Assortative mixing by degree makes a network more unstable

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We investigate the role of degree correlation among nodes on the stability of complex networks, by studying spectral properties of randomly weighted matrices constructed from directed Erdös-Rényi and scale-free random graph models. We focus on the behaviour of the largest real part of the eigenvalues, λₘax, that governs the growth rate of perturbations about an equilibrium (and hence, determines stability). We find that assortative mixing by degree, where nodes with many links connect preferentially to other nodes with many links, reduces the stability of networks. In particular, for sparse scale-free networks with N nodes, λₘax scales as Nα for highly assortative networks, while for disassortative graphs, λₘax scales logarithmically with N. This difference may be a possible reason for the prevalence of disassortative networks in nature.