It's hard to view the Flat Earth Society as a civilizational success. I can't understand how their minds can believe this in a world where telephones can cross time zones. But compare it to Boko Haram. They believe that the world is flat and that water doesn't evaporate (Allah creates the rain each time). Also, their popularity and death counts exceed that of ISIS (since 2015).

This isn't just because they're Muslim. If a group of Sunni Muslims murders lots of other Sunni Muslims, "religion" isn't a sufficient explanation. There are over three million Muslims in the US, this has been true since before 9/11, and they aren't constantly murdering people. 

This kind of self-destructive insanity happens for secular reasons too. The Hutu and Tutsi in Rwanda are so similar that people need to look at government-issued identification to find out which group someone belongs to, if they don't say themselves. This racial categorization was arbitrary from the beginning; Belgian colonizers measured nose shape and a couple other physical features, and assigned groups based on that. The sane thing would have been to reject the colonizer's absurd tool to keep them divided. Instead, the people in Rwanda internalized the division, eventually murdering each other until about a tenth of the population was killed (about a million). This was in 1994.

Sometimes the insane factions get political power. Lysenkoism is a theory of biology that in many points directly contradicts farmers' observations. It taught, for example, that seeds of the same type must be planted as close together as possible since members of the same class never compete, and that fertilizer is bad for plants. Enforcing Lysenko's methods wasn't the only reason that 30 million in Stalin's Soviet Union and another 30 million during China's Great Leap Forward died of starvation (these are estimates, the exact number is not known), but it was a major cause.

These were all as insane as the Flat Earth belief is today, and happened during the past century. Before the enlightenment this was even more common; I don't have statistics on how many people were martyred or massacred because people thought that their perfect and powerful god needed humans to murder it's mistakes for it, or something else, but it sure happened a lot.

Eliezer talked about how some pseudoscience has appeared where people think personality or diet is determined by blood types. This is wrong of course, but it makes as much sense as racism or India's caste system. Blood types would have resulted in those instead if it was discovered a thousand years ago. "Observe how, whenever our blood mixes with that of each other, it remains peaceful, but when our blood mixes with our enemies, it fights us. Our enemies are evil to the individual cells."

I bet this would have happened even if the original scientists urged against it.

What Fixed It

Humans have a tendency of destructive insanity. I don't think this can be fixed without superintelligent brain modification. But the great civilizational accomplishment of liberalism (in the freedom and tolerance sense, not left-wing policy) was to mostly limit this to individual harm. People might smoke cigarettes, go on stupid diets, or spend their money on church buildings with golden crosses, but they don't start inquisitions or execute people for using fertilizer. The Flat Earth society looks stupid trying to explain how the sun is above the horizon in some places but below it in others, but they aren't Boko Haram.

Even when a liberal society has a brutal conflict, people are far safer than usual. The Irish Troubles killed about 3,500 in a population of 1,500,000 during a three decade period; it was still very bad of course, but when compared to humans' normal behavior a 0.2% death rate during that much hatred is amazing. If enough people go crazy in the same way, a country with thousands of nukes might invade Iraq because they "oppose weapons of mass destruction", but most of the Iraqi were left alive and not enslaved. Default human behavior is something more like: "they utterly destroyed all in the city, both men and women, young and old, oxen, sheep, and asses, with the edge of the sword" (Joshua 6:21).

The natural tendency of humans is to have insane beliefs and to murder others for not sharing them. When you look at how insane people often are now, keep in mind that this is still a major improvement.