Without between-group variation, cultural group selection could not occur, as there would be no group differentiation to select for. While processes such as cultural drift, epidemics, and natural disasters increase between-group variation, migration and genetic mixing decrease between-group variation and increase within-group variation. Variation is only maintained when cultural groups have mechanisms that prevent the norms of outside groups from invading the cultural group. These ‘mechanisms’ are those uniquely human psychological traits and behaviours that encourage imitation, conformity, and in-group biases.
