During my lengthy career in finance, I picked up several market heuristics that have generally proven more useful than not. Every January, for example, I remember the “five-day rule” which holds that if the S&P 500’s accumulated performance is positive over the first five days of the new year, there is around an 80% probability that the overall market will end the year higher than where it started. So far, this analytical trick has worked pretty well, despite all of the world’s uncertainties.

With May comes a second rule of thumb: “Sell in May and go away” (and, for us Brits, “come back on St. Leger Day” which is 15 September). Th...