RESEARCH ARTICLE Modes of Interaction between Individuals Dominate the Topologies of Real World Networks

We find that the topologies of real world networks, such as those formed within human soci-eties, by the Internet, or among cellular proteins, are dominated by the mode of the interac-tions considered among the individuals. Specifically, a major dichotomy in previously studied networks arises from modeling networks in terms of pairwise versus group tasks. The former often intrinsically give rise to scale-free, disassortative, hierarchical networks, whereas the latter often give rise to single- or broad-scale, assortative, nonhierarchical net-works. These dependencies explain contrasting observations among previous topological analyses of real world complex systems. We also observe this trend in systems with natural hierarchies, in which alternate representations of the same networks, but which capture dif-ferent levels of the hierarchy, manifest these signature topological differences. For example, in both the Internet and cellular proteomes, networks of lower-level system components (routers within domains or proteins within biological processes) are assortative and nonhier-archical, whereas networks of upper-level system components (internet domains or biologi-cal processes) are disassortative and hierarchical. Our results demonstrate that network topologies of complex systems must be interpreted in light of their hierarchical natures and interaction types