If you were sitting in a waiting room and smoke began to billow out of a vent in the wall, you'd probably do something about it. At least, you'd report the problem to someone. Or maybe not.
In a famous experiment conducted by John Darley and Bibb Latané during the 1960s, Columbia University students were invited to share their views about problems of urban life. Those who expressed an interest in participating were asked to first report to a waiting room in one of the university buildings where they would find some forms to fill out before being interviewed. They had no idea that the urban-life study was just a cover story. The real experiment occurred in the waiting room.
As they filled out the forms, smoke began to enter the room through a small vent in the wall. By the end of four minutes, there was enough smoke to obscure vision and interfere with breathing. Darley and Latané examined how the students reacted to this smoke in two different conditions.
In the first condition, the students were alone. When this was the case, they invariably investigated the smoke more closely and then went out into the hallway to tell someone about it.
But in the second condition, the students were not alone. There were two or three other people in the room, who were secret confederates of the researchers. They had been instructed to not react to the smoke. They would look up at it, stare briefly, shrug their shoulders, and continue working on the forms. If asked about it, they would simply say, "I dunno."
In this setting, according to Darley and Latané, "only one of the ten subjects... reported the smoke. the other nine subjects stayed in the waiting room for the full six minutes while it continued to fill up with smoke, doggedly working on their questionnaires and waving the fumes away from their faces. They coughed, rubbed their eyes, and opened the window -- but they did not report the smoke."
That's the power of group pressure.
If you ever get the chance, check out Darley and Latané's book
The Unresponsive Bystander: Why Doesn't He Help?
It's not easy to find because it's out of print, but it's full of strange experiments like the smoke-filled room one.
Also check out this youtube video based on the smoke-filled room experiment. It's not the original study, but a later replication of it.