The bandwagon effect is a psychological phenomenon where people adopt certain behaviors, styles, or attitudes simply because others are doing so. More specifically, it is a cognitive bias by which public opinion or behaviours can alter due to particular actions and beliefs rallying amongst the public. It is a psychological phenomenon whereby the rate of uptake of beliefs, ideas, fads and trends increases with respect to the proportion of others who have already done so. As more people come to believe in something, others also "hop on the bandwagon" regardless of the underlying evidence.

A fad, trend, or craze is any form of collective behavior that develops within a culture, a generation or social group in which a group of people enthusiastically follow an impulse for a short time period.