Sad Man
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Sad older man.

Depression is more likely to strike in high-income countries than in poor ones, according to new research on depression rates across 18 countries worldwide.

The study, published July 25 in the open-access journal BMC Medicine, found that the average lifetime prevalence of major depression in the 10 high-income countries in the study was 14.6 percent. In the eight low- and middle-income countries, the lifetime prevalence of major depression was 11.1 percent.

Across countries, depression was linked to social factors such as age, marital status and income, though sometimes in complicated ways. In low-income and middle-income countries, for example, the average age of a first depressive episode was 24. In high-income countries, depression was likely to hit almost two years later, at 25.7.

The researchers speculate the wealthier countries experience more of the blues because richer countries also have more income inequality. In addition, depression may be a disease of the affluent, a phenomenon that isn't fully understood, they say. Figuring out the causes of depression around the world will help initiatives to combat the mental-health problem, which has been linked to Alzheimer's disease. In severe cases, depression can end in suicide, which leads to about 850,000 deaths per year, according to the World Health Organization (WHO).

World of sadness

Earlier studies had found variations in depression rates across countries, but this study was the first to assess depression with standardized surveys, making it less likely that differences in the way questions were asked account for the results.

As part of a WHO study, trained surveyors conducted face-to-face interviews with 89,037 people in 18 countries. The 10 high-income countries were: Belgium, France, Germany, Israel, Italy, Japan, the Netherlands, New Zealand, Spain and the United States. The eight middle- to low-income countries were: Brazil, Colombia, India, China, Lebanon, Mexico, South Africa and Ukraine.

The surveyors asked respondents questions about their major depression symptoms, including sadness and loss of interest in daily life, in order to diagnose the disorder. Respondents also answered questions about their age, income, marital status and other demographic data.

According to the study, here are the percentages of people who have experienced or will experience depression at some point in their lives:

  • Japan: 6.6 percent
  • Germany: 9.9 percent
  • Italy: 9.9 percent
  • Israel: 10.2 percent
  • Spain: 10.6 percent
  • Belgium: 14.1 percent
  • New Zealand: 17.8 percent
  • Netherlands: 17.9 percent
  • United States: 19.2 percent
  • France: 21 percent
Low- and middle-income:
  • China: 6.5 percent
  • Mexico: 8 percent
  • India: 9 percent
  • South Africa: 9.8 percent
  • Lebanon: 10.9 percent
  • Colombia: 13.3 percent
  • Ukraine: 14.6 percent
  • Brazil: 18.4 percent
Demographics of depression

Marital status was linked to depression in both low- and high-income countries, with people in high-income countries more likely to be depressed if they were separated or never married, and people in low-income countries more likely to be depressed if they were divorced or widowed. In France, Germany, New Zealand and the United States, the poorest respondents had double the risk of major depression compared with the richest respondents, but in middle- and low-income countries there was no link between individual income and depression.

Income inequality, which is larger in high-income countries, promotes a slew of chronic conditions, including depression, the researchers speculate.

One factor that held true across countries was the gender ratio of depression. No matter the nationality, women were twice as likely as men to have experienced depression.

The study had some weaknesses, including the fact that South Africa was the only African country included, the researchers wrote. Nonetheless, the research is important for understanding how depression affects people worldwide, study researcher Evelyn Bromet of the State University of New York at Stony Brook said in a statement.

"We have shown that depression is a significant public-health concern across all regions of the world and is strongly linked to social conditions," Bromet said. "Understanding the patterns and causes of depression can help global initiatives in reducing the impact of depression on individual lives and in reducing the burden to society."