Published: 24/02/2005, Volume II5, No. 5944 Page 38

It is highly likely that within the last 24 hours you have performed a behaviour which economists maintain makes little sense - you voluntarily increased the price of a service by adding a gratuity - commonly referred to as tipping.

Tipping service workers is particularly widespread in the hospitality industry and in some countries, like the US, it is customary to tip bartenders, bellhops, casino croupiers, parking valets, lavatory attendants and so on. US restaurants alone have a $26bn annual tipping bill.

So why is it acceptable to tip a waiter or hairdresser, but not a doctor?

At the heart of the psychology of tipping is a key assumption - tipping hinges on the idea that the worker has some control over quality of service.

The idea of variable quality at discretion is a deeply troubling notion in professions like medicine. It suggests an intriguing concept at the foundation of tipping: we only tip when it doesn't really matter - because if it did, like if it was brain surgery, we would expect the best service to be included in the price.

This is illustrated by a study on tipping which found that the server drawing a happy face on restaurant bills significantly increases tips. Imagine a brain surgeon drawing a happy face on their bill regardless of the outcome of the operation. . .

Managers often get obsessed with measurement in terms of comparing doctors with each other - whereas we tend to ignore the fact that there is bound to be variation in performance, and a key aspect of management should be to find out what determines poor or superior performance.

A large part of the stress of management is battling with underperforming doctors. A solution is understanding that everyone is subject to performance variability, and improvement usually hinges on grasping the particular determinants of skill in an individual doctor.

The reason this might be a managerial blind spot is because of the deeply ingrained belief that doctors are always doing their best. Since no-one else is (which supposedly is why we tip), why do we believe this of doctors?

Dr Raj Persaud is consultant psychiatrist at South London and Maudsley trust and Gresham professor for public understanding of psychiatry.