Being rich is great. But being rich in America? That’s even better.

With US stock benchmarks trading just below record highs, and Treasury yields not too far from the all-time lows reached last summer, the gulf between the world’s wealthy elite – those 18 million households worldwide with more than $1 million in assets – and everybody else is rapidly widening.

According to a new study by Boston Consulting Group via Bloomberg, these households - with a total head count of roughly 70 million people, or about 1% of the world’s population - control 45 percent of the $166.5 trillion in wealth. By 2021, they will control more than half, suggesting that, while wealth inequality in the rest of the world is simply accelerating, in America, it’s gone into overdrive. Right now, 63 percent of America’s private wealth in the hands of U.S. millionaires and billionaires, BCG said. By 2021, their share of the nation’s wealth will rise to an estimated 70 percent.

“The share of income going to the top 1 percent in the U.S. has more than doubled in the last 35 years, after dropping in the decades after World War II (when the rich were taxed at high double-digit rates). The tide shifted in the 1980s under Republican President Ronald Reagan, a decade when “trickle-down economics” saw tax rates for the rich fall, union membership shrink, and stock markets spike.”

In its report, BCG puts the global rate of wealth creation in 2015 and 2016 at 5.3 percent, though the consulting firm expects it to accelerate to about 6 percent annually for the next five years. Those gains will accrue almost exclusively to the wealthiest Americans, while wealth held by everyone else is just barely growing.

Of course, there’s a caveat: In America, most of these gains exist only on paper. More than 70% of new wealth creation is derived from the rising value of rich investors’ portfolios. The rest is what BGC describes as “new wealth creation” aka real money earned by workers and entrepreneurs. Globally, the share derived from asset valuations falls to about 50%.

“In the U.S., the creation of “new” wealth is a minor factor, making up just 28 percent of the nation’s wealth increase last year. It’s even lower in Japan, at 21 percent. In the rest of the Asia Pacific region, meanwhile, two-thirds of the rise is driven by new wealth creation.”

The richest Americans could receive an additional bump if/when President Donald Trump pushes the fiscal agenda on which he campaigned through Congress – an agenda that includes tax cuts, deregulation and trillions of dollars’ worth of new infrastructure projects. However, since the exact details of these policies are not yet widely known, it’s difficult to predict their specific impact, BCG says.

“’No one knows’ what kind of tax changes will become law, said BCG senior partner Bruce Holley. However, “this could buoy the [growth in U.S. wealth] that we are predicting.”

America remains home to the highest concentration of millionaires and billionaires in the world, and their ranks are growing fast: Today, about 7 million Americans are worth more than $1 million. BCG expects that number to balloon to 10.4 million by 2021 - an annual growth rate of 8 percent, or about 670,000 new millionaires each year.

China has the second most millionaires and billionaires, at 2.1 million, though its population is four times the size of the US.

Within the US, glaring disparities exist from region to region. As Fortune reported back in 2015, roughly two-thirds of America’s millionaires and billionaires live in 12 metro areas, mostly wealthy enclaves along the coasts.

Meanwhile, according to the Federal Reserve’s latest survey of household economics and decisionmaking, a quarter of American adults can’t pay all their monthly bills, and 44% have less than $400 cash on hand in case of an emergency.