Why more shares per dollar helps a saver
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.
How this maps across a whole investing life
The order of returns helps or hurts you depending on which way your money is flowing: an early crash is a gift to an accumulator with a small balance and decades ahead, roughly a wash for a mid-career saver with a large balance, and the single biggest danger to a retiree selling shares to live on. First Portfolio Builder › Sequence-of-Returns Risk maps that full lifecycle; this module stays on the accumulator's side of it, where the math is unambiguously in your favor.
How a price drop buys you extra shares
Extra Shares per Dollar = 1 / (1 - Drop%) - 1
How this connects to portfolio building later
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.
Write your plan for the next market drop
Why a young saver can welcome a downturn
Why the risk falls on sellers, not buyers
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?