As of 2020, same-sex marriage has been legalized at the national level in 28 countries. This dissertation examines the factors that make legalization of same-sex marriage (same-sex marriage) more likely. Using quantitative analysis and a customized index, The Rainbow Index, to capture overall country friendliness towards LGB equality, this dissertation shows that a combination of secular and emancipative values, a high degree of LGB mobilization capacity, and opportunities for transnational advocacy networks to connect with and support local activists make legalization more likely. Furthermore, this combination of variables, which together represent a country’s friendliness to LGB equality, is more predictive of same-sex marriage legalization than the effects of modernization. Case study qualitative analysis drawing from the cases of Colombia, Japan, Mexico, and Taiwan will also be conducted based on the quantitative findings.