Index Fund Concentration Raises Diversification Concerns
Regulators have long recognized a simple truth: diversification matters.
That is why funds and ETFs operate under rules designed to prevent excessive issuer concentration. Under the Investment Company Act of 1940, a “diversified” fund generally must keep 75% of its assets in cash, government securities, other investment companies, and securities where no single issuer represents more than 5% of total assets or 10% of that issuer’s voting securities. The tax rules for regulated investment companies impose their own quarterly diversification tests.
The policy intuition is clear: pooled vehicles sold broadly to the public should not expose investors to excessive single-company risk while presenting themselves as diversified investment products.
And yet, we now have an uncomfortable contradiction.
As U.S. cap-weighted indexes have become increasingly concentrated, regulators have shown a willingness to accommodate index funds that exceed traditional diversification limits when that concentration results from following the index methodology.
In other words, the fund may be permitted to remain true to the index even when the index itself becomes less diversified.
Why is preserving market-cap weighting treated as more important than preserving diversification?
Cap weighting is not a law of nature. It is a methodology. It rewards what has already grown the largest. It allows yesterday’s winners to become an ever-larger share of tomorrow’s exposure.
At modest levels of concentration, that may be acceptable. But at extreme levels, the investor experience begins to look less like broad diversification and more like a concentrated bet.
If diversification rules exist to protect investors from concentration risk, should the answer really be: “except when the concentration comes from an index?”
That seems backward.
The policy goal should not be to protect the purity of cap-weighted index construction. It should be to protect investors from risks they may not fully understand, especially when products are marketed and perceived as diversified core holdings.
A more logical approach would be to ask whether broad-market index funds should incorporate concentration guardrails. That could mean issuer caps, modified cap weighting, diversification bands or clearer labeling when an index crosses meaningful concentration thresholds.
None of this requires rejecting passive investing. It simply requires acknowledging that passive does not always mean diversified.
At some point, regulators, index providers, asset managers, advisors and investors need to confront the same questions:
When a “broad market” index becomes highly dependent on a handful of companies, is it still broad enough to deserve regulatory accommodation? Is the inherent rationale that nothing really bad can happen to the largest companies in the S&P 500? Might that be akin to saying in 2007, “real estate prices always go up?”
Investor protection should come before index purity.
