The arithmetic, in full. You contribute $500 in a month when the fund trades at $100 per share: you buy 5.00 shares. The next month the market has dropped 30% and the same fund trades at $70. Your next $500 buys $500 / $70 = 7.14 shares -- about 43% MORE shares than the $100 month bought you (7.14 / 5.00 = 1.43). You now own 12.14 shares for your $1,000. When the price recovers to $100 -- even if that takes two years -- your 12.14 shares are worth $1,214. Had both contributions bought at a flat $100, you would own 10 shares worth $1,000. The crash, plus the discipline to keep buying through it, left you about 21% ahead at the same recovered price. Nothing clever happened: the lower price simply let the same dollars buy more ownership.
| Where you are | What an early crash does | Why |
|---|---|---|
| Accumulator (22-year-old adding $500/month) | Helps you | The balance at risk is still small, and decades of future contributions get to buy shares at marked-down prices |
| Mid-career saver (large balance, still adding) | Roughly a wash | The existing balance takes a real hit, but ongoing contributions still buy cheap -- the order of returns starts to matter |
| Decumulator (retiree withdrawing each year) | The single biggest danger | Withdrawals sell shares at depressed prices, permanently shrinking the base the recovery compounds on |
Extra Shares per Dollar = 1 / (1 - Drop%) - 1
First Portfolio Builder module fpb-12 (Sequence-of-Returns Risk) maps this same idea across a whole investing lifetime, and pf-19 (decumulation and the 4% rule) shows the mirror image: for a retiree WITHDRAWING money, an early crash is the central danger, because selling into depressed prices permanently shrinks the base later growth compounds on. The inversion flips exactly when contributions stop and withdrawals start -- it is the direction of your monthly cash flow, not your age, that decides which side of sequence risk you are on. Two honest caveats. First, the math assumes the market eventually recovers: every broad US bear market so far has, though recovery can take years -- and an individual stock can go to zero, which is why this lesson applies to broad index funds, never to doubling down on a single name. Second, it assumes you will not need the money soon: a full emergency fund (pf-1) is what makes 'keep buying' possible instead of becoming a forced seller.
Sit with the ideas.
You are 23 and invest $500 a month in a total-market index fund. In month one the fund trades at $100 per share; in month two it has dropped 30% to $70. Two years later the price has recovered to exactly $100. Compared with a calm market that simply stayed at $100 for both purchases, where does the crash-and-recovery leave your first two contributions?