In restaurants in the US, customers pay out about $26bn in tips every year. And this is only one corner of the hospitality industry, where it is customary also to tip bartenders, bellboys, casino croupiers, chambermaids, doormen, parking valets, washroom attendants and others. But how do you know, in America or elsewhere, whether or when to tip?

Academics who study tipping have identified 33 jobs for which remuneration includes tips. Fittingly, arguably the world's leading authority on the psychology of tipping is himself a former bartender, bus boy and waiter. Michael Lynn is now professor of consumer behaviour at Cornell University in New York. He believes that tipping should be of interest to economists and psychologists, because it doesn't appear to make any economic sense, and may instead lie firmly in the realm of psychology. Few people aspire to pay more than is strictly necessary for goods and services, yet in tipping they do so, often quite voluntarily. So, what accounts for this largesse?