COPENHAGEN — The Nordic countries regularly appear at the top of an annual list of the world’s happiest nations, but their reputation as “happiness superpowers” masks the difficulties of a significant part of the population, a new analysis shows.

Finland, Norway, Denmark and Iceland led the 2018 ranking of the World Happiness Report, and Sweden wasn’t far behind, placing ninth. But in the five Nordic countries, an average of 12.3 percent of the population is “struggling” or “suffering,” according to a report by the Nordic Council of Ministers and the Happiness Research Institute in Copenhagen.

“Something doesn’t rhyme,” said Michael Birkjaer, an analyst with the Happiness Research Institute and an author of the report, titled “In the Shadow of Happiness,” referring to the uneven distribution of happiness across the Nordic countries. “It’s the youth driving the inequality to the highest degree.”

The report, based on research conducted from 2012 to 2016, asked people to assess their satisfaction with life on a scale of zero to 10. Those who answered seven or higher were categorized as “thriving,” those who responded five or six were classified as “struggling,” and those who said four or lower were deemed to be “suffering.” Most respondents in the Nordic countries reported satisfaction of seven to nine.