  Human societies include diverse social relationships. Friends, family,
business colleagues, and online contacts can all contribute to one's social
life. Individuals may behave differently in different domains, but success in
one domain may engender success in another. Here, we study this problem using
multilayer networks to model multiple domains of social interactions, in which
individuals experience different environments and may express different
behaviors. We provide a mathematical analysis and find that coupling between
layers tends to promote prosocial behavior. Even if prosociality is disfavored
in each layer alone, multilayer coupling can promote its proliferation in all
layers simultaneously. We apply this analysis to six real-world multilayer
networks, ranging from the socio-emotional and professional relationships in a
Zambian community, to the online and offline relationships within an academic
University. We discuss the implications of our results, which suggest that
small modifications to interactions in one domain may catalyze prosociality in
a different domain.
