In the same letter, Buffett explained the advice he gave to the trustee for a bequest to his wife in his will: "Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 fund. (I suggest Vanguard's.) I believe the trust's long-term results from this policy will be superior to those attained by most investors--whether pension funds, institutions, or individuals--who employ high-fee managers."
While it may not be appropriate for most investors to tilt so heavily toward U.S. equities, the benefits of index investing persist in a more diversified portfolio, according to a study by Richard Ferri, founder of Portfolio Solutions, and Alex Benke, VP of financial advice and planning at Betterment. Ferri and Benke constructed a portfolio that invested 40% of its capital in the Investor shares of Vanguard Total Stock Market Index Fund (VTSMX), 20% in Vanguard Total International Stock Index Fund (VGTSX), and the remaining 40% in Vanguard Total Bond Market Index Fund (VBMFX). They tracked the pretax performance of this portfolio from 1997 through 2012 and compared it with 5,000 portfolios of actively managed funds. These active funds were randomly drawn from a survivorship-bias-free universe of each of the following categories: U.S. equity, international equity, and U.S. bond funds. The funds in these categories received the same weightings as in the index portfolio. Ferri and Benke excluded sales loads and did not rebalance either the index or the active portfolios. If a fund merged or closed during the period, they replaced it on that date with another randomly selected fund from the category.
In 82.9% of the simulations, the index portfolio outperformed the active portfolio. When the active portfolios outperformed, they offered a median excess return of 0.53%. But when they underperformed, their median shortfall was 1.25%. These results were also consistent after controlling for differences in risk. In other words, the odds of randomly picking a winning fund aren't good, and the cost of selecting a losing fund can more than offset the payoff from a winning fund.
Not surprisingly, Ferri and Benke also found that the portfolio of index funds had a greater chance of outperforming a portfolio composed entirely of actively managed funds the longer it was held. This is because the cost advantage that index funds enjoy compounds over time, creating a bigger hurdle for active funds to overcome. Time also distinguishes luck from skill, which is scarce.