People will conceal views of opposition if they feel that they are outnumbered.

Elisabeth Noelle-Neumann : Political Theorist
This was the concluding theory which resulted from studies investigating the nature of public opinion conducted by the Public Opinion Research Center in Allensbach, Germany. The perception that one's critical opinion of the dominant regime is shared by a minority of the population made it less likely to be stated in public, thus biasing the public discourse.

Elisabeth Noelle-Neumann," The spiral of silence: a theory of public opinion". Journal of Communication, 24, 43-51.1974.