The American Dream of moving up to the middle class is looking more difficult than ever for those stuck on the bottom rungs of society.

Among developed nations, the United States has the third lowest social mobility, topping only Italy and the United Kingdom, according to research by University of Ottawa economist and current Russell Sage Foundation Fellow Miles Corak, CNNMoney reports.

Americans have a 47 percent chance of being stuck in the same economic class as their parents, the study says. That compares with 50 percent in Britain, 41 percent in France, 29 percent in Canada and only 15 percent in Denmark.

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Economists offer several explanations for why some countries offer less opportunity for their underprivileged citizens, CNNMoney notes.

First there's income inequality. The more unequal a society is currently, the more likely it is to stay that way. That's because the wealthy are able to provide benefits to their children, such as tutors, extracurricular activities and the time to engage in them, that the poor simply can't.

Access to strong education and a stable family life are important, too.

"Cities with strong families, civic support groups and a community-service orientation do well on social and economic mobility," author and CNN host Fareed Zakaria writes in his column in The Washington Post.

"That's why Salt Lake City — dominated by Mormons — has mobility levels that compare with Denmark's. This would also explain why America in general fares badly. The United States has many more broken families, single parents and dysfunctional domestic arrangements than do Canada and Europe."

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