What it means
The behaviour is rarely deliberate deceit and more often a slow drift driven by career risk. A manager who strays far from the benchmark and gets it wrong tends to lose the mandate, so hugging the index becomes the safe choice even though it defeats the point of paying for active management.
The cost of this to an investor compounds quietly. If an active fund charging 0.90% delivers index performance while a tracker charging 0.05% delivers the same thing, the gap over twenty years is enormous, and none of it is explained by any difference in what the two funds actually own.
Active share is the standard measure, calculated as half the sum of the absolute differences between the fund's holding weights and the index weights. A figure below roughly 20% is usually treated as closet indexing, 20% to 60% as moderately active, and above 60% as genuinely active stock selection.
Tracking error, the volatility of the difference between fund and index returns, is used alongside it. The two measures answer different questions, since a fund can hold quite different names but still track the index closely if those names behave similarly, so consultants look at both together.
There is a legitimate middle ground worth acknowledging. Enhanced index and risk-controlled strategies deliberately keep active share low and charge accordingly, and the objection is not to low active share itself but to charging a full active fee for it without saying so.
In practice
Real-world examples.
Example
A pension trustee board reviews its equity managers and finds one fund with an active share of 18% charging 0.85%. The board switches the mandate to a tracker at 0.07% and redeploys the saved fee into a genuinely concentrated manager with an active share above 80%.
Example
A retail investor compares a well-known active fund's top ten holdings with the index's top ten and finds nine names in common at almost identical weights. The performance chart confirms it, showing the fund shadowing the index and falling behind by roughly the fee each year.
Example
A financial regulator reviews the domestic fund market and writes to several managers whose funds combine low active share with full active fees, requiring them either to change how the funds are described or to reduce charges.
Formula
Calculation
Active Share = 0.5 x the sum of the absolute differences between each fund weight and the corresponding index weight, expressed in percentage points.
Consider an invented equity fund benchmarked against a broad market index. Adding up the absolute weight differences across every holding and every index constituent gives a total of 60 percentage points.
Step 1: Active Share = 0.5 x 60 = 30%.
Step 2: this means 70% of the portfolio is effectively an index replica, and only 30% represents genuine active positioning.
Now put a price on that active 30%. The fund charges an annual fee of 0.90%, while a tracker covering the same index charges 0.05%, so the extra cost of the active management is 0.90% - 0.05% = 0.85%.
Step 3: effective fee on the genuinely active portion = 0.85% / 0.30 = 2.83% per year. Paying 2.83% a year for the only part of the portfolio that can outperform is a very high bar, and it is the single clearest way to see why closet indexing destroys value.Case study
Seen in the real world.
This is an illustrative and fictional example. The Hartsmere Growth Fund, an invented equity fund, marketed itself on the skill of its manager and charged 0.90% a year. Over eight years it returned an average of 0.83% a year less than its benchmark, a gap that matched its fee almost exactly.
An adviser reviewing the fund for a client calculated an active share of 30% and found that the manager held 84 of the index's 100 constituents at weights within half a percentage point of the benchmark. Dividing the 0.85% fee premium by the 30% active portion produced an effective charge of 2.83% on the only part of the fund that was doing anything distinctive.
The client moved the money into a combination of an index tracker for the core and a small allocation to a concentrated fund with an active share above 85%. Total cost fell, and more importantly the portfolio now had a coherent story: cheap market exposure in one place and paid-for conviction in the other, rather than an expensive blend of the two.
Watch out
Common mistakes.
- Judging a fund as a closet indexer purely on its returns. Performance close to the index is a clue, but the diagnosis needs holdings data, active share and tracking error rather than a chart alone.
- Treating high active share as proof of skill. A high active share only means a manager is taking different positions, and being different is a precondition for outperformance rather than a guarantee of it.
- Ignoring the fee difference because it looks small. An 0.85% annual gap compounds relentlessly and is the most reliable predictor of long-run underperformance available to an investor.
Questions
People also ask.
What active share level counts as closet indexing?
There is no universal threshold, but below roughly 20% is widely treated as closet indexing and above 60% as genuinely active.
Is closet indexing against the rules?
It is generally a disclosure and value-for-money issue rather than outright fraud, though several regulators have taken action where a fund's marketing did not match its actual positioning.
How can an ordinary investor check for it?
Compare the fund's published top holdings and sector weights against the benchmark, look up its published active share or tracking error, and compare its fee to a tracker on the same index.
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