Choices and persuasion
Bandwagon effect
The more people adopt something, the more others want it too, simply because it's popular.
Popularity becomes a reason in itself. A product with momentum attracts people who want to be part of what everyone else is doing, or who fear being left out.
It's social proof at scale. It helps a product that is genuinely spreading, and it can make a trend look like quality when it isn't.
In practice
An app store listing that shows '1 million downloads' tends to draw more downloads than the same app with no count shown, other things being equal.
Common mistake
Mistaking it for evidence. Teams copy a popular pattern or tool because everyone else did, without checking it fits their users.
Where it comes from
Named in economics by Harvey Leibenstein in 1950, describing demand that rises because others are buying.