Stanford Prison Experiment

The Stanford Prison Experiment is a famous psych experiment where volunteers were split into groups of "guards" and "prisoners". While both sides knew it was fake, they quickly began treating it as though it was real, with both groups spontaneously falling into their "roles". The "guards" (and researchers) quickly becoming abusive and sadistic, while the "prisoners" attempted to riot and showed symptoms associated with long-term incarceration. The entire experiment had to be stopped after six days (it was originally planned to last two weeks), and is often used as an example of psychological research that cannot be repeated due to ethical concerns.

This scenario plays out on many reality TV shows. People in a staged environment start acting as though it were real, usually with dramatic changes in personality.

It is also an easily recognized plot premise for any kind of storytelling.

See also: What You Are in the Dark, Becoming the Mask, A Darker Me and Beneath the Mask

Film

 * German movie Das Experiment, which recreates the original Stanford experiment - but goes further. Things don't end well.
 * Likewise, the American remake, The Experiment, pretty much says Humans Are the Real Monsters.

Live Action TV

 * The Colony: This show has a psychology expert who frequently describes how the fake postapocalyptic scenario is being treated as real by the contestants.
 * Veronica Mars: One episode had a psychology class running the Stanford Prison Experiment in Veronica's university, though it doesn't turn out quite the same way. While the "Guards" quickly assume their roles, the "Prisoners" manage to use their brains to mess with the "Guards".
 * Community: A recent episode involved several of the cast members taking a sailing course. The sailboat was in the parking lot, but that didn't stop them from abandoning someone who fell overboard.
 * Life: In an episode, the Victim of the Week was a "guard" in one of these. The professor who was running it did it every semester for 5 days using college students. (One wonders how the student volunteers were able to ditch all of their classes for 5 days?)
 * Sabrina the Teenage Witch: One episode had her class simulate the Salem Witch Trials and of course Sabrina ends up getting persecuted by her classmates, led by the Alpha Bitch.
 * Star Trek: Voyager: In a possibly related example, The Doctor attempts to learn about family relationships by creating a home life in the holodeck with a holographic wife and kids. He ends up getting really emotionally invested in it, to the point where.

Video Games

 * Fallout: The Vaults were meant to provoke a Stanford Prison type of environment. Each with their own means of starting an experiment.
 * In Half Life 2, it's revealed that the lower levels of the Combine forces, "Civil Protection" are completely unmodified humans, who initially join for the added rations and other benefits. The position of power, anonymity and access to stun-sticks does the rest of work turning them into a bunch of sociopathic thugs.