This study examines whether bans on affirmative action across four states-- Texas (during "Hopwood v. State of Texas"), California (with Proposition 209), Washington (with Initiative 200), and Florida (with One Florida Initiative)--have reduced the enrollment rates of underrepresented students of color in graduate studies and in a cross-section of graduate fields: the natural sciences, engineering, social sciences, business, education, and humanities. Unlike most prior studies, which have examined the effects of an affirmative action ban in one state, this study adopts a cross-state approach to estimate the effect of multiple bans on the enrollment rates of graduate students of color. In this analysis, the outcome is defined as the "proportion" of first-year graduate students who are underrepresented students of color because the overall enrollment of graduate students changes over time; this measure has also been used in other studies (e.g., Howell, 2010; Hinrichs, 2009). This definition of underrepresented students of color includes students whose self-reported race or ethnicity is African American, Latino, and/or Native Americans/Alaska Natives, and who are not international students. The findings contribute to the mounting evidence about the detrimental effects bans on affirmative action have had on the representation of students of color in postsecondary education. Specifically, the bans in Texas, California, Washington, and Florida have reduced by about 12 percent the average proportion of graduate students who are students of color across all the fields of graduate study included in the evaluation. In "engineering," the bans have led to about a 26-percent reduction in the mean proportion of all enrolled graduate students who are students of color; a 19-percent decline in the "natural sciences"; a 15.7-percent drop in the "social sciences," and a 11.8-percent drop in the "humanities." Bans on affirmative action have also led to about a 13-per decline in "education," though the effect in this field is only marginally statistically significant. The author did not find an overall impact of affirmative action bans on the proportion of graduate students of color who are enrolled in the field of "business." In terms of individual students, these declines confirm an average of about 12 fewer students of color in engineering in total across these states; an average of 21 fewer underrepresented students of color in the natural sciences; an average of 10 fewer students of color in the social sciences; and an average of 8 fewer students of color in the humanities. These numbers reflect the already minimal representation of students of color in most of these fields, even with affirmative action policies. These findings also have broad implications for higher education institutions across the nation. Institutions located in states where they can still pursue affirmative action may be faced with ballot initiatives that seek to ban the practice. In these cases, information about the detrimental effects affirmative action bans have had at the undergraduate level, for the profession of law and, as documented in this study, across other graduate fields of study, needs to be considered by all stakeholders: the general public, policymakers, and institutional actors. These findings should also help inform pending challenges to the constitutionality of affirmative action practices, such as "Fisher v. University of Texas at Austin" (2011). They can also inform challenges to the constitutionality of bans on affirmative action in cases such as "Coalition to Defend Affirmative Action v. Regents of the University of Michigan" (2011). Simultaneously, postsecondary institutions and policymakers at the state-level need to continue to implement and explore effective strategies for increasing student of color representation and persistence to degree across fields of study. In light of the systemic inequities that contribute to the general underrepresentation of students of color at higher score percentiles in standardized tests, the findings should lead educators to reconsider their reliance on these measures in admissions, particularly in states where the tool of affirmative action is no longer available. Appended are: (1) Analytic Strategy; (2) Impact Across Graduate Studies; (3) Impact by Field of Study; and (4) Tables. (Contains 5 tables and 19 footnotes.)[The foreword was written by Gary Orfield.]