Spreading assets across many active managers feels like textbook prudence. You diversify single-manager risk, you respect capacity constraints, and no single bad year sinks the program. But there is a cost that rarely shows up in a manager-selection memo: as you add managers, their active bets increasingly cancel out, and the aggregate portfolio drifts toward the benchmark — while the fees stay resolutely active. This article unpacks the mechanics of overdiversification and a framework for spending an active-risk budget more productively.
Why the problem matters now
The backdrop sharpens the stakes. With forward equity returns widely expected to be lower than the recent past, excess return matters more, not less. When beta is doing less of the heavy lifting, paying active fees for index-like aggregate exposure is a more expensive mistake than it used to be.
The mechanics of cancellation
The intuition is simple. Each active manager holds positions that differ from the benchmark — that difference is the active risk (tracking error) and active share you are paying for. But different managers’ overweights and underweights are partly uncorrelated. One manager’s overweight in a name is another’s underweight. Combine enough of them and the offsets dominate: the aggregate book’s deviations from the benchmark shrink toward zero.
The evidence is stark. US defined-benefit plans hold a lot of managers — large plans (over $50bn) hold around 30 equity funds on average, and the all-pension average is around 17. A randomized study using the Morningstar equity mutual-fund universe (equal-weighted, on the order of 10,000 iterations) shows what that does: as the number of portfolios grows from 1 to 10, aggregate active risk falls by more than half (roughly from low-3% to low-1% tracking error) and active share drops sharply (from the low-70s to the high-30s).
The fees, meanwhile, do not fall. So the fee per unit of active risk — and per unit of active share — rises. You are paying progressively more for progressively less of the thing you hired active managers to deliver. That is the definition of overpaying.
A decomposition of where returns come from
To spend the active-risk budget well, it helps to decompose a manager’s return into three layers:
- Beta — exposure to the market itself, available essentially for free through an index.
- Factors — style premia such as value, momentum, quality, and low volatility, available systematically and cheaply through smart-beta vehicles.
- True alpha — the idiosyncratic, manager-specific skill that cannot be replicated cheaply.
Most of what overdiversification cancels out is the factor and alpha layer — exactly the part you are paying up for. What survives the cancellation is mostly beta, which you could have bought for a few basis points.
A menu of vehicles
Different vehicles occupy different points on the cost / active-risk spectrum:
| Vehicle | Active risk | Fee | Scalability |
|---|---|---|---|
| Index | None / very low | Very low | Very high |
| Smart beta | Low–moderate (systematic) | Low | High |
| Enhanced active | Low tracking error | Moderate | Moderate |
| Concentrated active | High active risk | High | Low |
The insight is that a few concentrated or enhanced-active portfolios can deliver the same aggregate alpha and active-risk profile as a sprawling roster of diluted managers — at a materially lower blended fee, because their bets are not being washed out against each other.
A framework for rebalancing the alpha budget
The solution is not “fire managers” but balance the sources of alpha against their fees:
- Replace some diluted active exposure with enhanced active or smart beta, capturing factor premia efficiently rather than paying full active fees for them.
- If risk tolerance allows, replace some pure index exposure with low-risk enhanced active to raise total active risk where it is cheap to do so — pushing the aggregate book back toward the intended active profile.
The governing principle: optimize fee productivity — alpha and active risk per dollar of fee — at the level of the total portfolio, not manager by manager.
Takeaways
- Manage the active risk of the aggregate equity portfolio — the plan’s true exposure — not the active risk of each manager in isolation.
- Watch the cost-benefit curve as the manager count rises: falling active risk and active share, rising fee ratios, and growing due-diligence overhead.
- Balance the sources of alpha (beta, factors, true alpha) and match each to the cheapest vehicle that can deliver it.
- Let plan size, alpha target, and fee budget drive the allocation — not the comforting instinct that more managers is automatically safer.
Diversifying across managers is sound risk management up to a point. Past that point, it becomes an expensive way to buy the index.
Educational commentary only. Figures cited reflect publicly described studies and are used illustratively. No proprietary models, client-specific material, or named holdings are reproduced. Nothing here is investment advice.