Greg Ip in The Wall Street Journal reports($) on new IRS income tax return data: The wealthiest 1% of Americans earned 21.2% of all income in 2005, according to new data from the Internal Revenue Service. That is up sharply from 19% in 2004, and surpasses the previous high of 20.8% set in 2000, at the peak of the previous bull market in stocks. The bottom 50% earned 12.8% of all income, down from 13.4% in 2004 and a bit less than their 13% share in 2000. [...] Jason Furman, a scholar at the Brookings Institution and an adviser to Democratic politicians, said: "We've had a 30-year trend of increasing inequality. There was an artificial reduction in that trend following the bursting of the stock-market bubble in 2000." The IRS data don't identify the source of increased income for the affluent, but the boom on Wall Street has likely played a part, just as the last stock boom fueled the late-1990s surge. Until this summer, soaring stock prices and buoyant credit markets had produced spectacular payouts for private-equity and hedge-fund managers, and investment bankers. One study by University of Chicago academics Steven Kaplan and Joshua Rauh concludes that in 2004 there were more than twice as many such Wall Street professionals in the top 0.5% of all earners as there are executives from nonfinancial companies. Mr. Rauh said "it's hard to escape the notion" that the rising share of income going to the very richest is, in part, "a Wall Street, financial industry-based story." The study shows that the highest-earning hedge-fund manager earned double in 2005 what the top earner made in 2003, and top 25 hedge-fund managers earned more in 2004 than the chief executives of all the companies in the Standard & Poor's 500-stock index, combined. It also shows profits per equity partner at the top 100 law firms doubling between 1994 and 2004, to over $1 million in 2004 dollars.