Summary

1. Income Inequality Declined from the early 20^th Century until the 1970s. Likely Causes:
   
   
   1. Higher Income and Estate Taxes
      
      
   2. The Depression followed by World War II
      
      
   3. "Stickiness" of American Political Institutions
      
      
2. Income Inequality has Steadily Increased Since the 1970s. Likely Causes (Provisional):
   
   
   1. Lower Income and Estate Taxes
      
      
   2. Increasing Returns to Education [debated]
      
      
   3. Increasing Immigration (Legal and Illegal)[debated]