The black-white IQ gap in the US is not scientifically disputed. [1]

US prisons are ~38% black. [2] Norwegian prisons aren't.

Not to mention that in general, and without taking into account race, you don't expect the brightest to be in there.

Ergo average IQ in US prisons being much lower than in Norwegian prisons.

[1] https://en.wikipedia.org/wiki/Race_and_intelligence#United_S...

[2] https://www.bop.gov/about/statistics/statistics_inmate_race....