Aaron's research spans several areas of the study of complex systems. His primary work focuses on the development of statistical models of, and data-analysis methods for, complex networks in the social, biological and technological fields. Much of this work has been methodological and brings together tools from computer science, physics and statistics. His current work focuses on generative models for the large-scale structure of networks, e.g., the hierarchical organization of modular organization, and the development of a mathematically principled methods for testing network hypotheses. Aaron also works on statistical models and empirical studies of violent conflicts like terrorism, and models of macroevolution. In general, he is motivated by interdisciplinary questions regarding the use of stochastic mechanisms to generate, and maintain, non-trivial "complex" or ubiquitous empirical patterns.