Did they separate the culture link from a potential genetic link? Maybe the association exists in a culture because it is a real effect, it just differs between groups.
NK has a lot of meth because the government started producing it for export in the 90s, and in the 2000s it became a sort of cottage industry and culturally normalized.
Imperial Japan was heavily dependent on amphetamines for wartime production and military stamina, and even gave the drug a patriotic-sounding name. It is definitely a useful tool for a totalitarian regime.
