Millions of Americans have far less money saved than they should have by this point in their lives. Whether they realize it or not, they're by default counting on Social Security to support them in a big way come retirement. That's a risky thing to do.

Social Security is a critical component of most Americans' retirement income, so it's well worth learning more about it in order to make informed decisions. Here are two reasons you shouldn't count on it, and one reason you should.

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1. It may keep working like it's currently working

For starters, it may not be around by the time you retire -- at least not in the form it is today. Headlines proclaiming that the program will run out of money soon are wrong, but there are problems and undesirable consequences looming.

According to the latest report from the Social Security and Medicare Boards of Trustees, "The Old-Age and Survivors Insurance (OASI) Trust Fund, which pays retirement and survivors benefits, will be able to pay scheduled benefits on a timely basis until 2034, the same as reported last year. At that time, the fund's reserves will become depleted and continuing tax income will be sufficient to pay 76% of scheduled benefits."

It can help to remember that Social Security has long worked by taking in more money from working people (via the taxes that are taken out of your paycheck) than it pays out to retired people.

Our nation's demographics are shifting, though, with people living longer and many people retiring sooner. Whereas the program used to run with a surplus, that surplus will end and the incoming tax dollars will only be enough to pay around 76% of retirees' expected (and entitled) benefits.

So Social Security is not going to suddenly disappear or stop paying out any benefits, but if nothing changes, it's on course to be paying a lot less in a little more than a decade.

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2. It won't be enough

Even if Social Security does keep operating as it is now, the monthly benefit checks you receive may be a lot less than you expected. The program was never designed to provide enough to live on -- just around 40% of your preretirement income. Recently, the average monthly retirement benefit was $1,555, or $18,660 annually.

That means you'd do well to save and invest during your working years to amass enough money to support you in retirement along with whatever Social Security benefits you collect. Growing a big nest egg will take some effort and determination. You may want to take on a side gig for a while, or cut back on your spending. Even just going out to eat at restaurants half as often may yield big savings.

As for investing, that doesn't have to be too complicated or intimidating. Just sticking with index funds can be good enough. Index funds have actually outperformed actively managed mutual funds over long periods.

3. It's not going away and it might get stronger

Now we come to why you (most likely) can count on Social Security: It's not going away anytime soon, and there are plenty of ways to strengthen the system. Indeed, just having every earned dollar taxed for Social Security can send a lot more dollars into the program's coffers, as can increasing the tax rate by just a percentage point.

Right now, only earnings up to $142,800 are taxed for Social Security -- which means that most or all of your earnings are taxed, but someone earning $1,142,800 will not be taxed on $1 million of earnings. These kinds of adjustments to Social Security can bring back surpluses instead of deficits, and may even be able to deliver more robust benefits, which most retirees could really use.

It's worth learning more about Social Security in order to be able to make better decisions about it and to get as much money out of the program as you can.