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Brainwashing is a colloquial term used to describe any process that either undermines a person's perceived "free will", or forces them to undergo a substantial personality shift. The term seems to originate from the Korean War (1950-1953), when many American GIs claimed that the Chinese attempted to force them to renounce America and capitalism. This led to scare movies like The Manchurian Candidate (1962 and 2004).
Of course, one person's brainwashing is another person's therapy. Is behavioral psychiatry brainwashing? Does it matter if the person signs up for it voluntarily? And, if these are the criteria, how is it different from what a cult does? It is unknown to what degree a person can actually be "brainwashed". For ethical reasons it is impossible to set up an experiment and test the process. Although it is apparently ethical for stage hypnotists to make people do wild things in the name of entertainment, professionals who are open about the subject (such as Derren Brown) admit that it is mostly people just acting up on stage for the attention. Whether this could also apply to non-staged brainwashing isn't known.
The theory of brainwashing may serve as a justification for "deprogramming", or attempts to force a person to give up allegiance to some religious group, usually a cult; this is often involuntary and may involve illegal practices such as kidnapping. Deprogramming took place during the American cult wars in the 1970s and 1980s, but has become rare.
Clearly, if conversion and adherence to a cult can be explained with more common, down-to-earth concepts, like socialization, social influence, and propaganda, then brainwashing theory should be rejected.
Due to the ethical issues of mind control, brainwashing techniques and their effectiveness is not well documented in the scientific literature. However, the controversial CIA project MKULTRA did test various forms of mind control and it turned out that they weren't that effective. Brainwashing theory in general have been criticized for being dehumanising and pseudoscientific. 
Appropriation by different groups
Politically speaking, some people on the fringe right tend to claim that the mainstream media is brainwashing people to be more left leaning.
"Evidence" of Brainwashing
- Voting Democrat
- Believing in Global Warming
- Tolerance to the Semitic Elite
- Tuning in to the big scary evil Mainstream media
- THEY'RE TURNING THE FREAKING FROGS GAY
- Joining RationalWiki (looking at you, yourself!)
It has also been used by anti-cult movements to explain why members join. It has also lead to treatment such as deprogramming which has been criticized for violating people's freedom. 
- Selective reporting
- Nationalism in history textbooks
- Reparative therapy