Soon the top 1% will own half the money in the world.

A new report by Boston Consulting Group has found that the world's millionaires are on track to control a majority of the world's wealth quite soon. Of the $166.5 trillion in global private assets, 45% is currently owned by the world's 17.9 million millionaire households.

By 2021, the report's authors expect that number to tip over the halfway mark to 51%.

This is a record breaking concentration of money, one in which North America leads the way. According to BCG's findings, 60% of the wealth in Canada, the U.S. and Mexico is held by top-1% households, by far the largest concentration worldwide. By contrast, Western Europe and Japan are the most egalitarian places to live, with 70 and 77% (respectively) of private assets held by households worth less than $1 million.

In fact, the only region which comes even close to North America's inequality is Africa and the Middle East, where millionaire households control 56% of private wealth.

Those are the facts. So what are the takeaways?

This is a global phenomenon driven by regional issues

The first lesson from BCG's report is that, despite the headline number, this is a far more regional phenomenon than it appears.

As noted above, North America has the most extreme results when it comes to concentration of wealth. Across Asia-Pacific, Japan, Latin America and Western Europe millionaire households own less than half the regional wealth, a cross-section which accounts for most of the world's countries and population.

At the same time, the U.S., Canada and Mexico account for nearly half of the world's millionaires.

North America has the most unequal distribution of wealth in the world. It also has the largest number of wealthy households and the most money out of any region, two facts that make the global wealth distribution appear far more skewed than it really is.

Inequality is a global issue, but one which varies widely, from Japan where the highest earning households control a tiny share of the money to Eastern Europe where those $100 million-plus households own nearly a fifth of the regional treasure. The same holds true of anticipated growth. Although most growth around the world will skew towards the top between now and 2021, exactly how that will play out differs from region to region.

In Asia-Pacific, for example, multimillionaire households will get richer at about twice the rate of the middle class. In North America, BCG expects households worth $20 million to $100 million to grow their wealth at nearly five times the rate of everyone making less than $1 million.

Although globally wealth is concentrating in fewer and fewer hands, it is an issue playing out very differently from place to place.

The Rise of the Global Middle

The share of adults living on a middle-class income is falling in the United States. According to data from the Pew Research Center, from 1991 to 2010 the middle class declined by about three percent across America.

Meanwhile, the size of the middle class around the world has exploded.

"In two to three years there might be a tipping point where a majority of the world's population, for the first time ever, will live in middle-class or rich households," wrote the Brookings Institute's Homi Kharas in a study on the subject. "The rate of increase of the middle class, in absolute numbers, is approaching its all-time peak."

"The global middle-class market is now clearly bifurcated," he also noted in this paper. "[A] slow-growing developed country middle class, and a fast-growing emerging economy middle class."

Even as wealth is concentrating in fewer and fewer hands, the global middle is growing, chiefly in India, China and other parts of Asia. By the end of 2016, according to the Brookings Institute, more than 3.2 billion people around the world had purchasing power and a standard of living that qualified them as middle class.

Part of this has to do with absolute growth. Since the early 1990s, global GDP growth has swung between 2% and slightly more than 4% (with one dip in 2009). Simply put, there's more money to go around.

However, as Kharas notes, that alone isn't enough to completely explain the growth of the global middle. A lot of it is also explained by different measures of inequality and by the success that developing nations have had in educating and mobilizing their workforce.

"We're in a period of very rapid change in the global economy," he said. "That's not a very surprising statement. But what's happening in the developed countries, essentially when you look at the middle class in large urban areas they're doing just fine. But when you look at the middle class in smaller towns or towns that are declining like Detroit and throughout the Midwest what you're finding is that their core economic basis and foundation has actually moved away and people have been slow to adapt to that change."

"The pace of domestic migration, just people moving from place to place, has actually slowed in developed countries whereas in developing countries you still have this massive internal migration across the workforce," he added.

The rich are getting richer but, globally speaking, so are the poor.

Income vs. Wealth

"There's a far greater concentration of wealth than there is a concentration of income," said Kharas. "And that actually has quite a separate effect and impact on the economy."

Although frequently confused, the difference between income and wealth is actually an important one. Income measures actual earnings and take-home pay, while wealth measures aggregate value. It's the difference between how much is in someone's paycheck compared against how much is in their savings account.

And, when it comes to big-picture inequality, this is a distinction with a lot of difference. Income to a very large degree defines living standards. Consumers tend to spend out of their earnings rather than their savings, so this is the measure that has far more to do with demand, housing, entertainment and other facts of daily life. Wealth is how much value a family has in total. It can measure how prepared the household is for retirement or a major emergency, but it often has less to do with quality of life or spending in the economy.

Although wealth and income inequality have continued to expand, incomes are growing more rapidly across the spectrum. This, ultimately, is why the global middle class has grown even while wealth has aggregated in a relatively few hands.

That, however, doesn't mean that a growing wealth gap means nothing.

In part, as Kharas said, wealth inequality can be an indicator that something is wrong either politically or economically. How the rich get richer matters.

"There is a very big difference between Bill Gates and Warren Buffett as individuals having accumulated a lot of wealth vs. the Queen of England and the royal family as individuals having a lot of wealth," he said. "And the impact of these two things is very different."

"The difference is that in one case, the wealth has come from a process of value creation for all of society," he said, "whereas in the other case wealth has come from some form of confiscatory powers or from pure rents."

When people get rich through a process of value creation, he said, that's a sign of a healthy economy. However that isn't always the case. Growing inequality of wealth is often suggestive of unproductive rent seeking, corruption and criminal activity.

That's a problem.

Wealth gaps also create problems at a societal level. "Inequality is divisive and socially corrosive," found one researcher. "A wide range of social problems are worse in societies with bigger income differences between rich and poor," including illness, educational failings and crime.

In other words, the difference between income and wealth matters, but so does the wealth gap. Especially since it's only going to get worse.

More from TheStreet:

More from Personal Finance

What Is Lionel Messi's Net Worth?

What Is Lionel Messi's Net Worth?

3 Ways for Retirees to Protect Their Portfolios In Volatile Stock Markets

3 Ways for Retirees to Protect Their Portfolios In Volatile Stock Markets

Credit Unions vs. Banks: What's the Difference?

Credit Unions vs. Banks: What's the Difference?

How Much Money Has Bill Gates Made Over Time?

How Much Money Has Bill Gates Made Over Time?

This Should Be Your Retirement Savings Plan When the Stock Market Crashes

This Should Be Your Retirement Savings Plan When the Stock Market Crashes