Deutsch, M., & Gerard, H. B. (1955). A study of normative and informational social influences upon individual judgment. The Journal of Abnormal and Social Psychology, 51(3), 629-636.
From PsychWiki - A Collaborative Psychology Wiki
The main purpose of this paper was to consider two types of social influence, normative and informational, and to report the results of an experiment bearing upon hypotheses that are particularly relevant to the normative influence. The results of the study indicate that the subject, even if not normatively influenced, may be influenced by the others in the sense that the judgments of others are taken to be a more or less trustworthy source of information about the objective reality with which he and the others are confronted.
This study is classic because prior experiments which have been concerned with group influence upon individual judgment have only incidentally been concerned with the type of social influence most specifically associated with groups, however, this study discovered that even when normative social influence in the direction of an incorrect judgment is largely removed, more errors are made by the subjects than by a control group of subjects making their judgment when alone.