In the late 1960s, behaviorism ruled psychology. If you wanted more of a behavior, you rewarded it, and B. F. Skinner had called money that “universal generalized reinforcer.”
Edward Deci, a graduate student, wanted to know what happens when you pay people for something they already enjoy. He chose the Soma puzzle, a set of three-dimensional pieces that can be fitted together to match figures on a sheet, some of them hard.
Students came in for three sessions, at least a day apart. In the middle of the first, the experimenter left the room for eight minutes, saying more material was needed. The students were told they could keep doing puzzles, read the magazines lying around, or do anything else. They were watched without knowing it, and the seconds they spent on the puzzles were counted.
In the second session, one group was paid $1 for every puzzle solved. In the third session, that group was told there wasn’t enough budget to pay them again. Both groups worked unpaid.
The experimenter stepped out once more, for another eight minutes.
The students who had been paid spent less of the break on the puzzles than they had in the first session, about 50 seconds less on average. The students who had never been paid spent about 28 seconds more.
Deci ran a field version at a college newspaper, paying one team of headline writers 50 cents a headline for three weeks. After the pay stopped, that team improved its speed less than the unpaid team, and showed a trend toward missing more work.
He also tried a different reward. In another study, students got praise instead of money, such as “You did very well in completing the task; many participants did not complete it.” Their interest held.
None of this was conclusive. The first experiment had 24 participants and its result was only marginally significant. Behaviorists attacked it. By the account of Deci’s later collaborators, it took years of replications, and a long fight over meta-analyses, before the finding became the field’s standard point of reference.
Go back to that room once more: puzzles on the table, no one else there, and the students who had once been paid spending less of their free time on them than they had the first time.