I think one of the biggest threats to humanity besides ignorance is fanaticism. Throughout history, we have seen clear examples of how fanaticism has blind seemingly smart, rational individuals, driving them into committing heinous crimes.
The danger starts when fans turn into fanatics. Like a drug addict or an alcoholic they think they are absolutely right and have complete control of their actions.
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Whether its religious, political sport related, etc; a fanatic won't reason, they believe in one truth and don't accept criticism or facts. Fanaticism is intoxicating, it can be negative and destructive. I've seen many people falling of the wagon and acting completely irrational when they are in this state of mind.
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Watch a football game for example and see what happens in the crowd when their team looses.
Try giving a fanatic facts about religion or politics, most likely they will get defensive and will completely shut you down because their believes are absolute and irrefutable.
In my opinion, a person that lacks common sense is more likely to succumb to others ideas, no matter how ludicrous they might be. Don't matter how many years of college or degrees people can have, a weak mind is easily persuaded.