What job does equity actually do in a portfolio?
Every asset class in an allocation is there to do a job. Confusing the jobs is how portfolios end up with six holdings that all do the same one. Equity's job is the easiest to name and the hardest to sit through.
The job — growth, paid for in variability
A share is a residual claim: shareholders are paid after employees, suppliers, lenders and the tax authority. That position at the back of the queue is exactly why equities have historically delivered the highest long-run returns of the major classes, and exactly why they deliver the deepest losses. You are not being paid for cleverness. You are being paid for standing last in line and staying there.
Two numbers make this concrete in a way that averages never do:
- Annual volatility for a broad developed-market equity index has historically run somewhere around 15%, about a long-run average nearer 10% — so single years anywhere from roughly −20% to +40% are unremarkable rather than exotic.
- Peak-to-trough drawdowns approaching or exceeding −50% have occurred several times in the last century (1973–74 ≈ −48%, 2000–02 ≈ −49%, 2007–09 ≈ −57%) — and the 1929–32 collapse reached roughly −86%.
The arithmetic that makes drawdowns matter
Losses and gains are not symmetric, and the asymmetry gets worse as losses deepen:
- Down 10% → needs +11% to get back.
- Down 20% → needs +25%.
- Down 50% → needs +100%.
A 50% fall requires a doubling to recover, and doublings take time. This single piece of arithmetic is why the equity share of a portfolio is treated as the main risk dial: it is the number that decides how deep the hole can get.
Why the equity sleeve dominates the risk even when it is a minority of the money
Here is the calculation that surprises people the first time. Take an illustrative mix of 60% equities and 40% bonds, with equity volatility of 15%, bond volatility of 5%, and — generously — zero correlation between them.
Each sleeve's contribution to portfolio variance is (weight × volatility)²:
- Equity: (0.60 × 0.15)² = 0.09² = 0.0081
- Bonds: (0.40 × 0.05)² = 0.02² = 0.0004
Total variance ≈ 0.0085, so portfolio volatility ≈ √0.0085 ≈ 9.2%.
Now look at the split: the equity sleeve accounts for 0.0081 ÷ 0.0085 ≈ 95% of the portfolio's variance, while holding 60% of the money. The bond sleeve holds 40% of the capital and supplies about 5% of the movement.
This is the single most useful fact in allocation arithmetic. Money weights and risk weights are not the same number, and the gap between them is usually enormous. When someone describes a portfolio as "balanced" on the basis of its money weights, they have described the cash, not the risk.
What this does not imply
It does not follow that any particular equity weight is right, too high, or too low. A portfolio whose owner needs the money in eighteen months and a portfolio funding a 40-year endowment can look at the same 95% figure and reach opposite conclusions — both correctly. The arithmetic tells you what the dial does; it does not tell you where to set it. That is the subject of Unit 2, and it does not have a universal answer.
Try it now
- A broad equity proxy over its full history is below. Find the 2007–2009 peak and trough, compute the percentage fall, then compute the gain required to recover it.
- Redo the variance split above for an illustrative 30/70 mix (same 15% and 5% volatilities). Equity contribution: (0.30 × 0.15)² = 0.002025; bonds: (0.70 × 0.05)² = 0.001225. What share of variance now comes from equity?
- Say the finding out loud in your own words — money weight and risk weight are different numbers. Note it as a fact about arithmetic, not as an instruction about what to hold.