How many managers should a portfolio have for adequate diversification?

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AcadiFi · Accepted Answer

There's no universal answer, but the analysis follows principles: 1. More managers reduces manager-specific (idiosyncratic) risk but dilutes active return and increases complexity/cost. 2. Diminishing marginal benefit: most diversification benefit achieved with ~5-8 managers within an asset class. Beyond that, correlations rise and effective active risk decreases faster than added information ratio. Formula for blended IR when combining N uncorrelated managers with equal IR: IR_blend = IR_individual * sqrt(N). But with correlation rho > 0, benefits shrink. With rho=0.3 and individual IR=0.4, blend of 10 managers: IR_blend = 0.4*sqrt(10/(1+9*0.3)) = 0.4*sqrt(10/3.7) = 0.66. Adding 5 more managers adds very little. By asset class: - US Large Cap: 3-5 managers (highly efficient, limited manager alpha available) - US Small Cap: 2-4 (capacity-constrained, higher alpha per manager, but each manager is more important) - International/EM: 3-6 (regional specialists + global) - Hedge Funds: 8-15 (strategy diversification across equity LS, macro, event-driven, relative value) - Private Equity: 5-8 vintages across years (vintage diversification more important than manager count) Example: Blackwood Pension Plan $3.5B. US large cap $800M → 3 managers (core + value + growth). International $400M → 4 managers. Small cap $150M → 2 managers. Hedge funds $300M → 12 managers (or via FoF given small allocation). ```mermaid flowchart LR A[1-2 Mgrs
High idio risk] --> B[3-8 Mgrs
Sweet spot] B --> C[10+ Mgrs
Index-like, high cost] ``` Over-diversification produces 'closet indexing' with high fees — an unattractive outcome vs direct passive implementation.

How many managers should a portfolio have for adequate diversification?

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