He has limited his data to the USA; but should we not also see increases in Europe and UK as well, given the similar shifts in society and the manner in which p

He has limited his data to the USA; but should we not also see increases in Europe and UK as well, given the similar shifts in society and the manner in which people choose their spouses? If there were other factors that were found in the USA but not found in other countries (whatever that may be, environmental or otherwise) then there would be statistically significant divergence.