Today, the top one per cent of incomes in the United States accounts for one fifth of US earnings. The top one per cent of fortunes holds two-fifths of the total wealth. Just one rich family, the six heirs of the brothers Sam and James Walton, founders of Walmart, are worth more than the bottom 40 per cent of the American population combined ($115 billion in 2012).

After thousands of scholarly and popular articles on the topic, one might think we would have a pretty good idea why the richest people in the US are pulling away from the rest. But it seems we don’t. As the Congressional Budget Office concluded in 2011: ‘the precise reasons for the rapid growth in income at the top are not well understood’. Some commentators point to economic factors, some to politics, and others again to culture. Yet obviously enough, all these factors must interact in complex ways. What is slightly less obvious is how a very long historical perspective can help us to see the whole mechanism.

In his book Wealth and Democracy (2002), Kevin Phillips came up with a useful way of thinking about the changing patterns of wealth inequality in the US. He looked at the net wealth of the nation’s median household and compared it with the size of the largest fortune in the US. The ratio of the two figures provided a rough measure of wealth inequality, and that’s what he tracked, touching down every decade or so from the turn of the 19th century all the way to the present. In doing so, he found a striking pattern.

You stopped just before the punchline!

From 1800 to the 1920s, inequality increased more than a hundredfold. Then came the reversal: from the 1920s to 1980, it shrank back to levels not seen since the mid-19th century. Over that time, the top fortunes hardly grew (from one to two billion dollars; a decline in real terms). Yet the wealth of a typical family increased by a multiple of 40. From 1980 to the present, the wealth gap has been on another steep, if erratic, rise. Commentators have called the period from 1920s to 1970s the ‘great compression’. The past 30 years are known as the ‘great divergence’. Bring the 19th century into the picture, however, and one sees not isolated movements so much as a rhythm. In other words, when looked at over a long period, the development of wealth inequality in the US appears to be cyclical. And if it’s cyclical, we can predict what happens next.

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.

I did not realize this:

This connection between the oversupply of labour and plummeting living standards for the poor is one of the more robust generalisations in history. Consider the case of medieval England. The population of England doubled between 1150 and 1300. There was little possibility of overseas emigration, so the ‘surplus’ peasants flocked to the cities, causing the population of London to balloon from 20,000 to 80,000. Too many hungry mouths and too many idle hands resulted in a fourfold increase in food prices and a halving of real wages. Then, when a series of horrible epidemics, starting with the Black Death of 1348, carried away more than half of the population, the same dynamic ran in reverse. The catastrophe, paradoxically, introduced a Golden Age for common people. Real wages tripled and living standards went up, both quantitatively and qualitatively. Common people relied less on bread, gorging themselves instead on meat, fish, and dairy products.

This makes sense now that I think about it, sadly.

In the US, there is famously a close connection between wealth and power. Many well-off individuals — typically not the founders of great fortunes but their children and grandchildren — choose to enter politics (Mitt Romney is a convenient example, though the Kennedy clan also comes to mind). Yet the number of political offices is fixed: there are only so many senators and representatives at the federal and state levels, and only one US president. As the ranks of the wealthy swell, so too do the numbers of wealthy aspirants for the finite supply of political positions.

Not all elites are the same.

Beside sheer numbers, there is a further, subtler factor that aggravates internal class rivalry. So far I have been talking about the elites as if they are all the same. But they aren’t: the differences within the wealthiest one per cent are almost as stark as the difference between the top one per cent and the remaining 99. The millionaires want to reach the level of decamillionaires, who strive to match the centimillionaires, who are trying to keep up with billionaires. The result is very intense status rivalry, expressed through conspicuous consumption. Towards the end of the Republic, Roman aristocrats competed by exhibiting works of art and massive silver decorations in their homes. They threw extravagant banquets with peacocks from Samos, oysters from Lake Lucrino and snails from Africa, all imported at great expense. Archaeology confirms a genuine and dramatic shift towards luxury.

I think this chart is fascinating:

The conclusion: We are prone to significant political instability and upheaval.

hat, then, explains the rapid growth of top fortunes in the US over the past 30 years? Why did the wages of unskilled workers stagnate or decline? What accounts for the bitterness of election rhetoric in the US, the growing legislative gridlock, the rampant political polarisation? My answer is that all of these trends are part of a complex and interlocking system. I don’t just mean that everything affects everything else; that would be vacuous. Rather, that cliodynamic theory can tell us specifically how demographic, economic and cultural variables relate to one another, and how their interactions generate social change. Cliodynamics also explains why historical reversals in such diverse areas as economics and culture happen at roughly similar times. The theory of secular cycles was developed using data from historical societies, but it looks like it can provide answers to questions about our own society.

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Three years ago I published a short article in the science journal Nature. I pointed out that several leading indicators of political instability look set to peak around 2020. In other words, we are rapidly approaching a historical cusp, at which the US will be particularly vulnerable to violent upheaval. This prediction is not a ‘prophecy’. I don’t believe that disaster is pre-ordained, no matter what we do. On the contrary, if we understand the causes, we have a chance to prevent it from happening. But the first thing we will have to do is reverse the trend of ever-growing inequality.

I believe this is the #1 national security issue facing America. Historically, around the world, when income inequality gets this wide, the results are not pretty.