It's good to have choices. But if you've got too many choices, it's easy to stress out about making the right one -- and to put off making any decisions at all.

Last year, a study looked at how people respond to having so many choices to make in their daily lives. The results showed that having too many choices made people less productive and more tired. In contrast, those who were given fewer choices and were told simply to act on the limited options available to them performed better.

As investors, we should take this lesson to heart. Although we face countless choices all the time in our investment decisions, narrowing the field to a few good options can help us act more clearly and decisively. That often means the difference between success and failure.

More choices than you can shake a stick at
For beginners and experts alike, the investing world can be overwhelming. With thousands of stocks and mutual funds available, choosing just a few seems like an impossible task.

Trying to break things down into categories doesn't really help. You might think of stocks in terms of growth versus value or large versus small. But often, that just forces you to make more decisions, such as whether you should invest in a particular category, or how you should divide your money among the categories that interest you.

The answer is to put together a portfolio of core mutual funds. With just a few funds, you can get all the market exposure you need. As Foolish fund expert Amanda Kish has discussed at length, what funds you choose for your core portfolio can make a huge difference to your investment results.

A bit of everything
While the rise of exchange-traded funds (ETFs) has given investors plenty of ways to make extremely narrow bets on specialized corners of the market, core funds generally seek to find broad exposure across traditional investing boundaries. Some core funds combine stocks and bonds within a single fund. Others hold just stocks but bring together different categories, like growth and value stocks or large- and small-company stocks. As a result, instead of having to buy a bunch of specialized funds to cover all your bases, a small number of core funds can make up the bulk of your portfolio.

Well-structured core funds can reduce your volatility and give you plenty of diversification. Large-cap core funds often hold the best-known, most secure blue-chip stocks, like Microsoft (NASDAQ:MSFT), Procter & Gamble (NYSE:PG), and ExxonMobil (NYSE:XOM). They typically don't pay off with dramatic returns -- most core funds have so many holdings that they stay pretty close to their benchmarks. But they will help you make it through bumpy markets without losing your nerve and making mistakes.

Taking chances
Once you have your core portfolio figured out, you can afford to take some risks on more specialized funds. But remember that these specialty funds can be more volatile than your core funds. For instance, the Tocqueville Gold Fund has gained more than 75% in 2009, largely because of the rise in precious metals and its effect on companies like Newmont Mining (NYSE:NEM) and silver-stream specialist Silver Wheaton (NYSE:SLW). Yet when the bottom fell out of commodities in 2008, the fund lost two-thirds of its value before rebounding sharply. Similarly, many emerging-markets funds that own stocks like Baidu.com (Nasdaq: BIDU) and Petroleo Brasileiro (NYSE: PBR) have done quite well lately -- but they've also reversed sharply in the past when those markets cooled off.

The beauty of owning good core mutual funds, though, is that if you don't want to, you don't have to buy anything else. It's a low-maintenance strategy that simplifies your life while getting you the returns you need to achieve your financial goals.

Don't let all the choices out there overwhelm you. Once you find some strong core funds, the rest of your portfolio will fall into place.

Learn how mutual funds can fit into your overall portfolio with a free 30-day trial of our Motley Fool Rule Your Retirement newsletter. You'll see fund analyst Amanda Kish's latest thoughts on funds and much more. Click here to get started.

This article was originally published May 28, 2008. It has been updated by Dan Caplinger, who doesn't own shares of the companies mentioned. Baidu is a Motley Fool Rule Breakers recommendation. Microsoft is a Motley Fool Inside Value recommendation. Petroleo Brasileiro and Procter & Gamble are Motley Fool Income Investor selections. The Fool owns shares of Procter & Gamble. Try any of our Foolish newsletters today, free for 30 days. The Fool's disclosure policy is always a good choice.