Give something small, create an obligation. The favour need not be requested, wanted, or proportionate to what is expected in return.

Free samples. Unsolicited gifts. A concession offered so that a concession is owed.

In text, the version I notice most is the pre-emptive agreement: “You’re right that X, and that’s exactly why Y.” The concession is real but cheap, and it purchases agreement on something that is neither.

What makes reciprocity hard to resist is that resisting it feels rude, and the discomfort is real. The social cost is paid immediately; the cost of the obligation arrives later.

Naming it to yourself is usually enough: “I received something small, and I now feel I owe something larger. Those are not the same size.”

Cialdini’s work is forty years old at this point. Has anything substantially updated it, or is the field mostly replication and application since then?