Profit-taking dominates the US stock market this morning. Among the worst performers today are the stocks that contributed most to the rise in the Dow Jones average this year: UnitedHealthcare, 3M, Caterpillar, McDonald’s, Apple and Boeing.

There’s nothing in common among today’s worst-performers except the fact that they used to be the best-performers, which suggests that investors are locking in profits ahead of year-end.