http://shirky.com/writings/group_enemy.html

Clay’s writings discuss the concept of a “critical mass” that Schelling discusses in Micromotives and Macrobehavior. The idea is that there is some threshold - be it percentage of membership, number of members actively attending, or so forth - beyond which a group is guaranteed to succeed, and below which a group is guaranteed to die out. Of course, “die out” and “succeed” are subjective measures and depend extensively on group metrics, as does the critical mass; that is, the measures of these metrics for one group are not correlated with those of another. For some, “die out” may signify that only a few core members remain; these are the people who are so loyal to the cause or idea behind the group that their attendance does not depend on how many other supporters are present. For others, “die out” may signify that a group has entirely dissipated; in this case, the reasons behind the group’s initial organization were probably not strong or significant enough to shift anyone into the classification of “core members.” Furthermore, “succeed” need not refer to a group becoming wildly popular and drawing new members at incredible rates. Rather, it may signify that everyone who is interested in the group’s ideals, or a percentage thereof, have attended. However, both outcomes are not significant because of the final attendance counts that occur under them. Rather, those two outcomes are significant because they are stable equilibria. Reduce the number of members below the threshold, and the group will “die out,” regardless of the exact meaning thereof, and it will remain at that state sans external influence. Bring the number of members above the threshold, and, similarly, the group will “succeed.”

Clay mentions W. R. Bion’s notion of “social stickiness” - that is, the idea that people will often stay in a group in order to maintain the appearance of being socially adept and to uphold ideals of politeness and propriety, even though they dislike being there. People also want to avoid feeling guilty about leaving their fellow group members behind by abandoning them. However, if enough people are united in their decision to leave, many more people follow them - the same people who were contemplating leaving before, but refused in order to assuage their consciences. This “paradox of groups,” as Bion puts it, is also responsible for the phenomenon that occurs at the end of class practically every day. If the professors continue their lectures past 12:05 pm, a few students start shuffling bags and papers, in an attempt to convince other students to do the same. A few individuals will cautiously start to stand up, to see if others will follow their lead, and, if others do not, will quickly switch to reaching for their bags in an attempt to save face and not appear to be too eager to leave. However, if even a few others do follow suit, often a large portion of the class will also begin to leave. Having acquired the moral support of others, as well as a sense of anonymity and deindividuation afforded by being in a large group, they now feel less accountable for leaving before the professors have finished talking. This also ties in to the lectures on information cascades, in that once enough people have decided to leave (i.e. received a High signal), (most of) the rest of the class will begin to follow their precedent, despite wanting to hear the rest of the lecture.