Birds of a feather sign together: Co-ratification patterns in the International environmental agreement network

International environmental agreements (IEAs) are a form of joint action that require coordination and cooperation. The action of ratifying an IEA is influenced by national level characteristics as well as agreement characteristics. However, these agreements can also be seen as being connected through the overlapping countries that ratify them, while countries can be seen as connected through the agreements they choose to co-ratify. The implicit bipartite structure of the IEA network, constituted by both countries and treaties, can give rise to two distinct, yet interacting one mode networks. Exploring the factors that influence each network can build on our understanding of IEA ratification in the present as well as the past, while expanding our understanding of agreement ratification generally. Moreover, the central sociological theories addressing IEA ratification have implications for how categories of countries co-ratify at different rates, both individually and together. The aim of this study is to explore how categories of both countries and agreements are associated with varying rates of co-ratification and how these associations have changed over time. Our analysis employs newly coded network data from over 840 multilateral IEAs between 208 countries, ranging in signing date from 1857 to 2022; using both period-based and lagged analyses, we consider the effects of past and ongoing mechanisms of treaty ratification. Our findings demonstrate only modest support for regionalism, while suggesting an evolution in the patterning of co-ratification, with a regime change from a world systems configuration to a world society one over the past 20 to 30 years. In addition to providing a picture of the evolving IEA system, this work demonstrates how the use of network methods in combination with a dual focus of countries and treaties can further our knowledge of various forms of agreement ratification.