Why people buy: six psychological triggers

Cialdini's six principles have been in circulation since 1984. Every marketing deck runs the same six, in the same order, with the same confidence.

I was going to repeat the list. Then I opened the papers.

The evidence behind the six is not equal. One experiment still stands fifty years on. Another one, the most cited of the group, vanished when it was run again in Germany, and in the second study it reversed. Using this list without knowing which is which is like carrying a toolbox without knowing which tool is broken.

All six are below, but they do not get equal space. The ones with solid evidence run long. The ones without run short. The imbalance is deliberate.

Reciprocity

When someone gives you something, you feel pressure to give something back. Free samples, free ebooks, the mint that arrives with the bill.

The cleanest measurement of this happened in a restaurant. In 2002, Strohmetz, Rind, Fisher and Lynn tracked tips for servers who left mints with the check. One mint raised tips by roughly 3 percent. Two mints raised them by 14.

The third condition is the interesting one. The server leaves a single mint, walks away, then stops, turns back, and hands over a second one with "for you nice people, I'll give you another." Same two mints, different delivery. That is where tips peaked. The quantity did not do the work. The gesture looking personal did.

But: the study ran in one 66-seat restaurant, across four dinner services, with 92 tables. Ninety-two is a small number. The direction is convincing. The size of the effect is not something a single study at that scale can settle.

Sweetening the Till, Journal of Applied Social Psychology, 2002

Social proof, and the real lesson in this list

Telling people that everyone else is doing it works. The most famous measurement of this involves hotel towels.

In 2008, Goldstein, Cialdini and Griskevicius placed different cards in hotel bathrooms. The standard environmental message, the one asking guests to help protect the planet, produced a 37.2 percent towel reuse rate. Descriptive norm messages, the ones saying most guests reuse their towels, pushed it to 42.8 percent. The highest number came from a card saying that 75 percent of the guests who stayed in this room reused their towels: 49.3 percent.

The finding was summarised, and travelled, as this: norms beat moral appeals, and local norms beat general ones.

In 2014, Bohner and Schlüter ran the same experiment in Germany. It did not hold.

In a four-star hotel, across 724 observations, the descriptive norm scored 81.9 percent against the standard environmental message at 83.7. The norm card lost. The "in this room" version scored 78.0 against 85.6 for the general norm, which is the original finding upside down. A second study in a three-star hotel, 204 observations, gave 93.3 percent for the standard message, 76.5 for the descriptive norm, and 64.3 for a control group with no card at all.

Note what survived: both messages beat having no message. So "put a message there" still holds. What collapsed is the claim that a norm message beats a plain one.

The authors' own reading is cultural. Baseline environmental commitment is already high in Germany, so reminding people of the norm may have less room to move them.

Here is what that leaves me with. The original experiments ran in the United States, with American hotel guests. Run in one European country, the effect disappeared. Nobody has run it in Turkey, which is the market I write for. If you are about to put "80 percent of our customers choose this" on a page, put it there knowing the evidence behind it has never been tested where you sell. That is not an argument for leaving it out. It is an argument against certainty.

A Room with a Viewpoint, Journal of Consumer Research, 2008 · The replication, PLOS ONE, 2014

Scarcity

In 1975, Worchel, Lee and Adewole handed cookies from a jar to 200 undergraduates. One group got a jar with ten cookies, another got one with two. The group looking at two rated the cookie as more valuable.

The real finding was in the third group. They were shown ten cookies, then before they could taste one, eight were taken away and two were left. That group rated the cookies highest of all. Being scarce mattered less than becoming scarce while they watched.

But: the sample was 200 female undergraduates in 1975. Half a century ago, one country, one demographic. This is the sampling problem social psychology is known for. It does not refute the finding, and it does not justify presenting it as a universal law either.

The practical part sits somewhere the experiment never went: invented scarcity backfires fast. People notice the page that says "only 3 spots left" and still says 3 the next morning. Scarcity works when it is true. When it is a lie it works once, and then you have lost that person.

Effects of Supply and Demand on Ratings of Object Value, JPSP, 1975

Consistency

Someone who agrees to a small request is more likely to agree to a larger one. Freedman and Fraser's 1966 experiments gave this behaviour its name: foot-in-the-door.

The everyday version is simple. A person who has given you their email address is closer to the next step than a person who hasn't. But treating this as a technique means trying to trap someone behind a decision they already made. I would rather use it the other way round: keep the small step genuinely small, and keep the exit easy.

Authority

This heading usually gets Milgram's obedience experiments attached to it. I don't buy that. Milgram measured obedience to authority, not a purchase decision. Bridging the two directly claims more than the work supports.

Authority online runs on something else anyway. Not titles, but showing your work. Someone who shows what they built beats someone who states what they know.

Liking, and the seventh one

We buy from people we like. That one needs no experiment; everyone knows it from their own shopping.

What makes it interesting is that Cialdini added a seventh principle in Pre-Suasion in 2016: unity. The distinction is thin but it matters. Liking is "they are like me." Unity is "they are of me." Cialdini describes it less as a separate lever and more as a multiplier on the others: with unity in place, scarcity and social proof both work harder.

So the title says six, but the count has been seven for a decade. Freezing the list at six says more about the habit of repeating it than about the principles.

What the list does not tell you

Six principles are not a checklist, and they are certainly not a setup procedure to run in order. Each one is a tendency measured in a particular context, on a particular sample.

Look at what those contexts have in common: one restaurant, one hotel, one university lab. None of it was measured in Turkey, none of it on an email list, none of it at the moment someone decides to buy a digital product. Carrying these into your own work means carrying a hypothesis, not a laboratory result.

And after watching the most-cited one fall apart in Germany, I think twice before saying "the science says so."

What I don't know

I don't know how these six behave with a Turkish-speaking audience. My own list is nowhere near large enough to test them. When it is, I will measure and write that up. Until then the only honest position is that it hasn't been tried.

There is a second thing I don't know: whether these effects wear out. People have seen "only 3 left" and "most of our guests" so many times that a cue which worked in 1975 may not do the same job in 2026. I have no measurement showing that. I just have a suspicion.