For years, statistics have depicted growing income disparity in the United States, and it has reached levels not seen since the Great Depression. In 2008, the last year for which data are available, for example, the top 0.1% of earners took in more than 10% of the personal income in the United States, including capital gains, and the top 1% took in more than 20%. But economists had little idea who these people were. How many were Wall street financiers? Sports stars? Entrepreneurs? Economists could only speculate, and debates over what is fair stalled. Now, a mounting body of economic research indicates that the rise in pay for company executives is a critical feature in the widening income gap.