Another angle.....
Cousin marriage relative to the advancement of Western Civilization....
https://marginalrevolution.com/marginalrevolution/2013/04/cousin-marriage-and-democracy.htmlSuch kin groupings may be extremely nepotistic and distrusting of non-family members in the
larger society. In this context, non-democratic regimes emerge as a consequence of individuals turning to reliable kinship groupings for support rather than to the state or the free market. It has been found, for example, that societies having high levels of familism tend to have low levels of generalized trust and civic engagement, two important correlates of democracy. Moreover, to people in closely related kin groups, individualism and the recognition of individual rights, which are part of the cultural idiom of democracy, are perceived
as strange and counter-intuitive ideological abstractions.
Just so happens that the Irish and Scottish Clans commonly married cousins