Debates have swirled around the legal definition of marriage, as U.S. states and indeed national governments consider the issue. This report draws on the longitudinal Portraits of American Life Study (PALS) to examine how the adult American public defines legal marriage, and whether that definition is changing over time. Interviewing the same 1294 Americans in 2006 and 2012, we track responses to the statement, “the only legal marriage should be between one man and one woman.” The findings include that in both years, the slight majority of adult Americans agree with the statement, and there was no significant overall change between 2006 and 2012. Yet, many Americans changed their minds over the period (some changing from agreeing to disagreeing, others from disagreeing to agreeing). The patterned manner in who changed their minds resulted in more division in 2012 than in 2006 in how Americans define marriage. Specifically, divisions have grown along educational, religious, and age lines. The patterns suggest a growing cultural divide across the nation.