Group polarization is a term introduced by social psychologists Serge Moscovici and Marisa Zavalloni in 1969, and refers to the tendency for a group discussion to result in the group members adopting a consensual position that is more extreme than the mean of the positions of the individual group members prior to the discussion.

Sources:

Moscovici, S., Zavalloni, M. (1969). “The group as a polarizer of attitudes”. Journal of Personality and Social Psychology 12 (2): 125–135

Gilbert, D. T., Fiske, S. T., Lindzey, G. (Eds.) (2010). “Handbook of Social Psychology, Volume Two (5th edition)” (pp. 1176). New York: McGraw Hill.

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