I suspect not. First, if they have wised up, it is only recently. 'Sell on May Day' worked well in the past two years, as the All-Share index fell between May Day and Halloween then.
Secondly, we've a strong theory behind the rule. This is that lighter evenings increase our appetite for risk - perhaps because they raise testosterone levels - thus causing shares to become overpriced in the spring. This big rise in share prices since mid-February is consistent with this theory.
Thirdly, even the best stock market rules fail sometimes simply because share prices are so volatile. Between 1966 and 2015, we saw 10 winters in which the All-Share index lost money in real terms, even though average winter returns in that period were a hefty 8.7 per cent.
And here's the thing. In the 10 summers after those winters, equities did badly. On average, real total returns were minus 6.5 per cent. That compares with a return of plus 0.8 per cent in the 40 summers following a winter in which real returns were positive. And these 10 summers saw six losses - which means that losses are almost twice as common after bad winters as after good ones.
This isn't because of any single outlier. Even if we ignore the summer of 1974 when the market lost more than 40 per cent, summers after a winter loss have seen real returns of minus 2.7 per cent on average.
History, therefore, sends us a clear message. The fact that the market did badly over the winter is no reason at all to expect it to bounce in the summer. In fact, the opposite has been the case on average.
Nor are low interest rates a reason not to shift into cash. Rates are low because central banks believe the prospects for economic growth are shaky. This is not a climate in which equities are guaranteed to prosper. A zero return is better than a negative one.
Of course, the same luck that causes shares to do badly in the occasional winter might also cause them to do well this summer. But this is an outside bet. Only 10 of the past 50 summers have seen a total real return of more than 10 per cent; losses of this magnitude have been equally common.
For me, all this reinforces the case for selling in May. Sure, there's a chance of missing out on a gain. But history suggests that such a profit, if it comes, will be small while there's a significant chance of a big loss.
