The paper "Polarized America: The Dance of Ideology and Unequal Riches" by Nolan McCarty, Keith T. Poole, and Howard Rosenthal examines the transformation of American politics over the past 25 years, focusing on the increasing polarization along ideological lines. The authors use historical data from roll call votes in Congress to demonstrate that the behavior of members of Congress has become highly polarized, with more Democrats adopting consistently liberal positions and more Republicans supporting exclusively conservative ones. This polarization began in the 1970s, reversing a long-term decline that started at the turn of the 20th century. The paper highlights five key trends: 1. Most political conflict in Congress is expressed in terms of liberal-conservative differences. 2. The dispersion of positions on the liberal-conservative dimension has increased. 3. The ideological composition of political parties has become more homogeneous. 4. The positions of average Democrats and Republicans have become more widely separated. 5. There is less overlap between the parties, with no longer any liberal Republicans or conservative Democrats. The authors also explore the causes of this polarization, including changes in the link between constituency interests and congressional voting, the Southern realignment, enhanced party leadership, congressional apportionment, and primary elections. They find that while constituency factors play a role, the surge in polarization cannot be fully explained by them alone. Instead, the increased link between constituency interests and congressional voting, along with the growing divergence between the parties, are key factors. The paper concludes by discussing alternative explanations for congressional polarization, such as the Southern realignment, and suggests that the economic and demographic changes in the South, particularly the migration of middle and upper-class Whites, have contributed to the convergence of southern politics with the rest of the country.The paper "Polarized America: The Dance of Ideology and Unequal Riches" by Nolan McCarty, Keith T. Poole, and Howard Rosenthal examines the transformation of American politics over the past 25 years, focusing on the increasing polarization along ideological lines. The authors use historical data from roll call votes in Congress to demonstrate that the behavior of members of Congress has become highly polarized, with more Democrats adopting consistently liberal positions and more Republicans supporting exclusively conservative ones. This polarization began in the 1970s, reversing a long-term decline that started at the turn of the 20th century. The paper highlights five key trends: 1. Most political conflict in Congress is expressed in terms of liberal-conservative differences. 2. The dispersion of positions on the liberal-conservative dimension has increased. 3. The ideological composition of political parties has become more homogeneous. 4. The positions of average Democrats and Republicans have become more widely separated. 5. There is less overlap between the parties, with no longer any liberal Republicans or conservative Democrats. The authors also explore the causes of this polarization, including changes in the link between constituency interests and congressional voting, the Southern realignment, enhanced party leadership, congressional apportionment, and primary elections. They find that while constituency factors play a role, the surge in polarization cannot be fully explained by them alone. Instead, the increased link between constituency interests and congressional voting, along with the growing divergence between the parties, are key factors. The paper concludes by discussing alternative explanations for congressional polarization, such as the Southern realignment, and suggests that the economic and demographic changes in the South, particularly the migration of middle and upper-class Whites, have contributed to the convergence of southern politics with the rest of the country.