This research note examines changes in attitudes toward same-sex marriage over time. Using OLS regression and decomposition techniques to analyze General Social Survey data, I explain individuals’ attitudes toward same-sex marriage from 1988 to 2004, 2006, 2008, and 2010. The influences on same-sex marriage attitudes differed substantially over time. Many of the characteristics commonly thought to increase opposition to same-sex marriage (including being African American, living in the southern United States, being an evangelical Protestant, and being Republican) are associated with attitudes only in the later years. In 1988, opposition was generally much higher for everyone; most respondents expressed at least some to strong disapproval of same-sex marriage in 1988, which was then reduced for the highly educated, urban residents, and those with less conservative or no religious affiliations. That is, support for same-sex marriage was fairly localized to specific subgroups in 1988. But, by 2010, support for same-sex marriage was much more broad-based, and opposition to same-sex marriage became more localized to specific subgroups—older Americans, southerners, African Americans, evangelical Protestants, and Republicans. The decomposition analysis finds that changing same-sex marriage attitudes are not due to demographic changes in the American population. Rather, the liberalization in same-sex marriage attitudes from 1988 to 2010 is due primarily to a general societal change in attitudes, as is evidenced by the large change in the constant. Taken together, the results suggest that changing attitudes toward same-sex marriage reflect a cultural shift.