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In Norway, oil prices have fallen, a threat to one of its main industries. Across much of the country, average daytime temperatures still hover around the freezing point.

And yet, Norway is the happiest country on Earth, said the 2017 World Happiness Report, an annual ranking of 155 countries published by the Sustainable Development Solutions Network, a United Nations initiative. The Scandinavian country unseated Denmark for the top spot on this year's list, published Monday in conjunction with the U.N.'s International Day of Happiness.

Not far behind Norway are Denmark, Iceland, Switzerland and Finland. Tied for ninth are the Netherlands, Canada, New Zealand, Australia and Sweden.

The Central African Republic, a landlocked nation that has seen increasing violence between warring factions, came in last.

The United States fell in at number 14, down a spot from last year. The U.S. has never cracked the Top 10 since the rankings were first published in 2012, when it came in at No. 11.

Why measure happiness? Some experts say that it is a better measure of a nation's progress, and that using social well-being as a goal drives better public policy, the report said. "It's the human things that matter. If the riches make it harder to have frequent and trustworthy relationships between people, is it worth it?" asked John Helliwell, lead author of the report and an economist at the University of British Columbia. "The material can stand in the way of the human."

The World Happiness Report rankings are based on data from the Gallup World Poll, which uses a simple measure called the "Cantril ladder." People are asked to envision a ladder, with their "best possible life" being a 10 on the top rung, and the worst possible life being a 0. Where does their life fall on that ladder?

Six key variables are then used to explain those happiness scores, the report said: "income, healthy life expectancy, having someone to count on in times of trouble, generosity, freedom and trust, with the latter measured by the absence of corruption in business and government."

All of the countries in the Top 10 scored highly in those six areas, with Norway as the leading example of how those factors contribute to the happiness of its residents.

"It is sometimes said that Norway achieves and maintains its high happiness not because of its oil wealth, but in spite of it," the report said. "By choosing to produce its oil slowly, and investing the proceeds for the future rather than spending them in the present, Norway has insulated itself from the boom and bust cycle of many other resource-rich economies. To do this successfully requires high levels of mutual trust, shared purpose, generosity and good governance, all factors that help to keep Norway and other top countries where they are in the happiness rankings."

Americans, on the other hand, have been reporting declining happiness over the past decade, the report said. While the United States has improved in two of the six variables used to calculate happiness — log income per capita and healthy life expectancy — it has suffered when it comes to the four social variables. U.S. citizens are reporting less social support, less sense of personal freedom, lower donations and more perceived corruption of government and business.

Economist and Columbia University Prof. Jeffrey D. Sachs said the United States' declining happiness is "a social crisis, not an economic crisis."

The United States, he concluded in the report, is looking for happiness "in all the wrong places." "The country is mired in a roiling social crisis that is getting worse," Sachs wrote. "Yet the dominant political discourse is all about raising the rate of economic growth. And the prescriptions for faster growth — mainly deregulation and tax cuts — are likely to exacerbate, not reduce social tensions. Almost surely, further tax cuts will increase inequality, social tensions, and the social and economic divide between those with a college degree and those without."