Can six simple levers explain why we say yes?

Cialdini argued that they do, but his levers have not all been tested equally hard. The psychologist Robert Cialdini spent three undercover years applying for and training at used car dealerships, fund-raising organisations and telemarketing firms, to watch persuasion in real life. His 1984 book argues that influence rests on six principles: reciprocity, commitment and consistency, social proof, authority, liking and scarcity. In 2016 he added a seventh, unity: the more we identify with others, the more they influence us.

The book's central idea is that, overloaded with information, we fall back on mental shortcuts that usually let us act correctly with little thought and time. The same shortcuts can be exploited by people who know them. A small free gift makes us feel obliged to give something back, even when we did not want the gift. Agreeing to display a small sign makes us more likely to accept a big one in the yard. Seeing a long list of earlier donors makes us more likely to give. A uniform or a degree can make us obey. An item that is hard to get makes us want it more.

Two of the tricks have been put to the test again. The door-in-the-face trick, where a huge request is refused and then a small one is made, was Cialdini's own 1975 experiment, and a 2020 study replicated it: that one held. The foot-in-the-door trick, a small request followed by a larger one, has mixed evidence: some studies find more donations, others find no statistically significant effect. A meta-analysis of 22 studies found no significant difference between the two.