Sloman and Fernbach take this argument further, positing that not just rationality but the very idea of individual thinking is a myth. Humans rarely think for themselves. Rather, we think in groups. Just as it takes a tribe to raise a child, it also takes a tribe to invent a tool, solve a conflict or cure a disease. No individual knows everything it takes to build a cathedral, an atom bomb or an aircraft. What gave Homo sapiens an edge over all other animals and turned us into the masters of the planet was not our individual rationality, but our unparalleled ability to think together in large groups.

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As Sloman and Fernbach demonstrate in some of the most interesting and unsettling parts of the book, individual humans know embarrassingly little about the world, and as history progressed, they came to know less and less. A hunter-gatherer in the Stone Age knew how to produce her own clothes, how to start a fire from scratch, how to hunt rabbits and how to escape lions. We today think we know far more, but as individuals we actually know far less. We rely on the expertise of others for almost all our needs. In one humbling experiment, people were asked to evaluate how well they understood how a zipper works. Most people confidently replied that they understood it very well — after all, they use zippers all the time. They were then asked to explain how a zipper works, describing in as much detail as possible all the steps involved in the zipper’s operation. Most had no idea. This is the knowledge illusion. We think we know a lot, even though individually we know very little, because we treat knowledge in the minds of others as if it were our own.

This is not necessarily bad, though. Our reliance on groupthink has made us masters of the world, and the knowledge illusion enables us to go through life without being caught in an impossible effort to understand everything ourselves. From an evolutionary perspective, trusting in the knowledge of others has worked extremely well for humans.

Yet like many other human traits that made sense in past ages but cause trouble in the modern age, the knowledge illusion has its downside. The world is becoming ever more complex, and people fail to realize just how ignorant they are of what’s going on. Consequently some who know next to nothing about meteorology or biology nevertheless conduct fierce debates about climate change and genetically modified crops, while others hold extremely strong views about what should be done in Iraq or Ukraine without being able to locate them on a map. People rarely appreciate their ignorance, because they lock themselves inside an echo chamber of like-minded friends and self-confirming newsfeeds, where their beliefs are constantly reinforced and seldom challenged.

According to Sloman (a professor at Brown and editor of the journal Cognition) and Fernbach (a professor at the University of Colorado’s Leeds School of Business), providing people with more and better information is unlikely to improve matters. Scientists hope to dispel antiscience prejudices by better science education, and pundits hope to sway public opinion on issues like Obamacare or global warming by presenting the public with accurate facts and expert reports. Such hopes are grounded in a misunderstanding of how humans actually think. Most of our views are shaped by communal groupthink rather than individual rationality, and we cling to these views because of group loyalty. Bombarding people with facts and exposing their individual ignorance is likely to backfire. Most people don’t like too many facts, and they certainly don’t like to feel stupid. If you think that you can convince Donald Trump of the truth of global warming by presenting him with the relevant facts — think again.