index funds are often described as safe. low cost. diversified. no fund manager risk.
all true.
but they have drawbacks. some are structural. some are behavioral. knowing them before investing matters.
concentration risk. the index is not as diversified as it looks
index funds are market-cap weighted. the biggest companies get the biggest weight. when a few stocks dominate the index, the fund is not as diversified as it appears.
by early 2025, the top 10 companies in the S&P 500 represented nearly 40% of its market capitalization . in 2023, the top seven stocks made up about 34% of the index, up from 20% in 2023 .
in 2022, as technology stocks corrected, the seven largest companies in the S&P 500 declined by more than 40% — roughly twice the decline of the broader index . investors who believed they owned a broadly diversified portfolio experienced a drawdown driven largely by exposure to a narrow group of stocks.
this is a global trend. at the end of 2025, the top 10 names accounted for 62% in germany, 57% in france, and 47% in china . the problem is not limited to the US.
hidden trading costs. index funds buy high and sell low
index funds have low expense ratios. but the total cost can be higher than the expense ratio suggests.
index providers announce changes in advance. sophisticated traders get ahead of these trades, pushing prices up before index funds buy and down before they sell . index fund managers systematically buy high and sell low.
looking across the decade ending in december 2023, stocks added and dropped from major indexes had prices deviate by an average of 3% to 4% relative to other stocks in the index . these trading costs can be as much as the expense ratio. the actual cost of an index fund can be up to twice what the expense ratio shows .
cash drag. index funds do not hold cash
the index has no cash. active funds often hold 2-5% cash for redemptions or opportunities.
index funds are fully invested. this is an advantage in bull markets. it becomes a disadvantage when markets fall. there is no cash buffer to limit losses .
nilesh shah, managing director of kotak mutual fund, noted that the cash held by active funds creates a drag on performance of 50 to 100 basis points compared to the index . but that same cash provides protection during downturns.
price discovery erosion. passive flows override fundamentals
passive funds do not read earnings reports. they do not care about valuation. they just follow formulas .
as passive ownership rises, the link between a company’s fundamentals and its stock price weakens . momentum and flows drive the action. not earnings or strategy.
research shows that stocks with high passive ownership have increasingly moved in lockstep . they become more sensitive to market-wide shocks. actively owned stocks act more independently, reacting to company-specific fundamentals.
market fragility. everyone sells at the same time
when everyone buys and holds the same stuff, they sell for the same reasons, too.
if one big index fund sees outflows, that pressure can ripple across portfolios holding the same stocks . this triggers a rash of selling that overwhelms fundamentals.
companies with high passive ETF ownership are more exposed to liquidity shocks . when the index sells, no one is there to buy. the result is more volatility, less liquidity, and a market that can go from calm to chaos in a heartbeat.
passive investing has overtaken active funds in total assets. passive funds now manage more money than active funds . that dominance brings hidden risks.
concentration and momentum. the self-reinforcing cycle
index funds create a self-reinforcing cycle. success attracts more money. more money sustains higher prices . smaller companies lose out on marginal capital and face downward pressure.
the same mechanism works in reverse. when investors pull money from index funds, the largest stocks must be sold . often at moments when market liquidity is strained.
the s&p 500 has by far the highest passive penetration. no index experiences more non-fundamental inflows or outflows . passive flows are often inelastic. price and valuation play no role.
limited flexibility. no active decisions
index funds give the investor what is in the index. nothing more. nothing less .
there is no automatic access to securities not included in the index. there is no ability to overweight securities where the investor has conviction. the fund cannot avoid stocks that appear overvalued.
FAQs
1. can an index fund lose money ?
yes. index funds track the market. if the market falls, the fund falls. in 2022, a 40% decline happened in a narrow group of stocks . the broader index also fell.
2. are index funds truly passive ?
not as passive as they appear. index providers make active decisions about which stocks to include. the trading around index rebalancing creates hidden costs .
3. what is concentration risk in index funds ?
a few large companies dominate market-cap weighted indexes. the investor is not as diversified as they might think. the index is heavily influenced by a small number of stocks .
4. are index funds still good for beginners ?
yes. they are simple and low cost. but the investor should understand that they are not buying a perfectly diversified portfolio. they are buying a market-cap weighted basket that can become concentrated in a few stocks .
5. how can concentration risk be reduced ?
consider equal-weighted index funds. they hold stocks in equal weights regardless of market cap . consider value-based funds. they limit exposure to expensive stocks. consider diversifying across regions and investment styles .
