What Maximum Drawdown Measures
Maximum drawdown (MDD) is the largest peak-to-trough percentage decline in the value of a portfolio or fund over a specified period. It captures the worst loss an investor would have experienced if they had bought at the highest point and sold at the lowest point before a recovery.
The calculation is straightforward: identify the portfolio's highest value (the peak), then find the lowest value reached before a new peak is established (the trough). The drawdown is expressed as: (Trough − Peak) / Peak. A fund that peaks at £100, falls to £60, and later recovers to £120 has a maximum drawdown of 40% — even though the investor who held throughout ended up with a profit.
Why Drawdown Matters More Than Standard Deviation for Behaviour
Standard deviation is symmetric and statistical. Maximum drawdown is asymmetric and experiential — it describes the actual worst-case loss that an investor sitting in that fund would have faced on paper.
This matters enormously because investor behaviour is driven by what they actually experience, not by abstract statistical measures. When a portfolio falls 40%, many investors panic and sell — not because they have made a rational calculation that it will fall further, but because the psychological pain of watching wealth disappear becomes unbearable. This is the behaviour that destroys long-term returns: selling at the trough locks in the loss permanently.
Research consistently shows that investor returns lag fund returns substantially because of this pattern of buying after strong performance and selling after drawdowns. Maximum drawdown is the measure most closely linked to the trigger point for this destructive behaviour.
Historical Equity Drawdowns
Major equity market drawdowns put the concept in perspective. US equities, as measured by broad market indices, experienced drawdowns of approximately 49% during the dot-com crash of 2000 to 2002, and approximately 51% during the global financial crisis of 2007 to 2009. UK equities experienced similar declines.
An investor who held through both of these periods ultimately recovered and went on to significant gains. But an investor who sold at the trough of either crisis, and waited in cash until they felt the coast was clear, would have missed much of the recovery and permanently impaired their long-term outcomes.
The Mathematics of Recovery
A feature of drawdowns that investors often underestimate is the asymmetry of losses and recoveries. A 10% loss requires an 11% gain to recover. A 20% loss requires a 25% gain. A 50% loss requires a 100% gain. A 75% loss requires a 300% gain.
This asymmetry is a direct consequence of percentage mathematics and is not intuitive. It means that deep drawdowns are disproportionately damaging — each additional percentage point of loss requires a larger and larger percentage gain to recover. A fund that occasionally drops 60% is not just 20% worse than one that drops 40%; it requires roughly three times the subsequent gain to return to the same starting point.
Using Drawdown in Portfolio Construction
Maximum drawdown should be a central input when constructing a portfolio for a specific investor. The question is not only "what is the expected return?" but "what is the worst loss this investor might face, and can they emotionally and financially sustain it?" If the answer is no — if a 40% drawdown would cause them to sell — then a portfolio with historical 40% drawdowns is inappropriate regardless of its long-run expected return.