A psychological phenomenon whereby people do something primarily because other people are doing it, regardless of their own beliefs, which they may ignore or override. The bandwagon effect has wide implications, but is commonly seen in politics and consumer behavior. This phenomenon can also be seen during bull markets and the growth of asset bubbles.

This tendency of people to align their beliefs and behaviors with those of a group is also called "herd mentality."

For example, people might buy a new electronic item because of its popularity, regardless of whether they need it, can afford it, or even really want it. In politics, the bandwagon effect might cause citizens to vote for the person who appears to have more popular support because they want to belong to the majority.