Roughly 62% of U.S. Adults Hold Stocks. Those Who Don't Could Be Missing Out on Millions.

There are plenty of ways to become a millionaire, like starting a successful business, being a professional athlete, or becoming a movie star.

For most ordinary people earning an average income though, the stock market is still their best bet for building meaningful wealth. Not only is investing a flexible and scalable endeavor, it's actually the lowest-risk way of amassing a fortune... provided you're patient.

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And yet, a sizable sliver of adults living in the United States don't own stocks. And of those who do, a bunch don't have much money in the market. That's a big mistake. Here's why, and what you can do about it if you're on the wrong side of the statistic.

By the numbers

As the headline explains, data from a recent Gallup poll indicates that only 62% of U.S. adults own stocks. Most of those holdings are in the form of equity mutual funds rather than individual equities; only about one-fifth of families living in the United States own individual stocks. And, in most cases these funds are found within a retirement account. Baby boomers are most likely to hold such investments, of course, although Generation X is catching up.

The number itself paints a somewhat misleading picture of the average American household, however. As the deeper look taken by the Motley Fool's in-house research arm adds, although 62% of U.S. adults may be in the market one way or another, the richest 1% own about 50% of the stock market's collective value, or roughly $23 trillion worth of equity investments. The next one-tenth of the population isn't doing too shabby either, holding just a little less than 40% of stocks' total value.

Then things get eye-opening fast. The remainder of the richest half of people living in the U.S. only own about 12% of stocks, while the bottom half of all American households (as measured by net worth) collectively hold on the order of $480 billion in equity investments. That's not much. Based on the Census Bureau's current count of 127.5 million households, that would be an average of less than $8,000 worth of stocks for each of these less affluent homes.

Just bear in mind the average is being dragged down by a whole bunch of households that don't own any stocks. Gallup's reported median figure is $52,000, which is clearly more, but still not a massive amount of money.

Why rich people own so much stock

These numbers aren't meant to alarm or discourage you (although there's no denying they could do both). Rather, the purpose here is to offer encouragement -- and a plan -- to those who may be investing little to nothing in the stock market.

Image source: Getty Images.

The stock market remains the top way for average, patient individuals to grow their wealth at an average inflation-beating annual pace of about 10%. That's why so many rich people make a point of owning stocks!

Here's the thing... you can steal a page or two from their proverbial playbooks, and adapt their strategies to work for you.

Starting small is better than not starting at all

See, whether you own just one share of Apple, or 1,000 shares, or 1 million, the net percentage return is the same for all of its shareholders. In other words, no matter how big or small these positions were, anybody who's owned a stake in Apple over the course of the past five years is sitting on a net gain of about 185% on their investment.

And contrary to a common assumption, you don't need a ton of money to get started in the stock market.

Most people -- and all newcomers -- are best served by owning a slice of the entire stock market in the form of an index fund meant to mirror the performance of the S&P 500 (SNPINDEX: ^GSPC). This investment will ebb and flow with the broad market, but it negates the risk and hassle of trying to pick individual stocks.

Your longer-term plan should be to keep devoting more money toward your wealth-building plans. Even if you're only depositing $50 or $100 per month into an IRA or a brokerage account, that's something to build on later when you can begin putting more money toward the effort. Just get started somewhere, since time is your biggest ally as investor.

This might help encourage you into action: Contributing just $300 per month to an S&P 500 index fund returning an average of 10% per year in an IRA would leave you with something in the ballpark of $1.1 million after 35 years. That's a doable plan for most people.

This plan would also leave you with about 4 times as much in your retirement account as mutual fund company Fidelity says is sitting in the average retirement-age investor's IRA right now, by the way. It would also make you richer than the majority of Americans.

Just keep in mind all of these figures will rise over time as inflation marches on, including the amount of money you can allocate toward the goal.

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James Brumley has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends Apple. The Motley Fool has a disclosure policy.