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.
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