The recent stock-market correction — the first in the US in two years — has invited substantial commentary about what investors should do, the role machines have played, and the implications for the real economy. But only some of what has been said is useful.

Many investment advisers have emphasised the need to think long term, rather than panicking when prices fall. They are right: the decline in early February is not a good reason to sell. What is a good reason to sell is that stocks are too high from a longer-term perspective.