Extremism: what’s the cultural history of this word?

The word is a simple construct derived from the adjective extreme and seems unlikely to have a strong cultural development as opposed to filling a linguistic niche for a suitable related noun. According to Google ngram its use has increased steadily since the early 1900s.

Extremism = "the fact of someone having beliefs that most people think are unreasonable and unacceptable"

Cambridge dictionary

This meaning accords with other dictionaries and with common usage. The beliefs, which may be of any form (for examples: leftist/rightist, religious/atheistic, nationalist/globalist, communist/capitalist), are not innate but are chosen by the believers.

Because extremism is related to behaviour that is by definition outside that condoned by most people it is not akin to Marxism, which has a set of particular beliefs open to as many people as choose to embrace it.

Nor is it reasonably likened to mutism or autism, both of which are human conditions characteristic of some people but not in any sense chosen by them.

Extremism is a construct (I agree) that has become increasingly prominent since 1900 (thank you for that info). If it does fill a semantic niche, it's a very elastic one, since extremism is a tendentious description. The elasticity has two sources (1) The concept depends on the speaker's perspective (as also does the word terrorism). (2) The suffix -ism sometimes implies an attribute that is characteristic, rather than definitive, of a category but only makes sense if the category's definition is accepted. The Catholic creed defines catholicism relatively unambiguously. Parkinsonism resembles Parkinson's disease, a category that can be defined objectively. Similar objective authority seems to attach to words such as extremism, barbarism and vulgarism, but there is no objective definition for categories such as these. It looks as though the rise in usage of the term extremism reflect the history of Anglo-American politics over the last 100 years.

OED: Extremism:

Etymology: < extreme adj., adv., and n. + -ism suffix.

Tendency to be extreme; disposition to go to extremes.

1865 Daily Telelegraph. 29 Dec. 2/1 These days of extravagance and extremeism.

1887 The American XIII. 276 It is..this extremism which makes any effective control of the traffic in liquors so nearly hopeless.

The 1865 quote may be a reference to the suppression of the Morant Bay rebellion by Edward John Eyre and the events that followed.

The lack of wording to describe good extremes is, likely, because there can not be any good extremes. The extremism, itself.

A person can be extremely normal, healthy, etc, in terms of mental acuity, and, physical strength, but, these traits, themselves, aren't extremes. Even the loftiest theories and other such works require a balanced view of things. Even great physical strength requires efficient muscles.

Being extremely generous, in the sense of handing out your life's savings, say, to strangers on a street corner, may be considered crazy behavior. Similarly, an extreme lottery win of tens of millions of dollars often leads to the complete demise of winners. Relatively extreme acts of charity pale in comparison to incremental sustained good in the community by keeping the overall wealth there. Some cures may be worse than others, as require more hardship by the patient, but, cures, themselves, aren't extreme. Trace amounts of poisons, in balanced amounts, not too much, or little, can cure us. Here, there is extreme accuracy, but, accuracy, itself, is about a delicate balance of things.

In general, it's much easier to quickly tear things down than to properly build them up with incremental or measured good acts. Haven't IQ's risen markedly with the overall health of the population, in the last hundred years? And, on the decline, again? If I recall, it takes millions of years for a species' genetic material to normalize on its own.

In other words, a wrong can be very wrong in many ways, whereas a right is just right in just one way. Although questions may be hard to answer, the answers, themselves, over long periods of time, remain reasoned out (in a balanced way). There are infinitely many ways to solve a given problem, but, each way admits only one overall solution to it, in which one solution may be more direct, elegant, etc, than another, but, each is still a matter of a balanced perspective and set of skills.