Why Oh Why is Income Inequality Increasing?

That increasing income inequality is a reality is widely acknowledged by economists, the debate has shifted to the source of the inequality, e.g. see Brad DeLong, Greg Mankiw, Paul Krugman, and Alex Tabarrok for openers. Is it due to a skill premium, non-linearities in returns to education, a rising oligarchy, a winner take all economy, luck, or some other factor?:

Brad DeLong: Optimal Tax Policy: An old New York Times column by Hal Varian:

Hal Varian: In the debate over tax policy, the power of luck shouldn't be overlooked: Those who argue for a more progressive income tax emphasize equity: a tax dollar paid by a rich person causes less pain than a tax dollar paid by a poor person. Those who argue for a less progressive system emphasize efficiency: the most productive people should face lower tax rates to give them strong incentives to work harder and produce more.... This formulation of the optimal income tax problem was first examined by the economist James Mirrlees of Cambridge University, who received a Nobel in economic science for his analysis. In the simplest version of the Mirrlees model... those at the very top of the income scale should face low marginal rates....

Of course, the fact that it pays to reduce the marginal tax rate for billionaires doesn't say much about what tax rates should be like for mere millionaires.... But the intuitive argument presented above is pretty compelling: if income depends only on ability, those at the very top of the income-ability distribution should face low marginal tax rates.

But perhaps this model is too simple.... So let's consider a different model: one in which differences in income are a result only of luck.... In this case, the optimal income tax may well involve taxing billionaires at very high marginal rates. True, aspiring billionaires won't work quite as hard.... But the chances of becoming a billionaire are pretty low anyway, so taxing billionaires at a high rate won't really discourage much effort by those hoping to become one.... This is about as far as theory can take us, but it highlights the critical question: How much income results from ability and how much from luck?...

Christopher Jencks, and his collaborators pointed out many years ago that income inequality among brothers, who share similar genetic and environmental characteristics, is almost as great as for the population as a whole....

If luck plays a substantial role... it makes sense to have a progressive income tax, creating a form of social insurance in which the lucky subsidize the unlucky. Perhaps the folk singer Phil Ochs had the best answer for why the upper half of the income distribution should pay so much more in taxes than the lower half: ''And there but for fortune, may go you or I.''