Does It Still Make Sense to Call Markets ‘Efficient’ When Most Money Is Passive?

Passive index funds now control a majority of assets in some major markets, and academics are increasingly debating whether that shift has changed how efficiently markets actually set prices.

Passive investing, once a niche alternative to actively managed funds, has grown to the point where index funds now hold a majority of assets in several major equity markets, a milestone reached faster than most in the industry expected even a decade ago. The appeal is straightforward: lower fees and, historically, performance that has matched or beaten most actively managed alternatives over long time horizons.

That growth has revived a long-running academic debate about price discovery — the process by which active traders analyzing company fundamentals push stock prices toward their true value. Critics of heavy index concentration argue that as fewer investors actively research individual companies, prices could become less responsive to genuine changes in a company’s underlying business.

The evidence remains genuinely mixed

Empirical studies on whether index fund dominance has actually degraded price efficiency have produced conflicting results, with some researchers finding measurable effects concentrated in smaller, less-covered stocks, and others finding limited impact on the broader market’s ability to price large, widely covered companies.

“Passive investing works precisely because enough active investors keep prices honest. The open question is how few active investors that actually requires.”

For now, the shift toward passive investing shows little sign of reversing given its cost advantages, leaving the price-discovery debate as more of an ongoing academic question than a factor most everyday investors are likely to notice in their own portfolios.