Too little education has been previously linked to poor mental health, but new research suggests the same problems for those with too much education, Live Science reported.

The researchers defined over-education as people having completed more years of school than their job requires. Such people, the study found, were at a higher risk of depression.

The study took information from 16,600 employed people in 21 different European countries, aged 25-60. Researchers presented their results at the annual American Sociological Association meeting in New York. They said they measured levels of depression with answers to a survey questionnaire.

The reason, Piet Bracke, a professor of sociology at Ghent University in Belgium, and his team believe over-educated people are more susceptible to mental stress is because they are not using the skills and knowledge acquired in school at work. In other words, they see the work they are assigned as being beneath them.

"At the country level, if the number of people with university education continues to rise, [and] if there isn't an equivalent upgrading of the labor market, it will deteriorate the mental health of the population," Bracke said.

Also contributing to mental distress was a lack of prestige or status in their job titles. Bracke also said some over-educated people were relying on others for support more than they could afford to give it.

Previous research has pointed to lower levels of education leading to more severe and frequent depression, but depending on the country, those levels of risk vary.

Bracke said when a country has a high level of over-educated people; it negatively affects anyone with a college degree. Even among people with a college degree and a job to match their skill levels, mental health still suffered.

"If the economic returns of education decrease, it affects the mental health of all the well-educated," Bracke said.

With the poor economy, it has become harder for college graduates to find a job in their field with the potential to move upward in a few years. In other words, graduates are longer remaining in positions that they are over-qualified for, heightening the risk of depression.

"At the country level, if the number of people with university education continues to rise, [and] if there isn't an equivalent upgrading of the labor market, it will deteriorate the mental health of the population," Bracke said.