America’s rich just won’t quit getting richer, according to a new study released in mid-June by the Boston Consulting Group (BCG), a global management consulting firm. The study, which seeks to analyze the global wealth management industry, as well as the evolution of private wealth, uncovered some startling statistics that suggest that global financial inequality will grow significantly by the year 2021.

The firm found that the already massive gap between the world’s wealthy elite – the approximately 18 million households that hold at least than $1 million in assets – and everyone else is continuing to widen at a remarkable rate. The estimated 70 million people who make up these households were found to control 45 percent of the world’s $166.5 trillion in wealth. And in just four more years, it is estimated that they will control more than half of the world’s wealth, despite representing less than 1 percent of the world’s current population.

However, while rising inequality is a global phenomenon, it is especially pronounced in the United States. While wealth inequality in the U.S. is by no means an unknown phenomenon, the U.S. is significantly more unequal than most other countries, with the nation’s elite currently holding 63 percent of the private wealth. The U.S. elite’s share of national wealth is also growing much faster than the global average, with millionaires and billionaires expected to control an estimated 70 percent of the nation’s wealth by 2021.

The U.S.’ high wealth inequality largely owes to post-World War II government policies that have seen almost a quarter of all national income go to its wealthiest residents. Meanwhile, wages for the majority of Americans have remained stagnant for decades – in contrast to the richest Americans, their future economic outlook is incredibly bleak by comparison.

The U.S. is also home to more billionaires and millionaires than anywhere else in the world, which partly explains how U.S. policy has come to favor them over the years. According to Bloomberg, two out of five millionaires and billionaires live in the United States – and their ranks are growing.

While the world’s richest citizens may be pleased by the results of BCG’s recent study, there is plenty for them to be worried about if history is any indicator. Indeed, history shows that societies with drastic wealth inequality are much more unstable and more likely to experience drastic economic failure or outright societal collapse.

For instance, a 2014 study conducted by the National Socio-Environmental Synthesis Center noted that over-consumption and wealth inequality have occurred in the collapse of every civilization over the last 5,000 years. That same study also warned that rising inequality could easily lead to an unsustainable use of resources and the “irreversible collapse” of global industrial civilization.

This warning seems particularly prescient, given that wealth inequality in the U.S. is well above that of past civilizations that eventually collapsed as a result of these factors. For example, at the time of the collapse of the Roman Empire, the top 1 percent of the Roman elite controlled just 16 percent of the society’s wealth, a measly figure compared to the percentage commanded by the 1-percenters of the U.S.

While the BCG study paints a rosy picture for the world’s millionaires and billionaires, particularly in the United States, they should be gravely concerned that their growing accumulation of wealth could have drastic consequences – not just for those poorer than them, but for everyone.

Feature photo | Vadim Ghirda | AP