Happy are the people of the Nordic nations — happier, in fact, than anyone else in the world. And the overall happiness of a country is almost identical to the happiness of its immigrants. According to a UN report released on Wednesday, Finland has overtaken Norway to become the happiest nation on earth. Norway, Denmark, Iceland, Switzerland, the Netherlands, Canada, New Zealand, Sweden and Australia follow in the same order.

Burundi and Central African Republic, both consumed by political violence, are the least happy countries for the second year in a row. This year, Central African Republic is slightly happier than Burundi; last year, their order was reversed.

The 2018 World Happiness Report also charts the steady decline of the US as the world’s largest economy grapples with a crisis of obesity, substance abuse and depression. The United States came in 18th out of 156 countries surveyed — down four spots from last year’s report and five from 2016’s, and substantially below most comparably wealthy nations. Though the economy is generally strong and per capita income is high, it ranks poorly on social measures: Life expectancy has declined, suicide rates have risen, the opioid crisis has worsened, inequality has grown and confidence in government has fallen.

For the first time the UN also examined the happiness levels of immigrants in each country, and found Finland also scored highest. “Finland has vaulted from fifth place to the top of the rankings this year,” said the report’s authors, although they noted that the other three Nordic countries (plus Switzerland) have almost interchangeable scores.

Explaining why one country is happier than another is a dicey business, but the report cites six significant factors: G.D.P. per capita, social support, life expectancy, freedom to make life choices, generosity and corruption levels.

The report, an annual publication from the UN Sustainable Development Solutions Network, said all the Nordic countries scored highly on income, healthy life expectancy, social support, freedom, trust and generosity. The rankings are based on Gallup polls of self-reported wellbeing, as well as perceptions of corruption, generosity and freedom.