Ulrike Heider, a syndicalist, grouped anarchism into left-wing anarchism, right-wing anarchism (anarcho-capitalism), and green anarchism. People who study anarchists, including Bryan Caplan and Murray Bookchin, do not agree with her. They say her research was not good enough. Left-wing anarchism is distinguished from free-market anarchism and right-wing anarchism (such as that of Murray Rothbard).