Gifts can have effects on worker productivity that cash can't match, according to a new study.

On two occasions, in 2007 and 2010, researchers posted an ad for jobs cataloging books in a German economics department. The pay was 12 euros ($16.50) an hour, for three hours' work, and there was no hint this was an experiment. Before the workers got started, some were told they'd get an extra €7—a random bonus. Others were given an attractive thermos, also worth about €7, wrapped as a gift.

Productivity was measured by counting the characters in the workers' database entries. Workers who got the gift were 25% more productive than a control group, while the cash group's small productivity boost was statistically insignificant. The effect persisted even when test subjects were explicitly told the value of the thermos.

On purely economic grounds, the workers' reaction was illogical, since €7 is a more efficient transfer of goods, allowing its recipient to purchase what he or she wants (like, say, a stylish thermos). But workplace gifts work as signals of kindness, or loyalty (though not exactly loyalty in this case, since there was no prospect of extended employment), or something extra.

In one experimental condition, workers themselves got to choose which bonus to take. As it happens, more than 90% picked the cash. But that group still worked 25% harder than the control group. (It really is the thought that counts.)