Despite the evasions of people who would like to pretend otherwise and dismiss it, fundamentalism is not just a bad name that some people call others they disagree with, but it is a very real mode and approach to understanding reality. Although the word may sometimes be used as a pejorative, fundamentalism is real. I know from experience.
Many of the fundamentalists I have known are good, nice and even educated people, not unintelligent or given to violence. As usual, the common stereotype presents an extreme caricature, and few people seem to realize that by definition the term "fundamentalism" covers an extremely large population that includes a wide range of people.
What all Christian fundamentalists have in common, however, is their existence as a fear-based reaction to modernism. Fundamentalism has its beginning in the early part of the twentieth century. Its initial products are the doctrines of infallibility and inerrancy as characteristics of Scriptural inspiration. The emphasis is on literalism, whether one is interpreting the Bible or tradition, and is applied in multi-varied ways and not usually consistently.
Modernism is the bogey-man of fundamentalist thought because it seems to provide a contradictory narrative of reality. For instance, if Darwinism presents a story about a universe that has been evolving for billions of years in which humans have been gradually shaping up for millions of years rather than just five or six thousand, a conflict arises when one purports a literal, infallible and inerrant understanding of the first chapters of Genesis.
Never mind if those chapters were never understood to be literal accounts and served other purposes in the histories of both Judaism and Christianity. If you are a fundamentalist, evolution challenges what you understand to be the basis of your faith, and as such is something to be feared and denied. As a result, fundamentalism distorts the actual history and content of the tradition from which it stems.
Fundamentalism as a fear-based reaction to modernism embodies many of the thought processes and characteristics that are described in psychological terms as cognitive distortions. In describing fundamentalism, I am borrowing a basic list from the Psych Central article, 15 Common Cognitive Distortions by John M. Grohol, Psy.D. The following describes the fundamentalist way of thinking rather well, though of course not exhaustively.
Polarized Thinking. Also known as "black or white" thinking, this plays out constantly in the fundamentalist need for certainty in all matters. There is no room for nuance or complexity, and those who do make such allowances are considered "wishy-washy." Certainty is a mechanism for allaying fear.
Overgeneralization. This is also a fallacy in which specific events are generalized to represent a more general or universal conclusion. For the fundamentalist, one appeals to an "essential" stereotype or phenomena, such as, for instance, the whole idea of secularism or modernism as monolithic thought systems, or singling out particular characteristics that are then applied to Darwinists, the LGBT community, liberals, atheists, and others.
Jumping to Conclusions. This one is obvious, and Grohol's definition characterizes fundamentalist behavior well: "Without individuals saying so, we know what they are feeling and why they act the way they do. In particular, we are able to determine how people are feeling toward us."
Catastrophizing. Fundamentalists think the rest of culture is out to get them, which leads to conspiracy theories based on spotty evidence, eschatological schemes that imply imminent end of the world scenarios threaded together and rationalized by dubious current events. Think: the so-called war on Christmas, or the notion that gay marriage somehow poses a threat to the institution of marriage itself. Modernism or "secular humanism" of course poses a serious threat when one catastrophizes.
Blaming. A hallmark of health and maturity is knowing that blame never produces any constructive or helpful results, whether one blames others for problems or oneself. Fundamentalists do both. In extreme examples, fundies like Pat Robertson. as well as others, may blame natural disasters on what they consider to be national sins. But this happens in a more subtle way as well. One's suffering, for instance, may be attributed to lack of faith or sin (cf. Job's pals), when many people suffer through no fault of their own.
Shoulds. Grohol's definition captures it: "We have a list of ironclad rules about how others and we should behave. People who break the rules make us angry, and we feel guilty when we violate these rules. A person may often believe they are trying to motivate themselves with shoulds and shouldn’ts, as if they have to be punished before they can do anything."
Fallacy of Change. Grohol: "We expect that other people will change to suit us if we just pressure or cajole them enough." Authoritarian fundamentalist sermons tend to be heavy on blame, threats, accusations and emotional appeals. The idea that we can get someone to change by shaming them into it is a popular notion, but extremely fallacious. It just doesn't work. Quite to the contrary, shame reifies unwanted behaviors because the shamed person identifies with them.
Always Being Right. Grohol, though not writing about fundamentalism but rather common cognitive distortions, sums up the fundamentalist need for certainty as a reaction to modernism well: "We are continually on trial to prove that our opinions and actions are correct. Being wrong is unthinkable and we will go to any length to demonstrate our rightness. For example, “I don’t care how badly arguing with me makes you feel, I’m going to win this argument no matter what because I’m right.” Being right often is more important than the feelings of others around a person who engages in this cognitive distortion, even loved ones." For the fundamentalist, being right is absolutely necessary for identity. Since his or her beliefs are assumed to be absolute truth, based on inerrant literal interpretations of the Bible, or in the case of Orthodox fundies, canons and cherry-picked writings of the fathers as well, to admit the possibility of being wrong is necessarily a betrayal of faith, and therefore a sin.
Rigidity Confused with Integrity. This one is mine, and not necessarily known as a cognitive distortion, though it is a distortion in fundamentalist thinking. The idea that one must stick to principles and be unbending, and that this constitutes integrity, is fairly popular, but it is an error. Genuine integrity has more do with understanding and accepting one's own flaws and limitations than it does with adhering to abstract principles. A real person with integrity can admit when she is wrong, or when she doesn't have an answer to a particular question, and can and should be flexible as a characteristic of humility and love.