The sound of alarm bells is becoming harder to ignore. America’s stockmarket finished the first quarter of 2024 on an astonishing tear, with its benchmark S&P 500 index having risen in 18 out of the preceding 22 weeks. No longer: it has fallen over each of the past three. Look at individual stocks, meanwhile, and it is clear just how far investors have swung from euphoria to twitchiness. Nvidia was the poster child of the S&P 500’s winning streak, seeing its share price more than double between October and March. On April 19th it fell by a gut-churning 10% over the course of a single day, wiping more than $200bn from the company’s market value. The awful news that precipitated the plunge? There wasn’t any.