Social proof
When people are not sure what to do, they look at what people like them did and they copy it. It is a shortcut that saves effort and usually works, which is why it runs below awareness and why you walk past the empty restaurant without slowing down. The pull gets stronger the less certain the buyer is and the more the crowd looks like them, and it gets stronger again when the proof is specific. "Popular" does very little. "847 sold this month" and "most ordered by tables of four" do the work.
The study
Goldstein, Cialdini & Griskevicius (2008), J. Consumer Research 35. A sign saying 75% of guests in this room reused their towels lifted reuse from 37.2% to about 50%, beating the standard environmental plea. doi.org/10.1086/586910. Real and published; no independent replication located yet.
Amazon, the badges on the tile
Open Amazon and look at what sits on a product tile before you have read a single specification. A Best Seller flag, an Amazon's Choice badge, a star rating and the count of people who left one. None of it describes the product. All of it tells you what other buyers did, which is the question your brain was asking anyway, and Amazon answers it before you can form your own view. Nancy Harhut points to it as the plainest case of a company letting the crowd do the recommending. A corner shop can do the same with a handwritten card.