People feel obliged to repay favours, concessions, and gifts, often disproportionately.
Regan's 1971 study: a confederate who bought an unrequested soft drink sold twice as many raffle tickets. The Hare Krishna flower and the free sample both work on the same obligation.
In practice
- A charity that encloses a pen with its appeal raises more than the identical letter without one, and the pen costs nine pence.
- A supplier's Christmas hamper costs £40 and shifts a procurement decision worth far more, which is why gift registers exist.