Solomon Asch's experiment on conformity. He set up a test wherein he would show 3 lines of different lengths to 5 or 6 individuals (I forgot the exact number) who had to state which line was the longest of the 3. The thing is, only the last individual is the participant and the others are actors paid to answer in a specific manner. For the first few questions, they choose the correct answer, but later on they start choosing the wrong one. The participants are conflicted as to whether they will say the correct answer or conform to the wrong answer as to not be judged by others or due to self-doubt of their own answers. In the end, most do conform.
It's really interesting since it shows how powerful conformity is in the face of doubt, up to a point that some even question their own sanity during the test.
Another variation of the experiment also had interesting results. It had the same set up with 5 individuals with the last person being the participant. However, this time some of the actors say the wrong answer while 1 actor says the correct one. There was an increase in participants who would choose the correct answer and avoid conformity. It shows how much doubt one can have on oneself when alone, but be brought back to self-confidence when they find outside support.
Edit: Conformity in participants might be caused by either being afraid others' judgement or due to self-doubt.