Market efficiency is a self-correcting equilibrium that survives even as passive investing dominates the landscape.

The Rise of the Average Investor

When index funds were first conceived in the 1970s, they were met with academic praise and industry derision. Critics labeled the strategy "un-American," questioning why anyone would settle for being merely average. Vanguard launched the first retail index fund in 1975, but for decades, active management remained the undisputed king of the financial world. That dominance has finally begun to erode. In the United States, passive funds doubled their market share in a single decade, climbing from 17% in 2006 to 34% by 2016. A similar, if slower, trend is visible in Canada and other global markets.

This shift is driven by a growing awareness of fees and a mountain of evidence suggesting that most active managers consistently underperform their benchmarks. As more capital flows into passive vehicles, a natural question arises: if everyone stops trying to beat the market and simply tracks it, does the market itself stop working? The fear is that without active participants to set prices, the stock market will become a mindless bubble, disconnected from economic reality.

The Engine of Efficiency

To understand why index funds won't break the market, one must understand how markets become efficient in the first place. Nobel Laureate Eugene Fama famously argued that in an efficient market, security prices reflect all available information. This occurs because sophisticated investors—not just "mom and pop" traders, but the largest institutional players—constantly research stocks in an effort to find an edge. When they buy or sell based on their findings, they inject that information into the price.

In this environment, an active manager can only outperform by accurately predicting the future, which is notoriously difficult to do consistently. Most proponents of this theory don't believe markets are perfectly efficient, but rather that they are efficient enough to make it nearly impossible to distinguish between a manager's skill and pure luck. Indexing is simply the logical response to this reality: if you cannot reliably beat the price, you should own the price at the lowest possible cost.

The Grossman-Stiglitz Paradox

The concern about "too much indexing" leads to a fascinating intellectual knot known as the Grossman-Stiglitz paradox. Introduced in 1980, the theory posits that perfectly informationally efficient markets are an impossibility. If a market were truly, perfectly efficient, there would be no profit available for those who spend time and money researching stocks. If there is no profit, no one will do the research. But if no one does the research, the market ceases to be efficient.

This creates a self-regulating equilibrium. If too many people move to index funds and prices begin to drift away from their fundamental values, the potential profits for active managers increase. These "mispricings" act as a homing signal for capital. Active managers will swoop in to exploit the opportunity, and by doing so, they push the market back toward efficiency. The market doesn't need everyone to be an active participant; it only needs enough participants to ensure that the cost of uncovering information is compensated by the potential for profit.

Quality Over Quantity

A critical nuance in this debate is who exactly is moving toward passive investing. In a 2005 paper, Eugene Fama and Ken French noted that market efficiency depends heavily on the sophistication of the remaining active traders. If the investors who choose to index are the "uninformed" or those with poor track records, the overall efficiency of the market actually improves. By removing the noise created by less skilled participants, the remaining pool of active managers becomes more concentrated with highly skilled, well-informed professionals.

Furthermore, the barrier to maintaining efficiency is lower than it used to be. The cost of uncovering and evaluating relevant data has plummeted in the digital age. As information becomes cheaper to process, it takes fewer active investors to keep prices accurate. Research has shown that even as index funds grow, the level of skill and competition among the remaining active managers is actually increasing. They are fighting over smaller scraps of alpha, which keeps the price discovery mechanism sharp.

The Persistent Allure of the Edge

Despite the logical superiority of indexing for the average person, active management is unlikely to ever disappear. There will always be a segment of the investing population motivated by the prospect of higher returns, regardless of the additional risks and costs. This human drive to outperform ensures that the "active" side of the equilibrium will always be populated.

Index funds are not a threat to the market's integrity; they are a refinement of it. They provide a low-cost haven for those who recognize the difficulty of the game, while leaving the task of price discovery to those most equipped to handle it. As long as there is a dollar to be made by being right when others are wrong, the market will continue to function, regardless of how many investors choose to simply sit back and enjoy the ride.