Although much of the Republicans'"Pledge to America" is given over to a discussion of economic issues, there is one topic that is never mentioned: the dramatic rise in income inequality. As with global warming, Republicans seem to have decided that the best way to deal with this fundamental challenge is to deny it exists.

If you asked Americans how much of the nation's pretax income goes to the top 10 percent of households, it is unlikely they would come anywhere close to 50 percent, which is where it was just before the bubble burst in 2007. That's according to groundbreaking research by economists Thomas Piketty, of the Paris School of Economics, and Emmanuel Saez, of the University of California at Berkeley, who last week won one of this year's MacArthur Foundation "genius" grants.

It wasn't always that way. From World War II until 1976, considered by many as the "golden years" for the U.S. economy, the top 10 percent of the population took home less than a third of the income generated by the private economy. But since then, according to Saez and Piketty, virtually all of the benefits of economic growth have gone to households that, in today's terms, earn more than $110,000 a year.

Even within that top "decile," the distribution is remarkably skewed. By 2007, the top 1 percent of households took home 23 percent of the national income after a 15-year run in which they captured more than half - yes, you read that right, more than half - of the country's economic growth. As Tim Noah noted recently in a wonderful series of articles in Slate, that's the kind of income distribution you'd associate with a banana republic or a sub-Saharan kleptocracy, not the world's oldest democracy and wealthiest market economy.

In trying to figuring out who or what is responsible for rising inequality, there are lots of suspects. Globalization is certainly one, in the form of increased flows of people, goods and capital across borders. So is technological change, which has skewed the demand for labor in favor of workers with higher education without a corresponding increase in the supply of such workers. There are a number of other culprits that come under the heading of what economists call "institutional" changes - the decline of unions, industry deregulation and the increased power of financial markets over corporate behavior. Over time, more industries have developed the kind of superstar pay structures that were long associated with Hollywood and professional sports.

And then there is my favorite culprit: changing social norms around the issue of how much inequality is socially acceptable.

Economists spend a lot of time trying to quantify precisely how much responsibility to assign to each of these, but in truth the death of equality is much like Agatha Christie's "Murder on the Orient Express": They all did it.

There are moral and political reasons for caring about this dramatic skewing of income, which in the real world leads to a similar skewing of opportunity, social standing and political power. But there is also an important economic reason: Too much inequality, just like too little, appears to reduce global competitiveness and long-term growth, at least in developed countries like ours.

We know from recent experience, for example, that financial bubbles reduce equality by siphoning off a disproportionate share of national income to Wall Street's highly-paid bankers and traders. What may be less obvious, but not less important, is that the causality also works the other way: Too much inequality can lead to financial bubbles.

The liberal version of this argument comes from former Labor secretary Robert Reich in his new book, "Aftershock." Because so much of the nation's income is siphoned off to the super-rich, Reich says, a struggling middle class trying to maintain its standard of living had no choice but to take on more and more debt. I have some problem with the argument that the middle class had no choice, but it's certainly true that the middle class and the economy as a whole would be in better shape today if households weren't burdened with so much debt.

The more conservative version of this argument comes from University of Chicago economist Raghuram Rajan. In his new book, "Fault Lines," Rajan argues that in order to respond to the stagnant incomes of their constituents, politicians took a number of steps to keep the "American Dream" within reach, including subsidization of home mortgages and college loans. He might have added that politicians also were quick to cut taxes for the middle class even when it meant running up the national debt to pay for popular entitlement programs and government services.

Concentrating so much income in a relatively small number of households has also led to trillions of dollars being spent and invested in ways that were spectacularly unproductive. In recent decades, the rich have used their winnings to bid up the prices of artwork and fancy cars, the tuition at prestigious private schools and universities, the services of celebrity hairdressers and interior decorators, and real estate in fashionable enclaves from Park City to Park Avenue. And what wasn't misspent was largely misinvested in hedge funds and private equity vehicles that played a pivotal role in inflating a series of speculative financial bubbles, from the junk bond bubble of the '80s to the tech and telecom bubble of the '90s to the credit bubble of the past decade.

The biggest problem with runaway inequality, however, is that it undermines the unity of purpose necessary for any firm, or any nation, to thrive. People don't work hard, take risks and make sacrifices if they think the rewards will all flow to others. Conservative Republicans use this argument all the time in trying to justify lower tax rates for wealthy earners and investors, but they chose to ignore it when it comes to the incomes of everyone else.

It's no coincidence that polarization of income distribution in the United States coincides with a polarization of the political process. Just as income inequality has eroded any sense that we are all in this together, it has also eroded the political consensus necessary for effective government. There can be no better proof of that proposition than the current election cycle, in which the last of the moderates are being driven from the political process and the most likely prospect is for years of ideological warfare and political gridlock.

Political candidates may not be talking about income inequality during this election, but it is the unspoken issue that underlies all the others. Without a sense of shared prosperity, there can be no prosperity. And given the realities of global capitalism, with its booms and busts and winner-take-all dynamic, that will require more government involvement in the economy, not less.