An international research, which surveyed nearly 90,000 people in 18 countries has found that some of the richest countries have the highest rates of depression. The survey was conducted through face-to-face interviews using World Health Organizations standard diagnostic test to assess the level of depression. Researchers said that in the 10 high-income countries, an average of 15% of participants said they had experienced depression in their lifetime. People in countries such as France, the Netherlands, New Zealand and the U.S. show even a higher rate of depression at more than 18%. On the other hand, the rate was only at 11% in the eight low- to middle-income countries surveyed, including India, Mexico, China and South Africa. Regarding the result, researchers believe that higher depression rates may have something to do with the more stressful environment and larger wealth gap in rich countries. They concluded that depression is to some extent an illness of affluence, while suggesting that more research is needed to be done on the link between social conditions and depression.