Hayek served in World War I during his teenage years and said that this experience in the war and his desire to help avoid the mistakes that had led to the war drew him into economics. At the University of Vienna, he studied economics, eventually receiving his doctoral degrees in law in 1921 and in political science in 1923. He subsequently lived and worked in Austria, Great Britain, the United States, and Germany; he became a British subject in 1938. Hayek's academic life was mostly spent at the London School of Economics, and later at the University of Chicago, and the University of Freiburg. Although he is widely considered a leader of the Austrian School of Economics, he also had close connections with the Chicago School of Economics. Hayek was also a major social theorist and political philosopher of the 20th century and as the co-founder of Mont Pelerin Society he contributed to the revival of classical liberalism in the post-war era. His most popular work, "The Road to Serfdom", has sold over 2.25 million copies (as of 2020).