The belief that people should be treated equally under the law is one of the cornerstones of political thought in the United States. Yet not all citizens have been treated equally throughout the nation’s history, and some are treated differently even today. For example, until 1920, nearly all women in the United States lacked the right to vote. Black men received the right to vote in 1870, but as late as 1940 only 3 percent of African American adults living in the South were registered to vote, largely due to laws designed to keep them from the polls.Constitutional Rights Foundation. “Race and Voting in the Segregated South,” http://www.crf-usa.org/black-history-month/race-and-voting-in-the-segregated-south (April 10, 2016). Americans were not allowed to enter into legal marriage with a member of the same sex in many U.S. states until 2015. Some types of unequal treatment are considered acceptable, while others are not. No one would consider it acceptable to allow a ten-year-old to vote, because a child lacks the ability to understand important political issues, but all reasonable people would agree that it is wrong to mandate racial segregation or to deny someone the right to vote on the basis of race. It is important to understand which types of inequality are unacceptable and why.