A new study found that rich countries have a higher incidence of depression than middle-income and lower-income countries.

On average, rich countries have 15 percent of their population depressed while lower income countries only have 11 percent of their population depressed, according to the sampling of 89,000 people the study interviewed.

The U.S., France, and the Netherlands are the most depressed countries, according to the study. 

"We were struck by the difference among high-income and low-income countries.  Why this may be the case is the $64,000 question. We don't know for sure," said Dr. Evelyn Bromet of State University of New York at Stony Brook, lead author of the study, reported ABC News.  The study is published in "BMC Medicine."

One explanation is essentially jealousy.  In poor countries, almost everyone is poor.  In rich countries, contrastingly, the poor are constantly confronted with the vast wealth of the rich.

There is an old joke that goes something like this. 

A genie granted a man whatever he wanted with the caveat that whatever he gets, his neighbor will get twice as much. 

When he wished for a house, for example, his neighbor got two.  When he wished for a car, the same thing happened.  Finally, the man became so enraged that he wished to be blind in one eye so his neighbor would become blind in both eyes.

While the above story is comical and ridiculous, scientific studies do back up this pattern of thinking.

A University of Warwick and Cardiff University study, for example, found that money only makes people happy if it makes them richer than their peers.

"The actual amount of income and the average income of others appear to have no significant effect. Earning a million pounds a year appears to be not enough to make you happy if you know your friends all earn 2 million a year," said lead researcher Chris Boyce from the University of Warwick.

The results of the study explain why raising societal wealth doesn't lead to happiness, stated a press release from the University of Warwick about the study.