Choices and persuasion
Social proof
When unsure, people look at what others do and follow them, so showing what others chose makes a choice easier.
Reviews, ratings, usage counts and 'people like you also' all work because others' behaviour is a shortcut through uncertainty. The more similar the others are, the stronger the pull.
Show real, relevant evidence: reviews from people in a similar situation, numbers that are true, and examples from customers people recognise.
In practice
Hotel cards saying most guests who stayed in this room reused their towels got more guests to reuse theirs than cards about the environment.
Common mistake
Fake reviews and inflated numbers. They're increasingly illegal, and social proof only works while people believe it.
Where it comes from
Robert Cialdini named it in Influence (1984), building on Solomon Asch's conformity experiments of the 1950s. The towel study is by Noah Goldstein, Cialdini and Vladas Griskevicius (2008).