Why Limit a Winning Trader?

Question

Your best trader has been profitable 18 months straight and asks the desk head to double his position limits: "limits only cap my P&L." Why do trading firms enforce hard position limits even on consistently profitable traders? Give at least three distinct reasons.

Accepted Answer

1. Track records are noisy; ruin is absorbing. Eighteen months of wins is consistent with modest edge plus luck, the standard error on a realized Sharpe over 1.5 years is large. Sizing must reflect uncertainty about edge (the fractional-Kelly argument), and the penalty is asymmetric: oversizing a true edge costs growth, but oversizing an imagined edge ends the firm. One blow-up erases the compounding that limits were protecting. 2. Size changes the game. Strategies are calibrated in normal regimes; large positions face market impact, crowding, and the tendency of liquidity to vanish and correlations to go to one exactly when you must exit. Losses scale superlinearly with size once you're the market. 3. Portfolio and capital logic. The firm runs many books against shared capital. Limits keep one desk from dominating firmwide risk and let management allocate capital to the best marginal risk-adjusted return rather than to whoever is loudest. 4. Bounding error, not just markets. A fat finger, a bugged pricer, a bad data feed: with no limit, the cost of a malfunction is unbounded. Limits are the firewall for operational and model risk. 5. Incentive asymmetry. The trader holds an option

Why Limit a Winning Trader?

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