Because Social Security will only provide enough income, if you're an average earner, to replace about 40% of your paycheck, and most seniors need around twice that amount to live comfortably, you'll need income outside of your benefits to ensure that you're able to make ends meet once you're no longer working. And chances are, that income will come from the savings you amass in a 401(k) or IRA.

But consistently funding a retirement plan is only part of the picture. You'll also need to invest the money in that account wisely to ensure that it's able to grow in a manner that outpaces inflation. If you don't, you'll risk a major shortfall once your career comes to an end.

Unfortunately, new data from Transamerica reveals that almost half of workers aren't putting enough thought into how their retirement cash is invested. In fact, 44% say they prefer not to think about or concern themselves with retirement investing until they get closer to the age at which they plan to exit the workforce for good.

Woman with serious expression at laptop

Image source: Getty Images.

Of course, obsessing over retirement plan investments is a hardly a healthy thing to do. Those who check their 401(k) or IRA investments daily are doing themselves a disservice, especially because market conditions can fluctuate from day to day and in the course of a decades-long savings window, overnight changes are virtually meaningless.

But the opposite end of the spectrum -- not caring about retirement investments at all -- isn't a great place to be, either. And if that's where you're at, you may have a financial struggle on your hands once the time comes to start dipping into your 401(k) or IRA.

Take control of your long-term investments

Why do you need to put thought into your retirement plan investments? It's simple: If you don't, you may not be happy with your end results.

Imagine you actively choose investments that deliver an average annual 7% return over the life of your savings window (that's a few percentage points below the stock market's average). If you set aside $300 a month in a retirement plan over 30 years, at that return, you're looking at accumulating $340,000. But if you leave things up to chance, you could wind up with a lower average annual return on investment -- say, 5%. And that, in turn, would leave you with just $239,000 in total savings by the time you retire, all other things being equal.

So how do you choose your retirement plan investments? For one thing, think about your age and the number of years you have until retirement might kick in. If you're looking at 10 years or more, it pays to go heavy on stocks, because they're more likely to generate substantial returns than bonds.

From there, you'll need to choose the right stocks. IRAs generally let you build a portfolio of individual stocks, but that requires research. With 401(k)s, you're generally limited to mutual funds and can't buy stocks individually. Either way, a good bet could be to load up on stock-focused index funds, which usually come with much lower fees than actively managed mutual funds.

Index funds track existing indexes like the S&P 500 and aim to match their performance, so when you add them to your portfolio, you're effectively investing in a bunch of different stocks at once. And that level of diversification is important when you're aiming to minimize losses on the road to growing wealth.

Of course, there are more specific strategies you can play around with, and those might also hinge on your personal appetite for risk. But either way, don't procrastinate on choosing investments for your retirement plan, and don't assume your investments don't matter. The right portfolio could set you up for a very enjoyable lifestyle as a senior, and the sooner you take control of your 401(k) or IRA, the greater your chances of accruing enough wealth to live out the retirement you've always dreamed of.