This article was co-published with The Washington Post.

Warren Buffett, the most successful investor of our time, is a huge fan of low-cost index funds — funds that replicate a market index rather than try to outperform it — as the way for the average investor to succeed in the stock market. "By periodically investing in an index fund … the know-nothing investor can actually outperform most investment professionals," he wrote in his 1993 letter to shareholders of his Berkshire Hathaway conglomerate. "Paradoxically, when 'dumb' money acknowledges its limitations, it ceases to be dumb."