Q: I am in my early 20s and am looking to expand my investment portfolio. I currently have a small 401(k) and an S&P 500 index fund. Should I keep building up my index fund or start diversifying into something with a higher return potential? — Caroline, California A: Participating in a 401(k) and investing in an S&P 500 index fund are a good start in your early 20s, notes Jim Ludwick, president of MainStreet Financial Planning. Index funds — which simply buy and hold all the stocks in a market index such as the S&P 500 — aren’t as flashy as actively managed portfolios, where stock pickers can choose only those shares that they think are promising. However, index funds are a simple and inexpensive way to gain exposure to the market. And there are years, depending on the market, where they can produce some sizeable gains. For instance, in 2013, the Vanguard 500 index fund, which tracks the S&P 500 index, returned more than 32%. That’s around three times the long-term average annual gain for stocks. However, index funds are only as diverse as the market they track. So a good way to expand at this point would be to invest in other index funds that go beyond the S&P 500 index of U.S. blue chip stocks. An index fund that tracks the Russell 2000 index, for example, would give you exposure to shares of faster-growing small U.S. companies which your existing portfolio lacks. You can add a Russell 2000 index fund to your mix to complement the S&P 500 fund, which gives you exposure just to large domestic companies. Or for simplicity, you could trade in your S&P 500 fund for a so-called total market index fund, which in a single portfolio gives exposure to both large and small U.S. firms. “Expanding into a whole market index,” Ludwick notes, “is a very effective way to do it.” Ludwick further highlighted the importance of investing in overseas markets for those seeking to expand and diversify. He noted that the Vanguard FTSE All-World ex.-U.S. ETF (ticker: VEU), which tracks stock markets outside the U.S., would be a good, low-cost complement to your U.S. holdings. When investing in index funds, it’s important to comparison shop among vendors — Vanguard, Fidelity, Charles Schwab, iShares, SPDR all offer index products — for fees. Keep in mind that the expenses you pay are deducted from the market returns the fund generates, so the less a fund charges in fees, the more of its returns you get to keep. Beyond index funds, you could branch out into actively managed portfolios. Studies have shown that over the long run, the majority of actively managed funds trail the basic indexes. Ludwick says active management is effective only in niche markets. That’s where the “insight really pays off,” he says. However, as with all funds, the lower you can keep the fees, the better off you’re likely to be in the long run.