The two economists measured the amount of cash in circulation in Sweden and found that 67 percent of all cash transactions were unexplained for the period in question.
The total value of coins and notes in circulation in Sweden amounts to 100 billion kronor ($14.3), almost half of which is made up off 1,000 kronor notes.
"We have assumed that the majority of economic activity that is not registered is carried out by people who don't want it registered," economist Gabriela Guibourg told Dagens Industri.
The Swedish National Economic Crimes Bureau and the tax authorities both share the Riksbank's conclusion that the black market sector has grown in recent year. The
trend is more or less the same in other Nordic countries.