The Quiet Math Behind a Market Selloff
Headlines call it panic. On a trading desk it is mostly arithmetic: discount rates move, and every future dollar gets repriced at once.
A falling market looks like chaos from the outside. Inside, it is closer to a spreadsheet recalculating. Every listed company is valued as a stream of future cash, and that stream is worth less today than it will be worth later. The rate used to shrink it — the discount rate — is the single most powerful number in finance, and it moves whenever expectations about interest rates move.
Why a small rate move creates a large price move
The further out a dollar sits, the more sensitive it is to the discount rate. A dollar arriving next year barely notices a one-point change. A dollar arriving in year fifteen notices a great deal. That asymmetry is the whole story behind why a modest shift in rate expectations can knock double digits off an index in a few sessions.
Illustrative. Higher discount rates compress distant cash flows disproportionately.
Growth stocks feel it first
Companies whose profits are mostly in the future have almost all their value in the distant part of the stream. Companies with steady profits today have much less. That is why the same headline can take eleven per cent off one index and three per cent off another on the same afternoon.
Price is what the discount rate says. Value is what the business does. They only agree by accident.
What this means for a long-term holder
- If your horizon is decades, a repricing changes the entry price, not the destination.
- Selling into the repricing converts a paper move into a realised loss.
- Regular contributions buy more units at lower prices — the mechanical benefit of not flinching.
None of this means every fall is a buying opportunity. Sometimes the cash flow forecast really has deteriorated. The useful discipline is separating the two questions: has the discount rate changed, or has the business changed?
Common questions
- Why do stocks fall when interest rates rise?
- Share prices are the present value of expected future cash. A higher discount rate shrinks that value, and the effect is largest for cash arriving many years from now.
- Does a selloff mean the companies are worth less?
- Usually not directly. Most selloffs are a repricing of the same expected cash at a different rate, rather than a judgement that the underlying businesses have deteriorated.
Sources
- Federal Reserve — Selected Interest Rates (H.15)
- Damodaran, Equity Risk Premiums: Determinants and Implications
Not financial advice — educational only. This article is general information, not a recommendation about any security or strategy. Consider your own circumstances and speak with a licensed professional.
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