Question

Economic productivity has declined in some countries as a result of:

Answer 1

Economic productivity has declined in some countries as a result of  : Bubonic Plaque
In the past, Bubonic plaque manage to kill about 25 - 30 % of the world population, these greatly decrease the number of workers that could contribute to production activity and reduce Overall economic productivity in several countries

Answer 2

As for me, I think that the main reason of the declining of economic productivity in some countries is a war.  In the conducting of military operations by the State in view of the mobilization of the whole array of the armed forces, the country's economy begins to unwittingly work on the process of the war as a whole. Often companies that previously engaged in manufacturing of any civil or technology things are changing skills and become involved in the manufacture of military items.  Also on war spending huge amount of money. Even taking into account the positive outcome of the final - winning - is not to say that the war - this is a positive factor for the economy.