The U.S. stock market has been transformed over the last twenty-five years. Once a market in which human beings traded at human speeds, it is now an electronic market pervaded by algorithmic trading, conducted at speeds nearing that of light. High-frequency traders participate in a large portion...

How stocks are traded in the United States has been totally transformed. Gone are the dealers on NASDAQ and the specialists at the NYSE. Instead, a company's stock can now be traded on up to sixty competing venues where a computer matches incoming orders. A majority of quotes are now posted by...