Did income inequality lead to the collapse of ancient Rome?

In 2013 a consensus developed among American liberals that the problem of income inequality should be a top priority. President Obama stated that income inequality was the 'defining challenge of our time'.

Income inequality was also a major issue when ancient Rome was at the height of its power.  Political struggles over income inequality destabilized the Roman system, and lead to irreversible political changes that fatally undermined Roman civilization.

The Roman city state was founded in the eighth century BC as a kingdom. After a particularly obnoxious king, the monarch was overthrown in 509 BC. Instead of replacing him with another king, the citizens of Rome did something revolutionary.  They swore an oath that no one man would ever again be allowed to rule Rome. They set up a system where no man had absolute power and where the government was accountable to the people. It was similar in many ways to our modern American system. They called it a Republic.

The Republic seems to have been very good for business. Modern science has given us some insight into Roman economic activity through the study of shipwrecks and ice cores. What is really interesting is that both unrelated datasets tell the same story. The Roman economy grew for about 500 years from 500 BC to a peak at 1AD, and then shrank away to nothing by 500 AD.

The data on lead production comes from Greenland ice cores. Lead smelting released pollution, which found its way to the Greenland icecap. The Romans were mining lead so they could use that lead to extract silver from its ore. The Romans needed silver because their monetary system was based on silver coins.

The growth in the Roman economy roughly coincides with the existence of the Roman Republic, which started with the overthrow of the king in 509 BC  and came to an end in 27 BC when the Republic was overthrown. Good institutions are vital to economic growth in the third world today, and the Republic was likely key to the success of Rome.

Much has been written about why Rome fell. What the modern data reveals, is that the economy had been shrinking for centuries before the final collapse of the Roman state. When the last Roman emperor was overthrown in 476 AD, the economy had withered away. The data also shows that the economy peaked around 1 AD, so the cause of the eventual collapse of Rome dates to that era.  The replacement of the Republic with emperors seems to be the most likely reason. Kings and emperors are very common in history, while republics are rare.

Why did the Republic fall?

What seems to have destabilized the Republic was growing income inequality.  By the middle of the second century, the economic situation for the average Roman was declining. The backbone of Rome was small farmers who owned their own land. These small farms started going bankrupt, and they sold their land to aristocrats who set up  large estates worked by imported slave labor. The displaced farmers went to the cities looking for work, but they didn't find many jobs, and ended up dependent on government welfare.

In 133 BC these unemployed people elected a populist called Tiberius Gracchus who promised them land reform. The wealthy elite resisted the reforms, and Tiberius was assassinated. This started a series of political assassinations and military coups which lead to the dismantling of the Republic by 27AD. The whole process took over a hundred years.