The news media typically covers stock markets using one exceedingly simple frame: Market goes up, good! Market goes down, bad!

And this much is true: If you’re in a stage of life when you are pulling money out of the market, like a retiree living off accumulated savings, a higher stock market is indeed great news. But some simple experiments with four hypothetical people show why that calculus is a lot more complicated for younger people. The key lesson: The dramatic runup in the American stock market over the last five years is actually bad news for many young adults who are just embarking on saving for retirement.

When you invest in some broad index of the stock market, such as any of numerous vehicles that let you buy into the Standard & Poor’s 500 stocks, you are in effect buying a claim on the future profits of all major listed companies. When the stock market goes up, you have to pay more for that future stream of profits; when it goes down, you are paying less.