By Allan Hall and Berlin
PEOPLE who smile a lot and say "have a nice day" are headed to an early grave, while the grumpy stay fit.
Researchers at a German university — a land where surliness is an Olympic discipline — say enforced jolliness on the job is likely to make people fall ill.
They found that flight attendants, sales personnel, call centre operators, waiters and others in contact with the public for long periods risked seriously harming their health.
Psychologists at Frankfurt University said the fake friendliness led to depression, stress and a lowering of the immune system, which in turn can trigger more serious ailments.
"Every time a person is forced to repress his true feelings, there are negative consequences for his health," researcher Dieter Zapf said.
The study tested students working in an imaginary call centre who were subject to abuse from clients. Some could answer back, while others always had to be polite and friendly.
Those who stood up to clients had a rapid heartbeat for a brief period, but those who had to remain friendly had racing hearts long after the client had hung up.
Psychologists concluded that "being friendly against one's will causes nothing but stress".
Flight attendants, shop assistants and carers also took part in the study.
"We are all able to rein in our emotions," Professor Zapf said. "It becomes difficult when you have to do this over a protracted period, as cabin attendants are forced to on long-haul flights.
"We have to get away from the 'customer is king' attitude and show more respect to those working in the service industries."