Over 50 percent of Americans are making this colossal mistake

Saving for the future isn't just a matter of putting money aside each paycheck and calling it a day. While committing to saving money is definitely a good start, if you don't invest that money wisely, you risk coming up short by the time retirement rolls around.

It turns out that more than half of us are missing out on a major opportunity to grow our savings. According to a study by Bankrate, over 50% of American adults have no money invested in the stock market. And while avoiding stocks makes more sense for older Americans approaching retirement, younger savers should be capitalizing on the growth stocks offer -- but they're not. In fact, Bankrate found that only a quarter of adults under 30 are buying stocks. But while investing in stocks is indeed riskier than leaving money in a savings account, there's a danger to the latter approach too.

Why Americans aren't buying stocks

So why are so many of us avoiding stocks like the plague? A big part of it boils down to fear. While there's no such thing as a risk-free investment, the stock market in particular is known for its perpetual volatility.

But in reality, if you employ the right investment strategy and choose the right stocks, there's no reason to fear the stock market at all. First of all, the stock market has a strong history of rebounding. Between 1965 and 2015, the S&P 500 underwent 27 corrections of 10% or more, and guess what? It recovered every time. If you put money in stocks with the goal of cashing out your investments in a year or two, then yes, you stand a pretty good chance of losing money. But if you're willing to invest for a decade or more, you'll have time to ride out the market's ups and downs to ultimately come out ahead.

Another reason Americans tend to stay away from stocks is that they feel they don't know enough about investing to get in the game. In reality, you don't need to know a whole lot about stocks to do well as an investor. If you stick to index funds, which simply follow existing indexes like the S&P 500, as opposed to buying individual stocks, you get automatic diversification without the guesswork. Remember, even the so-called experts aren't always successful in picking the best investments, so you certainly shouldn't let your lack of knowledge hold you back.

Looking to make more money? Check out the slideshow below:

20 unusual ways to make quick money
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20 unusual ways to make quick money

Dog-sitting, babysitting, or house-sitting

These jobs are always in high demand, and the best part: you can name your price and create your own schedule! Post an ad on craigslist, or use your friends' and family's connections to get your name out there. 

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Rent out your space 

List your apartment on Airbnb or another rental site, and make some easy cash by staying at a friends and renting out your place for the weekend.

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Share your space

Just as you can rent out your full apartment or house, you can also post a free room (or even just your couch!) on sites like Craigslist or Airbnb. This way you can split your living expenses -- and maybe even make a new friend!

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Sell your body parts

Now here's a weird one: Donate your hair, breast milk, or even plasma for a profit. According to Grifols, if you're healthy and weigh above 110 pounds, you can earn up to $200 a month donating your plasma to life-saving medicine. 

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Sign up to participate in medical tests and clinical trials. 

Universities constantly need volunteers to test new medicines and treatments -- and because the pool of willing participants is limited, there is typically a large compensation for being a guinea pig. 

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Participate in a focus group

Companies and organizations will pay you to join a focus group. These can be conducted in person, online, or via phone. You will most likely be reimbursed in cash or gift cards -- plus, you often get to test out fun new products! 

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Take online surveys

Similar to focus groups, you can get paid to give your time and insights on an online questionairre. Plus, you can do this from the comfort of your couch. 

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Bank on your sperm

Although we don't necessarily recommend this option, there is a very high demand for healthy sperm donors. Keep in mind some of the obvious drawbacks, but sperm donation is non-invasive and highly compensated. 

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Crowdfund your dreams

Crowdfunding allows you to raise monetary contributions from a large group of people who want to support your venture. Post your project or idea on a crowdfund site, like GoFundMe.com, and see the cash pile up.

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Become a tutor

If you're qualified, post an ad online or on a community board to tutor children on their school courses or for the upcoming SATs.

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Get a part-time job

Capitalize your free time (on the weekends or after work hours) by working a part-time job. A bartender, waiter, or Uber driver are all great options for an additional source of income -- and great tips! 

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Resell tickets

Take this suggestion at your own risk: If you're staying within legal limits, buy tickets low and sell high as an effective way to source additional money. (Just make sure to check your state and local laws first!)

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You can sell anything on the internet these days... including your companionship! Get paid to go on a platonic outing for a few hours and enjoy your afternoon with a new friend. 

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Rent out your parking spot

Make sure to check with your landlord first, but if you have the option to park your own car further away, lend or share your parking space or driveway for the hour, day, or even month! 

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Keep a coin jar 

This one takes patience before a big pay out, but keep a spare jar or drawer for loose change that you usually toss anyway. It will keep it all in one place -- and those quarters do add up! 

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Make something to sell 

If you have a knack for arts & crafts, create jewelry or other handmade gifts to sell on sites filled with other thrifty vendors like Etsy

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Sell items online

This effective strategy requires low effort with a high return. Post photos of your used or non-used items on sites like eBay or Craigslist, and let the bidding begin! 

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Have a yard sale

Sell clutter you've been meaning to get rid of right in your front yard. This simple tactic is convenient, and guarantees a wad of cash right to your pocket.  

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Return past purchases

This tip may seem obvious, but is often overlooked: Take your recently-purchased items that are laying around back to the store for either store credit or a full refund. 

Recycle scrap metal and cans

Collect cans and scrap metal out your own garbage, basement, and street and bring to your local recycler to exchange your findings for money.  

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A real opportunity for growth

So just how much money are you potentially losing out on by avoiding the stock market? Let's run some numbers and see.

Imagine you're 30 years old and your goal is to retire at 65. Let's also assume that you're a strong saver who puts $500 in the bank every month. Now if you leave that money to sit in a savings account paying 1% interest (which, by the way, some of today's banks aren't even hitting), over the course of 35 years, you'll grow your savings to roughly $250,000 -- not too shabby. But watch what happens if you invest that money in stocks instead and score an average yearly return of 8% on your investments (which is actually a bit below the market's average). In 35 years, you'll have $1 million to your name!

Even if you split your savings down the middle, investing half in stocks and keeping the other half tucked away in a savings account, over the course of 35 years, you'll still come away with about $500,000. And while that's not a move I'd recommend for someone as young as 30, it's a decent compromise if you're the type who's really, really terrified of losing money in stocks, even if just on paper.

If you're already over the hump of saving money, don't sell yourself short by locking your cash away in a savings account. Investing in stocks offers a real chance to grow your money into a sizable retirement nest egg, and the sooner you start, the more you stand to gain.

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