People infer correct behaviour from what similar others are doing, especially under uncertainty.
Cialdini's canned-laughter example. It works hardest under uncertainty and when the others are similar, which is why testimonials specify occupation and circumstance.
In practice
- A hotel card saying most guests in this room reuse their towels works better than one appealing to the environment.
- A queue outside one restaurant and an empty one next door persist all evening, and the food was never the variable.