  Why is our society multicultural? Based on the two mechanisms of homophily
and social influence, the classical model for the dissemination of cultures
proposed by Axelrod predicts the existence of a fragmented regime where
different cultures can coexist in a social network. However, in such model the
multicultural regime is achievable only when a high number of cultural traits
is present, and is not robust against cultural drift, i.e. the spontaneous
mutations of agents' traits. In real systems, social influence is inherently
organised in layers, meaning that individuals tend to diversify their
connections according to the topic on which they interact. In this work we show
that the observed persistence of multiculturality in real-world social systems
is a natural consequence of the layered organisation of social influence. We
find that the critical number of cultural traits that separates the
monocultural and the multicultural regimes depends on the redundancy of
pairwise connections across layers. Surprisingly, for low values of structural
redundancy the system is always in a multicultural state, independently on the
number of traits, and is robust to the presence of cultural drift. Moreover, we
show that layered social influence allows the coexistence of different levels
of consensus on different topics. The insight obtained from simulations on
synthetic graphs are confirmed by the analysis of two real-world social
networks, where the multicultural regime persists even for a very small number
of cultural traits, suggesting that the layered organisation of social
interactions might indeed be at the heart of multicultural societies.
