[Correction Notice: An erratum for this article was reported in Vol 119(2) of Psychological Review (see record 2012-06153-001). In the article, incorrect versions of figures 3 and 6 were included. Also, Table 8 should have included the following information in the table footnote "P(A V) = probability of acquittal given unanimous verdict." All versions of this article have been corrected.] Social influence rises with the number of influence sources, but the proposed relationship varies across theories, situations, and research paradigms. To clarify this relationship, I argue that people share some sense of where the "burden of social proof" lies in situations where opinions or choices are in conflict. This suggests a family of models sharing 2 key parameters, one corresponding to the location of the influence threshold, and the other reflecting its clarity--a factor that explains why discrete "tipping points" are not observed more frequently. The plausibility and implications of this account are examined using Monte Carlo and cellular automata simulations and the relative fit of competing models across classic data sets in the conformity, group deliberation, and social diffusion literatures. (Contains 18 footnotes, 10 tables, and 11 figures.)