Here's a question that should keep multi-strategy traders up at night: your individual strategies each look fine in isolation, but together the portfolio feels like it's wearing concrete boots. The volatility is higher than expected, the Sharpe ratio is lower, and you can't figure out why. Welcome to the gap between perceived diversification and actual diversification.
The honest answer is that most traders never decompose their portfolio volatility at all. They eyeball correlation matrices, nod approvingly at numbers below 0.5, and assume diversification is doing its job. It isn't always. The real culprit — correlation drag — often hides in plain sight, quietly inflating realised volatility while your position sizing model remains blissfully unaware.
The mechanics start with the variance decomposition formula. Portfolio variance equals the weighted sum of each strategy's variance plus twice the weighted covariance terms between every pair. That second chunk — the covariance terms — is where correlation drag lives. Traders often size strategies using standalone volatility targets, then wonder why the book's realised vol consistently overshoots the plan.
The practical fix is to calculate each strategy's marginal volatility contribution — the partial derivative of portfolio volatility with respect to that strategy's weight, multiplied by its weight. Think of it like pinpointing which housemate is actually running up the electricity bill, rather than splitting it evenly. Strategies that look cheap on a standalone basis can be shockingly expensive once correlation to the rest of the book is priced in. Risk budgeting by marginal contribution — rather than by standalone vol — is the approach institutional desks use to keep the aggregate risk profile honest. For deeper grounding on the mechanics, portfolio variance and how it aggregates across positions is well documented, as is the concept of Modern Portfolio Theory which formalised the covariance framework, and risk budgeting which translates theory into operational position sizing.
Start today by pulling your strategy return series, calculating the full covariance matrix, and comparing each strategy's standalone vol to its marginal contribution. Where those numbers diverge sharply, you've found your correlation drag.
Diversification that only works on paper isn't diversification — it's decoration.
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