The bandwagon effect is a well documented form of groupthink in behavioral science and has many applications. The general rule is that conduct or beliefs spread among people, as fads and trends clearly do, with "the probability of any individual adopting it increasing with the proportion who have already done so". As more people come to believe in something, others also "hop on the bandwagon" regardless of the underlying evidence. The tendency to follow the actions or beliefs of others can occur because individuals directly prefer to conform, or because individuals derive information from others.
The metaphor suggests that a bandwagon goes through town and people's inclination to jump on is dependent on how many people have already jumped on board. The implication is that the sole motivating factor is the fact that others have already joined. The more who join, the more the motivation despite the fact that nothing changed about the bandwagon and nothing changes in individuals' preferences for riding on a bandwagon.
Connect with Schelling and critical mass models.
Explain that can be analyzed graphically by starting with frequency histogram and converting to a cumulative frequency diagram that will likely take form of an S curve. With a 45 degree line on the chart we can identify stable and unstable equilibria.