The card on the hotel bathroom rail says that most guests who stayed in this room reused their towels. You had not thought about towels. Now you are thinking about the other guests, and about what kind of person throws a clean towel on the floor. You hang it up.
That card is the product of a research programme, and it is also the product of a book. In 1984 Robert Cialdini, a social psychologist at Arizona State University, published Influence. He had spent roughly three years as a participant-observer inside the world of professional persuaders: sales training courses, fundraising operations, advertising agencies. He came out with a taxonomy. Six principles, he argued, account for most of what compliance professionals do. A seventh, unity, was added in the 2021 edition.
The six are worth stating plainly, because they are still the most useful map of ordinary persuasion that exists. They are also forty years old, and the field they came from has been through a reckoning since. Some of the map holds. Some of it does not.
Reciprocity. A favour creates an obligation. Dennis Regan showed this at Stanford in 1971: a confederate who brought a participant an unrequested soft drink later sold them more raffle tickets, whether or not the participant liked him. The free sample and the charity's address labels work the same way.
Commitment and consistency. Once people have taken a position, they act to stay consistent with it. Jonathan Freedman and Scott Fraser's 1966 foot-in-the-door study is the anchor: residents who had agreed to a small request were far more likely to agree to a large one later.
Social proof. People decide what is correct by watching what others do, most of all when they are uncertain. Solomon Asch's line-judgment experiments in the 1950s are the classic demonstration. The hotel towel card is the modern one.
Liking. We say yes to people we like, and we like people who are attractive, similar to us, familiar, and complimentary.
Authority. Symbols of expertise and rank produce deference. Stanley Milgram's obedience experiments at Yale are the foundation stone.
Scarcity. Things seem more valuable when they are less available. Stephen Worchel and colleagues showed in 1975 that cookies from a nearly empty jar were rated more desirable than identical cookies from a full one.
In 2015 the Open Science Collaboration reported an attempt to reproduce one hundred published psychology findings. Fewer than half replicated with a statistically significant result. Effects that had been taught as settled turned out to be fragile, small, or absent. The field has spent the decade since sorting what survives from what does not.
Cialdini's principles were mostly built on older work, and older work has done better than the priming-era studies that caused the most embarrassment. But not all six have been tested equally hard, and not all have come through equally well.
Authority is the most secure. Milgram's basic finding has been reproduced many times, including Jerry Burger's 2009 partial replication at Santa Clara University, which stopped at the 150-volt mark for ethical reasons and found obedience rates not much lower than Milgram had reported in the 1960s. The interpretation of why people obey is contested, and a separate post covers that. The behaviour itself is not in doubt.
Social proof in its classic form is also solid. Rod Bond and Peter Smith's 1996 meta-analysis of Asch-style conformity studies found the effect real across dozens of studies, stronger in collectivist cultures, and somewhat weaker in later American studies than in the original ones.
Reciprocity has been repeatedly demonstrated in the field. David Strohmetz and colleagues showed in 2002 that a mint delivered with the restaurant bill raised tips, and a second mint raised them further. The effect is reliable. It is also not large.
Commitment and consistency is real but weaker than the book implies. Jerry Burger's 1999 review of the foot-in-the-door literature concluded that the effect exists but is modest, and that it depends on conditions: the first request has to be big enough to matter and not so big that people refuse it, and the two requests need to feel related.
Social-norm messaging, the towel card specifically, has a mixed record. Noah Goldstein, Cialdini and Vladas Griskevicius reported in 2008 that a card citing other guests' behaviour beat a standard environmental appeal. Gerd Bohner and Lena Schlüter tried the same intervention in German hotels in 2014 and found no advantage for the social-norm message. Whether that reflects culture, hotel type, or a smaller effect than first reported is not settled.
Scarcity is well established in the laboratory. Its size outside the laboratory varies a great deal with the product, the audience and the plausibility of the claim. Countdown timers on shopping sites are built on it, but the evidence that they move real buyers is thinner than the confidence with which they are deployed.
Liking is the hardest to falsify because it is the broadest. Attractiveness, similarity and compliments each have their own literatures, and each shows real effects. Bundled together as one principle, it explains almost anything after the fact.
Cialdini's six were never a theory. They were an organised set of observations about what persuaders do, each backed by a strand of research, and the book's lasting value is that organisation. It does not predict how large any effect will be, and the effects are typically small in absolute terms: shifts of a few percentage points in compliance, on average, across a population. None of the principles is a switch that works on a given individual.
That is the right way to read them. A scam that leans on scarcity, authority and reciprocity at once is not guaranteed to work on you. It is guaranteed to work on more people than a scam that leans on none of them. The principles are a description of where the pressure comes from, and that is enough to make them worth knowing.