The United States is a federal republic and a representative democracy with three separate branches of government, including a bicameral legislature. It is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and other international organizations. It is a permanent member of the United Nations Security Council. Considered a melting pot of cultures and ethnicities, its population has been profoundly shaped by centuries of immigration. The U.S. ranks high in international measures of economic freedom, quality of life, education, and human rights; it has low levels of perceived corruption. However, the country has been criticized for inequality related to race, wealth, and income; use of capital punishment; high incarceration rates; and lack of universal health care.