CHAPEL HILL, N.C. (MarketWatch) — The stock market is no easier to beat today than it’s ever been.

If anything, it may be more difficult.

In 2014, only 15.6% of the several hundred advisers monitored by the Hulbert Financial Digest did better than the broad stock market, as measured by the Wilshire 5000 Index. Five years ago, the proportion stood at 33.1%.

It's important for us at any time to be reminded of how hard it is to beat the market, but especially in light of recent fund-flow data showing that the Vanguard Group, the largest index-fund provider, received $216 billion in new money last year. According to the Wall Street Journal, that sum represents a record not just for Vanguard, but for any mutual fund company ever.

The surge in index-fund popularity has prompted some to wonder if the market has become less efficient, and therefore easier to beat. After all, as more investors give up trying to beat the market, won’t it become easier for those who persist in trying?

Eventually, yes. But index funds would have to become a whole lot more popular than they are today before the odds would shift in your favor.

That, at least, is the opinion of Lawrence G. Tint, chairman of Quantal International, a risk-management firm for institutional investors. Tint is uniquely qualified to comment on market efficiency, since he is the former U.S. CEO of Barclays Global Investors, the firm that created iShares and helped to popularize index-based exchange traded funds. Before that, he was president of Sharpe-Tint, a consulting partnership with William Sharpe, the 1990 Nobel laureate in economics.

In an interview this week, Tint estimated that the stock market would remain substantially efficient even if as much as 75% of all equity assets were invested in index funds. We’re nowhere close to that number now.

The accompanying chart gives you an idea of how far we’re away from 75%. A total of 19.9% of equity mutual fund assets are invested in index funds, according to the Investment Company Institute, the mutual fund industry’s trade association.

This percentage would be higher if we included exchange traded funds, but still low: According to fund tracker Lipper, 31.4% of combined open-end and ETF equity fund assets are currently benchmarked to an index. And even this probably overstates the case since it includes ETFs benchmarked to narrow sectors rather than to broad market indices. (Lipper’s database doesn’t differentiate between broad-based and sector-based index funds, so an exact percentage isn’t available.)

Tint recommends that we not focus on the amount of money invested in index funds, but instead on the behavior of those investors who choose not to index. “For stocks to become mispriced as indexing grows in popularity,” Tint told me, “it would have to be the case that most of those who continue trying to beat the market are irrational in one direction — with insufficient assets available at managers who are willing or able to arbitrage away others’ irrationalities.”

“That’s always possible, but seems a very unlikely scenario,” he said. “That’s because you’d expect the remaining active investors to jump at any opportunities to exploit others’ irrationalities.”

The investment implication: Beating the stock market is as difficult as it ever was, if not more so. Investing in an index fund, unexciting as it may be, is your best choice unless you have overwhelming reasons to believe you can beat the odds.

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