On the relationship between interindividual cultural transmission and population-level cultural diversity: a case study of weaving in Iranian tribal populations

It is often assumed that parent-to-child cultural transmission leads to similarities and differences among groups evolving through descent with modification (“phylogenesis”). Similarly, cultural transmission between peers, and between adults and children who are not their offspring, is widely believed to result in groups exchanging cultural traits (“ethnogenesis”). However, neither of these assumptions has been examined empirically. Here, we test them using ethnographic data on craft learning in Iranian tribal populations and the cladistic method of phylogenetic analysis. We find that parent-to-child transmission dominates learning during childhood, but the other two forms of interindividual transmission become more important in later periods. The latter do not, however, appear to have resulted in extensive exchange of cultural traits among tribes. Instead we find that most of the variation among the tribes' craft assemblages can be explained by descent with modification. This can be accounted for by the fact that weavers usually only share their knowledge with members of their own tribe and are prevented from interacting with women from other groups by social norms. These findings demonstrate that the relationship between processes of cultural evolution at the level of the individual and processes of cultural evolution at the level of the group is more complex than is usually acknowledged, and highlight the need for more integrated studies of the processes operating at both scales.