In Not Born Yesterday author and researcher studying social cognition Hugo Mercier makes the argument that, in spite of popular belief, people are not actually gullible or credulous. 

The author begins by arguing against the theory that credulousness is associated with a lack of sophistication or "intelligence" (e.g. the less educated are more gullible). The "arms race" theory explains gullibility as predator and prey evolving mechanisms to outwit one another, resulting in one eventually having a temporary advantage over another. However, he claims this is a false way of looking at the issue, and that it is more likely that humans have evolved not from being more gullible to being less gullible over time, but that, in order to survive, the opposite is true.
So far, it seems like he's making a semantic argument over the definitions of "sophisticated," "intelligence," and "reliable sources."

Humans are social creatures, and a common natural imperative is to seek approval from others. As such, Mercier argues that people do not hold erroneous or extreme beliefs because they are "intuitive beliefs," but because they are using them to seek approval. For example, innocent people will confess to crimes they did not commit because, depending on the scenario, they may believe that confessing to the crime and seeking forgiveness is better than continuing to be badgered by the police. Or people who publicly proclaim their extremist views might be doing so because they are purposefully seeking the disapproval of those outside of their circle of believers in order to signal how dedicated they are to their group. This, of course, escalates over time until people are making increasingly irrational statements. Mercier argues that none of this has to do with credulity or gullibility, but are merely manifestations of our innate desire to belong to social groups, whether they are mainstream or fringe.

Since communication is the key to our social success, we place a lot of weight in the "social points" we gain from successful communication attempts. If we can give someone a piece of information and that piece of information is accepted, then that person's opinion of us has increased slightly. If that piece of information then turns out to be false, then that person's opinion of us will be damaged. Therefore, the author argues, rumor and false information spreads so easily because they are low-cost social points. If they turn out to be true, then we seek to gain social standing, but if they are false we do not lose as much as if we had been confidently stating a fact. 

This also ties into why "fake news" spreads so easily. "Fake news" often takes the form of a threat. Something terrible is happening to someone somewhere. Immigrants are committing crimes, politicians are involved in human trafficking rings, or any other threatening problem. We are cognitively inclined to be more receptive to threatening information because the consequences for not accepting the information could be worse than accepting it. If someone tells you the house is going to collapse with you in it even though it seems perfectly fine, the potential outcome ignoring the warning is far more disastrous than if you followed the person out of the building.

People find it easier to espouse views that have virtually no apparent consequences on their lives. If someone tells you that there is another planet beyond Pluto, it really doesn't matter if you believe them or not. The same thing goes for many matters of public policy in the government. The administration may tell you that there a particular foreign regime is bad, but whether you have a favorable or negative view of that foreign regime really doesn't change your life in any meaningful way. Therefore, the consequences for proclaiming that view can be measured by their more immediate "social point" implications rather than being an intuitively held belief.

I have to admit, while I went into this book with a healthy dose of skepticism. However, by the end of the book, I can't help but feel that Mercier made some compelling arguments. It doesn't really change the end result—people will vote against their best interests, false rumors will spread, and conspiracy theories abound. But the conclusion can be distilled down into the idea that humans are simply not that easy to persuade. In fact, that's our weakness; we are too stubborn in our beliefs to listen to or trust those who might know better. Mercier offers no easy answers to these issues, but perhaps looking at it from a different perspective will help convince some people that it's worthwhile to think more critically and earnestly listen to what others have to say.