Even if Korean reunification happens now, a declining population both in the South and the North is expected to lead to an overall drop in the population starting in 2034.

There have been expectations that North Korea's young population could solve South Korea's low birthrate following reunification. But the latest study by Choi Joon-wook at the Korea Institute of Public Finance pours cold water on such hopes.

The main reason for the population decline is that North Korea is already suffering from a low birthrate and aging population. As of 2015, North Korea's birthrate stood at 1.94 children by the UN tally. That is far less than the average birthrate of 4.75 in other underdeveloped countries.

"Countries with low per-capita GDP usually have high birthrates, but North Korea is unique by not being part of that trend," the study says.

The development is mainly due to the absurdly extended conscription of young North Korean men, which delays the marrying age, and a large number of North Korean women who work. The birthrate in the North declined much sooner than its economic level would suggest.

The report said South Korea and China saw rising populations in phases of high growth, which resulted in economic expansion. But that is unlikely to happen in North Korea.