Denmark has a problem: it may soon be unable to afford offering such a good deal to its people.

Free health care for all, a $757 monthly stipend for college students and robust safety nets for the less fortunate all cost money – Denmark devotes slightly more than 30 percent of its gross domestic product to social spending, one of the highest levels in the rich world.

According to the Organisation for Economic Cooperation and Development, such a "generous welfare system requires robust public finances." And while Denmark's "seem sustainable" for now, the assumptions that underpin that view carry a high level of uncertainty, the OECD warned in a recent survey. 

Take labor market participation.