I think this is a good point and worth emphasizing.
The studies are focused on studying variation across populations—if everyone in the studied population is equally affected by the cultural forces in question, then this will not show up in the results.
This still means that in practice deliberate cultural interventions are less appealing. In this interpretation, you cannot work towards improving the values of a subpopulation and hope that they will persist through the time—the forces of dispersal and diffussion, as you say, will slowly wilt away the differences.
In order to effect deliberate change, you will need either a very convincing reason to believe that the change you are introducing will survive cultural diffussion, or a mechanism for affecting a very large population.
I think this is a good point and worth emphasizing.
The studies are focused on studying variation across populations—if everyone in the studied population is equally affected by the cultural forces in question, then this will not show up in the results.
This still means that in practice deliberate cultural interventions are less appealing. In this interpretation, you cannot work towards improving the values of a subpopulation and hope that they will persist through the time—the forces of dispersal and diffussion, as you say, will slowly wilt away the differences.
In order to effect deliberate change, you will need either a very convincing reason to believe that the change you are introducing will survive cultural diffussion, or a mechanism for affecting a very large population.