Compound interest and management fees.

Humans are horrible at understanding compound interest, and it's making our golden years much less so.

Think about your 401(k). The first thing you probably look at when you pick your funds is their returns. It's only human nature. Everybody likes to think about their nest eggs growing and growing and growing—especially if they're growing a little bit faster than everybody else's. But, in this case, human nature is costing you hundreds of thousands of dollars.

The sad fact is that returns aren't certain, but fees are. Now, maybe everything will go according to plan, and your 401(k) will be partying like it's 1999. Maybe the 1 percent—or more—that you're paying in fees will actually buy you market-beating returns. But probably not. You can see this in the chart to the left from Vanguard. It shows the percentage of actively managed funds that have underperformed index funds over the short and longer hauls, net of fees. Which is to say, most of them. It's hard enough for funds to beat their benchmarks over just one to three-year periods. But that gets damn near impossible the longer you go. Once you account for survivorship bias—that bad funds go bust, and disappear from the sample—almost 80 percent of actively managed funds don't beat simple index funds over 10 to 15-year periods.

In the meantime, you're stuck paying fees. Those fees don't sound too bad—just 1 percent!—but this is where our total lack of intuition for how compounding works really hurts us. Let's try an example: what's 0.99 to the 40th power? It's not exactly a calculation you can do in your head. It's not even one you can estimate. But it's the kind of calculation that you need to do to figure out how much your 401(k) fees are costing you.

The answer is a lot more than you think.