In one especially elegant and effective experiment, psychologist David Strohmetz and his colleagues arranged for waiters to hand customers their bills with or without sweets, and examined the impact on tipping. In the control condition, diners were unlucky enough to receive their bills without any sweets at all. A second group was given a single sweet. Compared to the control group, this simple gesture of kindness resulted in a measly 3 per cent increase in tips. A third group of customers received two sweets each, and, compared to the control group, gave 14 per cent larger tips. Not bad. However, here comes the really clever bit. In the fourth and final condition, the waiters were asked to present the bill to customers along with one sweet each, then, just as they were turning away from the table, reach into their pocket and quickly hand everyone a second sweet. In terms of sweets per customer, everyone ended up with exactly the same number of sweets as those in the third group. But psychologically speaking, this was very, very different. The waiter had just carried out an unnecessary and nice favour, and, because of that, tipping increased by an impressive 23 per cent.