust and What We Believe, Mercier challenges the pervasive idea that humans are fundamentally gullible, revealing the sophisticated cognitive tools we use to defend ourselves against misinformation.

Narrator: The central argument of the book is a direct rebuttal to the common belief that people are easily fooled. Mercier posits that humans are not credulous by nature. Instead, we possess a suite of cognitive mechanisms he terms "open vigilance." These mechanisms act as a sophisticated filter, allowing us to be open to beneficial information while remaining vigilant against manipulation and falsehoods. This system constantly evaluates three things: the source of the information, its coherence with our existing beliefs, and the quality of the arguments presented. This perspective reframes human belief not as a story of passive acceptance, but of active, and often effective, evaluation. The reason some mistaken beliefs spread isn't because of successful persuasion, but because they tap into our intuitions or serve social purposes, a crucial distinction for understanding the modern information landscape.

Narrator: Despite our evolved defenses, history seems to offer a powerful case for human gullibility. Scholars point to events like the Children's Crusade of 1212, where thousands of children marched toward Jerusalem on a futile quest, or the tragic Xhosa cattle killing in the 1850s, where a prophetess convinced her people to destroy their own livelihood in the hope that their ancestors would rise and defeat the British. These events, alongside modern conspiracy theories like Pizzagate, paint a picture of a manipulable populace. This view is bolstered by classic psychological studies. Solomon Asch's experiments showed people conforming to a group's obviously wrong answer, while Stanley Milgram's infamous experiments revealed that ordinary individuals would administer seemingly lethal electric shocks simply because an authority figure in a lab coat told them to. These examples form the traditional, powerful narrative that Mercier’s work sets out to dismantle.

Narrator: To understand why we aren't inherently gullible, the book looks to the evolution of communication itself, where conflicting interests are the norm. Communication is fragile. Even a pregnant mother has reasons to mistrust the chemical signals sent by her fetus, which is trying to extract more resources. Honesty must be enforced. Consider the spotted bowerbird. The male builds an elaborate bower to attract a mate, a signal of his quality. But what stops a low-quality male from faking it? It’s the vigilance of his rivals. If a researcher artificially enhances a bower with extra decorations, other males quickly notice the fraudulent display and vandalize it. The signal remains honest not because it's intrinsically costly, but because the community enforces honesty. Similarly, human communication would collapse if we were truly gullible. Our vigilance is what forces speakers to be trustworthy, making communication a stable and cooperative enterprise.

Narrator: One of our primary vigilance tools is plausibility checking. We constantly, and automatically, compare new information to our existing web of beliefs. This is an efficient filter, but it has a well-known vulnerability: the backfire effect. In a famous study, political scientists presented conservative Americans who believed Iraq had WMDs with authoritative evidence to the contrary. Instead of changing their minds, the participants often became more convinced of their original belief. However, Mercier argues this effect is the exception, not the rule. More often, people adjust their views, even if only slightly, when faced with credible counter-evidence. The real power lies in argumentation. A good argument can help us accept a conclusion we would have otherwise found implausible, like when Socrates guided a slave boy through the Pythagorean theorem. It works by resonating with our pre-existing inferential mechanisms, making us both vigilant and open-minded.

Narrator: When our own knowledge is insufficient, we must decide who to trust. We do this by looking for cues of competence and benevolence. One powerful cue is expertise demonstrated through past performance. For example, researchers studying the Hadza hunter-gatherers in Africa found that the community members could accurately rank the hunting skills of their peers, aligning almost perfectly with objective measurements. We also rely on majority opinion, a heuristic with deep evolutionary roots. However, our trust is not blind. We weigh these external cues against our own beliefs. The most critical factor, however, is not competence but trustworthiness, which hinges on aligned incentives. We don't trust a used car salesman, not because he lacks knowledge about cars, but because his incentive—to make a sale—is not aligned with ours. This focus on diligence and shared interests, rather than trying to detect lies, is the true foundation of trust.

Narrator: The 20th century gave rise to the myth of all-powerful propaganda, capable of bending entire populations to a leader's will. The book systematically debunks this. Take the supposedly masterful Nazi propaganda machine. Historical analysis reveals it had a surprisingly minimal effect. In regions of Germany with a long history of anti-Semitism, Nazi radio propaganda amplified existing hatred. But in areas with low pre-existing anti-Semitism, it had no effect, or even backfired, creating more opposition. The same holds true for demagogues like Hitler. He didn't persuade Germans to become anti-Marxist; he rose to power by championing an anti-Marxist ideology that was already widely held by elites and the church. Mass persuasion, whether from preachers, politicians, or advertisers, rarely creates new beliefs. It is most effective when it reinforces what people already want to believe.

Narrator: If we're not gullible, why do false rumors and fake news spread so easily? The book argues it’s because they often have low stakes and high social value. In 1969, a rumor swept through Orléans, France, that Jewish shopkeepers were abducting women. Despite official denials, the rumor persisted for months. Yet, almost no one changed their behavior. People didn't stop shopping or barricade their doors. This reveals a gap between what people profess (a reflective belief) and what they act on (an intuitive belief). People spread the Orléans rumor not because they intuitively feared for their safety, but because it was a titillating story that tapped into existing anxieties and prejudices, making it socially relevant to share. In contrast, rumors with high personal stakes, like those in a workplace or a military unit, tend to be remarkably accurate because individuals have a strong incentive to verify them. False rumors persist not because we take them too seriously, but often because we don't take them seriously enough to act on them.

Narrator: Sometimes, people profess beliefs that are not just false, but patently absurd. During the Salem witch trials, Tituba, an enslaved woman, confessed to riding on a stick and conspiring with the devil. Why would anyone believe such a confession? Because it is profoundly self-incriminating. Our vigilance mechanisms are wired to find such statements credible. But why would someone make such a statement? The book suggests that professing repugnant or absurd views can be a powerful social signal. By publicly adopting a belief that makes one an outcast to the mainstream, a person can "burn their bridges" and signal unwavering loyalty to a specific fringe group. This can be seen in the over-the-top flattery of dictators like Kim Jong-il or the extreme statements made by political partisans. The belief isn't about truth; it's a costly, and therefore credible, signal of commitment to the tribe.

Narrator: Ultimately, Not Born Yesterday makes a compelling case that the story of human belief is not one of weakness, but of strength. We are not passive sponges soaking up whatever we are told. We are active, discerning evaluators of information, armed with sophisticated cognitive defenses that, for the most part, serve us well. The greatest threats to our understanding arise not from simple gullibility, but from the complex social dynamics of trust, the misalignment of incentives, and the modern information environment that can push our evolved vigilance mechanisms to their limits.

The book leaves us with a critical challenge: if we are to combat misinformation, we must stop blaming supposed human irrationality. Instead, we must focus on strengthening the institutions—from science to journalism to our own social networks—that provide the reliable arguments and trustworthy signals our vigilant minds are built to recognize. The question is not how to make people less gullible, but how to create an environment where their inherent vigilance can thrive.