Historians Arthurs Schlesinger Sr. and Jr. have proposed that United States history goes in cycles of liberalism and conservatism. The liberal periods often have many progressive reforms, while the conservative periods usually do not undo many of them. AS Jr. noted that liberal and conservative periods may be interpreted as public-purpose and private-interest periods, an alternation between willingness to sacrifice personal gain and interest for the greater national good, and an unwillingness to do so.