Over at Breviosity, Martin Hewson links to an essay by Peter Turchin that was published in Aeon Magazine last month. Mr. Turchin is one of the founding fathers of cliodynamics, a fledgling branch of science that approaches economic, social, and political history by means of mathematical module.  His essay presents some of his recent research (to be presented more rigorously in an upcoming book) on cycles of inequality, changing living standards, and violence in the United States. He begins by comparing American inequality with earlier antecedents:
Peter Turchin. Aeon Magazine. 7 February 2013.
"A historically based model might shed light on what has been happening in the US over the past three decades...The rest of Turchin's essay goes on to compare varying levels of social "well being" and inequality across American history. The info graphic presented therein is worth a few minutes reflection in and of itself. The essay's only real flaw is the comparison of modern America, medieval England, and classical Rome that I have quoted above.
Much the same pattern [present in American history] can be seen during the secular cycle of the Roman Principate. The population of the Roman Empire grew rapidly during the first two centuries up to 165AD. Then came a series of deadly epidemics, known as the Antonine Plague. In Roman Egypt, for which we have contemporary data thanks to preserved papyri, real wages first fell (when the population increased) and then regained ground (when the population collapsed). We also know that many grain fields were converted to orchards and vineyards following the plagues. The implication is that the standard of life for common people improved — they ate less bread, more fruit, and drank wine. The gap between common people and the elites shrank.....
As the slice of the economic pie going to employees diminishes, the share going to employers goes up. Periods of rapid growth for top fortunes are commonly associated with stagnating incomes for the majority. Equally, when worker incomes grew in the Great Compression, top fortunes actually declined in real terms. The tug of war between the top and typical incomes doesn’t have to be a zero-sum game, but in practice it often is. And so in 13th-century England, as the overall population doubles, we find landowners charging peasants higher rents and paying less in wages: the immiseration of the general populace translates into a Golden Age for the aristocrats." 
The problem inherit in these sort of comparisons is highlighted in my essay, "Notes on the Dynamics of Human Civilization: The Growth Revolution." During the early 1800s world energy consumption, population, and GDP began to grow exponentially. Before this exponential age human resources were essentially static. (One year of economic growth or energy use on the part of a modern, industrialized country exceeds centuries of economic growth and energy use on the part of premodern economies.) Because their resource base was static, the only real avenue to wealth was the labor of others, seized by means of taxes, tribute, slavery, plunder, or conquest. Wealth was not shared; it was stolen. 
But that is not how wealth is created today. Modern economics is not a zero sum game. The prosperity of modernity is not built upon limited human or animal labor, but upon the seemingly limitless engines of steam, coal, oil, and gas. This is one of the central differences between our age of exponential growth and the earlier age of static resources. In pre-modern societies wealth creation was primarily extractive. This is no longer the case.
Image Credit: Kevin Phillips, Wealth and Democracy, (New York: 2002), pg. 205-206.
Notice that before the late 1700s all fortunes - that amassed by the Peruzzi banking company being the partial exception - were made outside the market.
This is the flaw that handicaps comparisons between modern inequality and its ancient precedents. The unequal distribution of new wealth created by modernity's fantastic economic growth is the primary cause of modern inequality. Ancient inequality was different. It was not caused because proceeds of growth were spread unevenly across the system, for the system did not have enough growth to spread around. Economic inequality in pre-modern societies happened because pre-modern elites extracted their wealth from the rest of the population. The dynamics of the two systems are simply not the same.
 http://cliodynamics.info/ has many resources for those interested in further study of cliodynamics. I have not read Mr. Turchin's vanguard works, War and Peace and War: Life Cycles of Imperial Nations (New York: 2006) and (co-authored with Sergei Nefedov) Secular Cycles (Princeton: 2009). Those of you with tight wallets (like me!) will be glad to know that much of Mr. Turchin's work is available in the public domain. The following article is an excellent example of his methods and arguments:
"A Theory For Formation of Large Empires." Journal of Global History (2009). 4 pp. 191–217
Significantly, this article restricts itself to 3,000 BC to 1800 AD, avoiding the problems of comparing static and exponential age empires.
 Peter Turchin. "Return of The Oppressed." Aeon Magazine. 5 February 2013. Mr. Turchin provides the data behind the essay in a post written on his blog, Social Evolution Forum:
Peter Turchin. "Double Helix of Inequality and Well-Being." Social Evolution Forum. 8 Febuary 2013.
 These ideas are presented with far more rigor and depth in the original essay. Please read it in full if the idea is new.
T. Greer. "Notes on the Dynamics of Human Civilization: The Growth Revolution, pt. 1." The Scholar's Stage. 4 August 2010.