Religious Fundamentalism
"Fundamentalism is always an expression of repressed doubts." ~Brian Herbert
Repressed homosexual tendencies is generally understood to manifest itself in violent gay-bashing behavior. As long ago as Shakespeare it was accepted and understood that those who "protest too much" do so because they are denying truth. "The lady doth protest too much, methinks" is used to imply that Queen Gertrude really did participate in the murder of Hamlet's father.
When 100,000 Millerites discovered on October 22, 1844 that Jesus didn't return after all, many were undeceived and returned to their homes, but the phenomenon of interest was those who continued to believe even more vehemently and actively proselytized unbelievers to convert to the Millerite faith.
Soren Kirkegaard, in Fear and Trembling, notes that there is no particular merit in believing what you know is true. True meritorious faith is to believe in something you know to be false, even impossible. It seems obvious once you think it through, that people with doubts will be the most aggressive in trying to convince others to agree with them, as this reinforces whatever it is they want to believe. If I can convince someone else to agree with my foolishness, I don't seem quite so foolish. It's human nature to resist admitting you have made a mistake. Especially after you have invested not only emotional baggage, but money, family and public ridicule into the error.
Why would creationists go so dramatically far to "disprove" evolution, even lying under oath in a court of law, swearing that "Intelligent Design" is not creationism, even though their own published words contradict them, and they risk prosecution for purgery? Methinks they doth protest way too much.
The man who is in denial, who is repressing his doubts, is going to be the most virulent and assertive in pushing his beliefs on someone else. He cannot even entertain the possibility that he is wrong. If he would look at the matter objectively and open-mindedly, his doubts would pop out and overwhelm him. There is no way to force good people to bomb abortion clinics, lynch black men, or be a suicide bomber unless they are emphatically denying reality.
In practical application, this means that the most assertive fundamentalists are the ones who have the most repressed doubts. They know darned well they are wrong, but refuse to admit it, and require you and I to agree with them in order to validate their false world view. It is hard enough to admit error when you have nothing of yourself invested in the decision, but if you have wagered "your life, your fortune and your sacred honor" it takes an enormous man to admit error, then. Most people are just not that big.
History teaches us that the deep and violent schizophrenia of the American South concerning slavery which suppressed the immanent fear of slave revolt under the pretense that the blacks loved being slaves and were so much better off than they had been in Africa is largely responsible for the dramatic excesses of the Ku Klux Klan. This denial also explains why students are not taught in high school in the South that Haiti had a successful slave revolt, that the US refused for 100 years to recognize the black government of Haiti and that the US has invaded Haiti with bloody results time and again. The denial of guilt and fear created an entire society of irrational and abnormal behavior. Good people don't lynch their neighbors. Or burn crosses on their lawns.
People who live in a fantasy world both fear exposure of their doubts and feel guilt about their irrational behavior. This makes their denial powerful enough to make them go to great lengths to keep their fantasy and to lure others into it. The ozone layer comes to mind. The hole in the ozone layer was discovered, measured and catalogued in the international geophysical year (1957), before the wide-spread distribution of chlorofluorocarbons. But died-in-the-wool environmentalists continue to believe that it was not discovered until 1984. Mind you, ocean water evaporation puts 8 or 9 million tons of chlorine ions into the atmosphere every year, but the annual "depleting" of the Antarctic ozone layer could not be caused by any natural process, it has to be man-made. They also try to conceal the EPA definition of "hole" which is "50 per cent density". They ignore the fact that direct sunlight on the ionosphere creates ozone, and that Antarctica is in darkness for half of every year, so you would naturally expect the ozone layer there to thin out every winter and return every spring. And yet today you are forbidden by law to use Freon in your air conditioner, no matter how many people die of malnutrition for lack of refrigerated transport. The indirect murder of the third world hungry is something that rational people would not countenance unless repressed doubt forced them to behave irrationally.
Every Republican administration since Eisenhower has increased the size and spending of government, and yet those people who are in favor of smaller government keep irrationally voting for them rather than wake up from their fantasy world.
It is just a matter of emotional maturity to face facts and take responsibility for your own decisions, but some people never do grow up. The source of fundamentalism then is emotional immaturity and the unwillingness to admit that you are wrong even if your face is rubbed in it. Better to deny the real world and kill people than admit I could be wrong.