Any time share prices are slumping, it is worth looking at a chart of America’s S&P 500 index that goes back to 1987. That year on October 19th, or “Black Monday”, it plummeted by 20% in a single day—a crash not equalled before or since. The shock was so great that regulators subsequently devised “circuit breakers”, which automatically halt trading after a big enough drop, to prevent a repetition. Pull up a chart stretching from then to today, however, and Black Monday is barely visible, dwarfed by the scale of the subsequent returns. For long-term investors, what felt like an earth-shaking event at the time turned out to be little more than a blip.