Graduates from Columbia University's School of Business hold a sign as they cheer during university's commencement ceremony in New York

With Thomas Piketty’s book, Capital in the 21st Century, rising to number 1 on best-seller lists, inequality has become central to the public debate over economic policy. Piketty, and much of this discussion, focuses on the sharp increases in the share of income and wealth going to the top 1 percent, .1 percent and .01 percent of the population.