Mental health problems have overtaken musculoskeletal disorders such as back pain as the main reason for incapacity benefit claims, researchers have said.
Experts, writing in Occupational Medicine, looked at new benefit awards for both kinds of conditions in Britain from 1997 to 2007.
Claims for musculoskeletal disorders fell by 50% over the 11-year study, while mental health claims were steady.
Social views of illness might explain the change, the team said.
There are 2.6m people of working age in the UK currently claiming incapacity benefit.
The ratio of new claims for mental illness to those for musculoskeletal disorders more than doubled between 1997 and 2007, Department for Work and Pensions data showed.
Mental illness claims remained at around a quarter of a million while musculoskeletal disorders fell from 181,820 in 1997 to 84,420 in 2007.
The change occurred across the country, but the difference was more significant in north-east England and Scotland than in the South East.
The researchers say such large changes cannot be explained by changes in working practices linked to musculoskeletal problems, and that there were no changes in the criteria used to assess claims.
Instead, David Coggon, Medical Research Council professor of occupational medicine at Southampton General Hospital who led the study, suggested it may be to do with people's beliefs and expectations.
The pattern seen during the period of the study runs contrary to that seen between 1950 and 1990, when the number of IB claims for back pain increased eightfold.
However, during that time there was a widespread belief that back pain could be long-term and could seriously incapacitate people.
Now, people are aware that if they strain a muscle they can be better in a few weeks, Professor Coggon said.
He added: "In a particular country, or in a particular occupational group, at a particular time, there will be certain illnesses that everyone knows you can get.
"I'm not saying people aren't ill or disabled. But there are complex causes."
Professor Coggon suggested one way to tackle the level of mental health claims would be to change the approach to stress in the workplace.
"If you say you're trying to tackle hazards linked to workplace stress, it sends a message that people are exposed to 'bad things' and that affects reactions."
He said it was better to stress positive approaches, such as the introduction of good management and not overloading people with work.
Dr Olivia Carlton, president of the Society of Occupational Medicine, said: "A life on incapacity benefit means that many people lose their sense of self worth, identity and esteem and also places a huge financial burden on the country."