Preferential Attachment, Homophily, and the Structure of International Networks, 1816-2003

This study examines the extent to which network formation processes in international relations parallel models that characterize the formation processes of physical, biological, and social networks. I introduce two influential models from networks sciences: Preferential Attachment (PA) models state that the probability of a new node forming a link with an existing node is a function of the latter node's centrality. Networks that form through a PA process tend to have a power-law degree distribution. The Homophily (HO) model states that nodes tend to attach to similar other nodes. Such networks evolve into a set of homogenous subgroups. An analysis of alliance and trade networks over the 1816 (1870)-2003 period reveals strong evidence that alliance networks are affected by homophily processes. Trade networks form via a preferential attachment process. The tendency of international networks to evolve according to such processes increases over time. I discuss the implications of these results.