People who live in wealthy countries are slightly more likely to be depressed than those in low- to middle-income countries, a new study of global depression rates suggests.

In the study, close to 15 percent of people in high-income countries said they experienced depression at some point in their lives. That compares with 11 percent in low- and middle-income countries.

However, across nations, women were twice as likely as men to suffer from depression, the researchers said.

And no matter where a depressed person lived, the condition affected his or her ability to function in everyday life, the researchers said.

"In every single country, depression was related to impairment," said study researcher Evelyn Bromet, a professor of psychiatry and behavioral science at the State University of New York at Stony Brook. "What [this] says is that, whatever depression means across the world, it has an impact on people's lives," Bromet said.

Global depression stats

About 121 million people worldwide suffer from depression, and the condition is the fourth leading cause of disability, according to the World Health Organization.

Bromet and colleagues reviewed interviews of about 89,000 people from 18 countries 10 high-income countries, including France, Germany, Japan and the United States, and 8 low- and middle-income countries, such as Brazil, India, China and Mexico.

The researchers asked participants questions about their symptoms, and diagnosed major depressive episodes according to criteria in the Diagnostic and Statistical Manual of Mental Disorders .

Depression was estimated to have its highest prevalence in high-income countries, including France (21 percent) and the United States (19 percent.)

Some of the lowest rates were in low- to middle-income countries, including Mexico (8 percent) and China (6.5 percent).

However, some high-income countries had low rates of depression, such as Japan (6.6 percent), and some low- to middle-income countries had high rates, such as Brazil (18 percent).

When asked if they had experienced depression within the last year, the numbers were more similar: 5.5 percent of those in high-income countries had, and 5.9 percent in low- and middle-income countries had.

The effect of age on depression differed across countries. In the United States and some European countries, depression rates went down with age. "People get happier as they get older," Bromet said. But in some of the low- and middle-income countries, such as Ukraine, the opposite was true, she said.

Rich-poor dichotomy

It's not clear why high-income countries have higher rates of depression. Rich countries tend to have greater income disparities between the very rich and very poor, which could play a role in the development of depression, the researchers said.

It's also possible that the study underestimated depression rates in low- and middle-income countries, the researchers said. People in these countries have a lower life expectancy, and those with depression may die even earlier. The study also included only one country in Africa.

Unlike previous estimates of global depression rates, the researchers of the new study ensured that the study protocols were conducted exactly the same in all cases. However, the study used a definition of depression developed in Western countries. Creating diagnostic criteria for depression based on the culture nuances of each country may increase the ability to detect depression cross-culturally, the researchers said.

The study will be published in an upcoming issue of the journal BMC Medicine. It was conducted in conjunction with the World Health Organization World Mental Health Survey Initiative and led by Ronald Kessler of Harvard Medical School.

Pass it on: Depression is a global problem, but rates vary considerably depending on where you live.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner.