Raper vs. Rapist; Why the shift in suffix?

I’ve always been vaguely aware of raper as an alternative to rapist, as a vaguely wrong sounding, possibly archaic formulation.

Nowadays, it’s most often heard from speakers of English as a second language, for whom the construction is slightly irregular compared to other similar verbs. But I’m also aware that I've seen the -er version of the word in older texts.

Well, today I happened to be curious while sitting at a computer and fed it into an ngram, and...

That is not the sort of graph I expected to see; the use of -ist appears seemingly out of nowhere after over a century of dormancy in the mid 20th century, and by the mid 1960s has skyrocketed in usage. Now I would expect an increase in general at this time due to the rise of feminism and increased attention to sexual violence, but I'm curious as to the specific choice of the -ist form over the -er variant; particularly in light of the sharp increase in the latters usage during World War II in 1940, presumably connected to growing reports in the west of the Japanese atrocities at Nanking.

Is there any documentation or research explaining how the -ist formulation came to be dominant in current usage? What could be the reason for that sudden shift?

Solution 1:

An explanation for the shift in usage between raper and rapist as shown in the OP's Ngram Viewer

If one reads the results in the OP's Google Ngram link the first three pages for 1700-1776 refer to names of authors, Raper, all spelled with a capital letter. Similar results appear for Google books dated 1777-1957. The first three pages, again, all refer to people named Raper, spelled with a capital letter.

In order to find an early references to raper (spelled with a lowercase letter) and not the result of some OCR error (paper, taper and even super were among those I found) one has to start looking through books published in the 20th century. One of the earliest instance of "the raper" I found was in a poem written in 1914 by Arturo Giovannitti, entitled The Walker

I, who have never killed, think like the murderer; I, who have never stolen, reason like the thief; I think, reason, wish, hope, doubt, wait like the hired assassin
the embezzler, the forger, the counterfeiter, the incestuous,
the raper, the drunkard, the prostitute, the pimp, I, I who
used to think of love and life and flowers and song and
beauty and the ideal.

While I agree that raper and rapist mean the same thing, and there are instances of usage of the former; rapist with its suffix -ist, is and has always been the much preferred one. The instances of raper which clearly mean someone who forces another to have sexual intercourse are limited and not nearly as frequent as suggested by the OP's original Ngram chart but are more accurately reflected in the one below, dated 1900-2008.

Finally, one more graph showing the importance of a capital letter. Does this result suggest that the term, Raper, is much preferred? No, it doesn't.

References
_{When OCR Goes Bad: Google’s Ngram Viewer & The F-Word
Example of an OCR error super misread as raper
The Walker, a poem by Arturo Giovannitti
Ngram Viewer a raper/a rapist/the raper/the rapist/rapers/rapists}

Solution 2:

Technically, one who forces another to have sexual intercourse (with reference to a specific occasion or incident) should be raper, not rapist. Many dictionaries agree on this usage.

On the other hand, rapist would be a habitual offender, again, technically.

However, Raper is an honorable surname.

Raper is an Ango-Saxon name. The name was originally given to a rope-maker.
…
The spelling variations under which the name Raper has appeared include Raper, Wraper, Rapper, Rapier and others. First found in Sussex.
(c) Swyrich Corp.

Naturally, it is to be avoided in a negative connotation.