Sorry I don't give Webster the last word but "science fiction" is a totally meaningless term. It has no definition.

Incidentally, here's how Webster's defines "science fiction"
(Simple definition) stories about how people and societies are affected by imaginary scientific developments in the future
(Full definition) fiction dealing principally with the impact of actual or imagined science on society or individuals or having a scientific factor as an essential orienting component.
They're somewhat contradictory (or at least one is broader), but this movie isn't either.

Okay, sure: psychiatry is a branch of medicine, which is generally considered a science, or at least scientific.
So, presumably these are also science fiction:

• Gray's Anatomy
• ER
• Scrubs
• Trainwreck (one of the two main characters is a doctor, and that's central to the plot)
• Patch Adams
• Girl, Interrupted (it's even psychiatry)
• Dr. Zhivago

Psychiatry is science and the story is fiction. Plain and simple. No Martians or talking robots necessary.

Thriller, political thriller, conspiracy thriller all describe it. Science fiction does not.

I agree with Odilwerk. The setting of the film was the early 60s (I think), and the brainwashing techniques were not the focus of the film. There are ways to break people's will. "Conspiracy thriller" does it for me.

I'd consider it more of a conspiracy thriller.

Brain washing is possible but not to the extent that was shown in the movie. If a new drug is developed there may be things that can happen that exceed what happened in the movie.
A few real life examples of things that people were made to do thru brain washings.
People that were part of the Heaven's Gate religious group were convinced to willingly cut off their testicles they convinced 38 followers to commit suicide so that their souls could board the supposed space ship hidden behind the Comet HaleBopp.

Jim Jones convinced people to give everything they owned to come live with his group Rainbow Family located in Jones Town Guyana. When Congressman Leo Ryan when over to investigated they killed him and they convinced the members of his cult to drink Kool Aid mixed with poison

David Koresh who was the leader of the Branch Davidians religious sect, convinced his members that he was the final prophet. when the FBI surrounded his compound he persuaded his members to die fighting rather tan surrender to the FBI. I would be willing to bet that there are government agencies in many different countries that have developed drugs that are powerful enough to make people do things against their will.

Brainwashing isn't real, thus that aspect is science fiction.
It ain't the Ganges, but you go with what you got." ~ Ken Talley, "The Fifth of July"

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What is science fiction about it?

I've always thought "science fiction" was a lame expression. The words "speculative fiction" are sometimes used for the same genre and it's a little more descriptive. Of course, the brainwashing effort has to do with medical technology which is science, but that is a lesser plot point. The overall story would have to be called a "thriller" or a "political thriller" or "suspense" or something like that. I've seen the phrase "straight-faced farce" in print describing this film. You have to admit Iselin is a very funny character.