FRANKFURT, Germany, May 16 (UPI) -- A German scientist has proved that people forced to smile and take on-the-job insults suffer more and longer-lasting stress that may harm their health.

Dieter Zapf of the Johann Wolfgang Goethe University in Frankfurt studied 4,000 volunteers working in a fake call center. Half were allowed to respond in kind to abuse on the other end of the line while the other half had to suck it up, The Telegraph reports.

He found that those able to answer back had a brief increase in heart rate. Those who could not had stress symptoms that lasted much longer.

"Every time a person is forced to repress his true feelings there are negative consequences," Zapf said. "We are all able to rein in our emotions but it becomes difficult to do this over a protracted period."

In an interview with the German healthcare magazine Apotheken Umschau, Zapf suggested that people who must keep smiling on the job should get regular breaks to let it out.