Millions of investors have adopted a simple philosophy: If you can't beat 'em, join 'em. That's the basic principle behind passive index investing, in which you try merely to match the overall performance of an index rather than seeking to outperform it. Given how many active stock mutual funds fail to match the performance of their respective stock market benchmarks, that philosophy has worked well for stock investors.
But one key you need to understand about index investing is that it's only as good as the index you follow. In many markets, index investing creates problems that actually cost passive investors in lost returns.
When friction kills
A lot of attention about the downsides of index investing have focused on stocks and the ability to take advantage of massive index funds that are required to buy or sell particular stocks when they get added or deleted from the indexes they track. For instance, many point to the run-up in Facebook
But the problem goes beyond large-cap stocks. Other investments have also had mixed success with the passive approach.
Take, for instance, the U.S. Natural Gas Fund
Not all commodity ETFs have that problem, though. With SPDR Gold
The trouble with bonds
Passive investing with bonds can get even trickier. As an article in this weekend's Barron's noted, many of the bonds that bond ETFs have to buy and sell don't have enough liquidity to handle the volumes involved when crisis situations arise. The result is that ETF shares can trade at premiums or discounts to their actual value, costing those who buy or sell at the wrong time. In particular, high-yield bond funds SPDR Barclays High Yield
High-yield bonds aren't the only area that's open to such danger. If tax increases take hold next year, then any increase in popularity of municipal bonds could trigger a similar phenomenon. Foreign bonds also have the same bottleneck potential, as most foreign bond markets have nowhere near the liquidity that Treasuries have.
Where stocks fear to trade
Once you get past the top companies in the market, many stocks are very illiquid. Tiny micro-cap stocks have so little interest that even a small institutional trader can greatly disrupt their trading. High spreads between bid and ask prices and thin trading raise transaction costs, making index funds that specialize in micro caps far less efficient than their large-cap counterparts.
As foreign stocks have gotten more popular, they also raise concerns. Big international markets are largely as efficient as U.S. markets, but some of the smaller emerging markets have struggled to handle the onslaught of investor interest in their listed companies.
The right strategy for the right stocks
Index investing is a great way to produce fine long-term results, but it isn't a panacea. Before you commit to a purely passive strategy, make sure you understand the downsides of indexing and take steps to protect yourself from its biggest failings.
One answer is to add some individual stocks to your index-fund portfolio. For some good prospects, you may want to consider U.S. companies that have achieved a big presence not just domestically but throughout the global economy. Learn the names of three of the best in our latest special report: "3 American Companies Set to Dominate the World." This report is completely free, so don't miss out!
Fool contributor Dan Caplinger has made his share of mistakes of all sizes. He doesn't own shares of the companies mentioned in this article. You can follow him on Twitter @DanCaplinger. The Motley Fool owns shares of Facebook. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool's disclosure policy tells you all, big or small.