First, the scale of what happened
The shift to passive is not a trend anymore; it is the market's new default. By 2026, passive strategies hold roughly 54% of US domestic equity fund assets, and indexed funds now hold $21.82 trillion, surpassing the $18.75 trillion in active funds. The flows are lopsided to the point of being one-directional: active equity funds saw more than $1 trillion in outflows in 2025, the eleventh straight year of net outflows, while passive vehicles kept taking money in.
The reason is not fashion. It is arithmetic, and it is close to irrefutable.
The math that started it all
In 1991 William Sharpe laid out what he called the arithmetic of active management, and it is worth stating because it is the bedrock the whole passive case rests on. Active managers, in aggregate, own the market. Therefore, before fees, they must collectively earn the market's return, because they are the market. After fees, the average active dollar must underperform the average passive dollar by roughly the difference in cost. This is not a claim about skill or markets or any particular year. It is an identity, true by definition.
The real-world data has confirmed it about as thoroughly as finance data ever confirms anything. Over 15 years, 94% of active funds underperform their index, and in large-cap US equity, just 14% beat the S&P 500 over that span. Over the 10 years ending in 2025, only 21% of active funds survived and beat their passive counterparts. The longer the horizon, the worse active looks, which is exactly what Sharpe's arithmetic predicts, because fees compound.
That is the case for passive, and it is strong enough that the burden of proof sits squarely on anyone arguing for active. So the interesting question is not whether active underperforms on average. It does. The question is whether the rise of passive is quietly changing the conditions in a way that could shift the odds, which is the sabotage thesis worth examining honestly.
The theory that passive breaks the market
Here is the active industry's best argument, presented at full strength because it deserves to be.
Markets work because buyers and sellers who have opinions about value push prices toward fundamentals. An index fund has no opinion. It buys a company because the company is in the index and at whatever weight the index assigns, regardless of whether the stock is wildly overpriced or a bargain. As passive ownership grows, a larger share of all buying and selling carries no information about value, which should, in theory, let prices drift further from fundamentals before anyone corrects them.
If that were happening, two things would follow. Individual stocks would move more on index flows than on their own merits, and the mispricings that active managers exist to exploit would grow larger and more frequent. A world with more passive money should, on this logic, be a world where skill pays better, because there is more distortion to correct.
It is a clean theory, and there is a specific version of it that has real force: concentration. The top 10 companies in the S&P 500 now make up roughly 40% of the index, which means passive flows pour disproportionately into a handful of mega-cap names simply because they are already large, potentially inflating them further in a self-reinforcing loop that has nothing to do with their fundamentals. That is a concrete, plausible distortion, and it is the strongest piece of the sabotage case.
Why the theory keeps failing its own test
And yet. If passive investing were making markets less efficient and handing active managers a richer opportunity set, the last decade, during which passive overtook active, should have been a golden age for stock pickers. It was the opposite. Active underperformance got worse, not better, over exactly the period when the distortion was supposedly building.
That is the fact the sabotage thesis has to explain and mostly cannot. The theory predicts active managers should be thriving as passive grows. They are doing the reverse. Something in the elegant argument is not matching reality.
The likely reason is that the marginal price is still set by active traders, and it does not take many of them. Sharpe's arithmetic has a subtle corollary: passive funds are price takers, so prices are set entirely by whoever is still actively trading. As long as enough informed capital, hedge funds, active managers, arbitrageurs, remains to pounce on genuine mispricings, the market stays roughly efficient even if most money is passive. Price discovery does not require that most investors are active. It requires only that enough are, and that they are the ones actually setting prices at the margin. So far, that condition appears intact, and the mispricings the sabotage thesis promised have not materialized at a scale that shows up in active returns.
There is also an uncomfortable selection effect. As passive absorbs the least skilled and most fee-insensitive money, the active managers who remain are competing mostly against other serious professionals rather than against naive retail money. The game got harder, not easier, because the easy marks left the table for index funds. Passive did not create a target-rich environment for active managers. It removed the targets.
Where active genuinely still earns its fee
None of this means active management is pointless everywhere, and the honest version of the passive case has always conceded specific exceptions. The academic and practitioner evidence points to conditions where active has a real edge.
Market efficiency is not uniform. Active tends to do better in less efficient corners: small-cap stocks, emerging markets, and fixed income, where information is less evenly distributed and a diligent manager can find things the crowd has missed. PIMCO's research documents that active management has consistently outperformed in fixed income over the past decade while failing to do so reliably in US large-cap equity. Bonds are a genuinely different game, because index construction in fixed income has real flaws that a thoughtful manager can exploit.
Active also does better when stocks stop moving in lockstep. Wilmington Trust identifies high return dispersion, stocks moving independently rather than together, as a condition favoring active managers, and there are early signs of it: the Magnificent Seven have underperformed the wider S&P 500 year-to-date in early 2026, with broader market participation returning. When securities diverge, there is more for a stock picker to distinguish between. Whether that persists long enough to matter is unknown, but it is the environment in which active has its best case.
This is why most professionals do not treat it as a binary. A large majority of financial advisers combine both approaches, using cheap passive funds for efficient large-cap US equity, where active reliably loses, and deploying active selectively in fixed income, small caps, and international, where the odds are better. That hybrid is not a fence-sitting compromise; it is the position the evidence actually supports, matching the tool to the efficiency of the market.
The honest answer to the headline
Is passive investing sabotaging active managers? The evidence says no, or at least not in the way the theory predicted. Passive did not make markets so inefficient that stock picking became easy. If anything it did the reverse, by draining away the unsophisticated money active managers used to beat and leaving them to fight each other for a shrinking pool of genuine mispricing.
But the theory is not dead, and intellectual honesty requires holding the door open. The concentration concern is real, and nobody knows the level of passive ownership at which price discovery genuinely degrades, because markets have never been this passive before. It is possible there is a threshold out there past which the elegant theory finally comes true and the distortions become large enough to exploit. We simply have not hit it yet, and the people confidently predicting we are about to have been predicting it, wrongly, for a decade.
The practical takeaway is unglamorous and well supported. For efficient large-cap exposure, the arithmetic is close to decisive and passive wins. For less efficient markets, active still has a real case. And the grand theory that indexing would eventually rescue the stock pickers remains what it has been the whole time: elegant, plausible, and still waiting for the data to agree.