  Since the publication of 'Complex Contagions and the Weakness of Long Ties'
in 2007, complex contagions have been studied across an enormous variety of
social domains. In reviewing this decade of research, we discuss recent
advancements in applied studies of complex contagions, particularly in the
domains of health, innovation diffusion, social media, and politics. We also
discuss how these empirical studies have spurred complementary advancements in
the theoretical modeling of contagions, which concern the effects of network
topology on diffusion, as well as the effects of individual-level attributes
and thresholds. In synthesizing these developments, we suggest three main
directions for future research. The first concerns the study of how multiple
contagions interact within the same network and across networks, in what may be
called an ecology of contagions. The second concerns the study of how the
structure of thresholds and their behavioral consequences can vary by
individual and social context. The third area concerns the roles of diversity
and homophily in the dynamics of complex contagion, including both diversity of
demographic profiles among local peers, and the broader notion of structural
diversity within a network. Throughout this discussion, we make an effort to
highlight the theoretical and empirical opportunities that lie ahead.
