“All the bits of evidence we have tend to say that this trend is continuing,” Professor Lemieux said. In 2003, the authors note, 44.5 percent of workers at Fortune 1000 companies received some form of performance-based pay, up from 34.7 percent in 1996. And think of the growing legions of self-employed — people selling items on eBay, mortgage brokers and real estate brokers, freelance journalists and consultants of all types — for whom all pay is performance-based. Among these growing cadres, the dispersion of incomes is rather large.

“When you look at the self-employed and contractors,” Professor Lemieux said, “inequality is much higher.”

Aside from corporate compensation policies, public policies have played a significant role in contributing to the growth of income inequality. That’s the argument made in a recent, brilliant National Bureau of Economic Research working paper by Professor Levy and Peter Temin, the Elisha Gray II professor of economics at M.I.T. The paper, which is more narrative than quantitative — Professor Temin is a distinguished economic historian — argues that the rise of income isn’t simply a byproduct of the free market working its wonders.

Professor Levy and Professor Temin divide the second half of the 20th century into two periods. In the first, 1955 to 1980, a grand bargain between labor and corporate America involving New Deal-era protections for workers and high marginal tax rates (the top rate was 90 percent in the 1950s) led to what economists have called the Great Moderation. The middle class grew dramatically, income inequality decreased, and corporations generally enjoyed labor peace.

Since 1980, they argue, it’s been a different story, thanks in part to a shifting political environment. Unions have weakened, the minimum wage hasn’t come close to keeping up with inflation, and marginal income tax rates have been cut — the top marginal rate is now 35 percent, down from 70 percent in 1980. A result has been declining bargaining power for workers and the rise of a winner-take-all environment.

“The last six years of federal tax history have involved an inhospitable politics in which winners have used their political power to expand their winnings,” the authors say. In other words, if capital has lately been prevailing in the centuries-long battle with labor, it is doing so with a substantial assist from the government.

Professor Saez agrees with the broad argument, but says that the impact of tax policy in recent years has been minor. When the top income tax rate was 5o percent in the 1970s, he says, “the force to pay according to talent was much weaker” because most of the excess pay would be eaten up by taxes. “Once the very high tax rates for big fortunes were removed in the 1980s,” Professor Saez says “then the market could drive up the compensation at the top.”