Sometimes, investing can seem tricky, if not overly complicated. That's because there are no shortage of investment opportunities for Americans to choose from. In no particular order, you could buy:

  • Stocks
  • Corporate bonds
  • Treasury bonds
  • Banks CDs
  • Real estate
  • Mutual funds
  • Hedge funds
  • Index funds
  • Exchange-traded funds (ETFs)
  • Precious metals or commodities like oil
  • Cryptocurrency (tread carefully)

You could also stick with a cash-equivalent investment, such as a savings account or high-yield checking account, or choose to invest your money into a business. As I said, the choices are plentiful, especially considering the number of stocks, corporate bonds, ETFs, mutual funds, and real-estate options available.

A surprised young woman holding a fanned pile of cash.

Image source: Getty Images.

However, the real question is: Do any of these investment vehicles actually work? In other words, do any of these investment opportunities offer a guaranteed way to make real money?

While nothing in investing can ever be a 100% guarantee, statistically, one investment opportunity has never let investors down, as long as one criterion is met.

Understanding the difference between a nominal and real return

Before we dive into that specific investment vehicle, you'll note my decision above to italicize the word "real" when discussing "real money." This is to bring your attention to the fact that while many of these investments will generate a positive nominal return, not all of them have sufficiently outperformed inflation over time. And if your investment isn't keeping pace with the rising cost of goods and services, then you're technically losing real money.

For example, bank savings accounts, bank CDs, and Treasury bonds are among the safest investments on this list. Money held in a bank is generally protected by the Federal Deposit Insurance Corporation, and the U.S. government has yet to fail to meet an interest payment or fully redeem a maturing bond.

Yet, yield often correlates with risk. Since these are very low-risk investment tools, they also have relatively low yields. Good luck finding a bank savings account at the moment that comes anywhere close to topping the existing inflation rate, according to the Bureau of Labor Statistics, of 2.9%, based on the Consumer Price Index for All Urban Consumers over the past 12 months. 

Two businessmen shaking hands after signing mortgage paperwork, with one holding a replica home in his left hand.

Image source: Getty Images.

While some investment tools do offer inflation-topping potential, the results tend to be negligible. According to data found in Robert Shiller's book Irrational Exuberance, the appreciation in home prices between 1950 and 1997 outpaced the average inflation rate by (drum roll) 0.08% per year. In fact, prior to 1998, inflation-adjusted home prices were hardly above par over the previous 107 years! If you're OK with pretty much keeping pace with the inflation rate, then buying a home has done a pretty good job of doing that. But it's not going to guarantee you a real money return. 

The same can be said for commodities, cryptocurrency, and even investments in businesses, which run the risk of going belly-up.

This near-surefire investment has yet to let investors down

On the other hand, the stock market has historically kicked inflation's behind! Whereas inflation has averaged more than 3% over the trailing 100-year period, the stock market has historically returned an average of 7% a year historically, inclusive of dividend reinvestment, and when adjusted for inflation.

Now, to be clear, this doesn't mean individual stocks can't go down. In theory, a stock has just as much of a chance of falling as rising over the long run.

Similarly, the U.S. stock market regularly undergoes corrections, whereby its major indexes fall by at least 10%. Data from market analytics firm Yardeni Research shows that there have been 36 stock market corrections in the S&P 500 (SNPINDEX:^GSPC) since 1950, or about one every two years. In short, if you pick individual stocks, you could still lose money, making your real-money return not guaranteed.

A businesswoman critically examining data in a financial newspaper.

Image source: Getty Images.

However, one such investment tool has (thus far) offered a guaranteed real-money return, as long as one criterion is met: the SPDR S&P 500 ETF (NYSEMKT:SPY).

This index fund, which very closely mirrors the movement of the broad-based S&P 500, minus microscopic asset management fees, exposes investors to 500 high-quality businesses with a single asset. While picking out individual stocks can be somewhat of a crapshoot, the S&P 500 has generally trended upward over time as top-performing companies outweigh the laggards.

The criterion that needs to be met is simply time. Pick any rolling 20-year period in the S&P 500, and at no point have investors lost money. Tack on the ability to reinvest your dividends (currently a 1.8% yield), and it becomes possible for investors in the SPDR S&P 500 ETF to compound their wealth even faster.

As further evidence, the S&P 500 has completely erased the declines associated with all previous stock market corrections since 1950, excluding the latest stock market correction for recency. That's 35-for-35! Again, while nothing is 100% guaranteed, the data doesn't get more suggestive -- or predictive -- than that.

If you're looking for a near-guaranteed real-money return, the SPDR S&P 500 ETF looks to be a solid choice if you're willing to hold it over the long term.

Sean Williams has no position in any of the stocks mentioned. The Motley Fool has the following options: short January 2019 $285 calls on SPDR S&P 500 and long January 2019 $255 puts on SPDR S&P 500. The Motley Fool has a disclosure policy.