Index funds are the only rational alternative for almost all mutual fund investors.

It is very hard, if not impossible, he wrote in his study, to justify active management for most individual, taxable investors, if their goal is to grow wealth. And he said that those who still insist on an actively managed fund are almost certainly deluding themselves.

Assuming that the future is like the past, you can outperform 80 percent of your fellow investors over the next several decades by investing in an index fund-and doing nothing else. But acquire the discipline to do something even better: become a long-term index fund investor.