Economics: Over time. the earnings yield on stocks has trended downward. That means that pretty much any market timing rule for the equity proportion of your portfolio will fail. Price-earnings ratios that appear high today from the perspective of past generations will, if this trend continues, appear low in a generation. In this context, the sheer size of the equity return premium is going to make any but the most finely tuned and crafted equity-share market-timing portfolio strategy into a loser. And the finely tuned and crafted strategies that are winners on historical data are very unlikely to be properly robust:

John Authers: It’s Dangerous to Stay Out of Stocks: ‘Research from Barclays provides fresh evidence that you should almost always have some money in equities…. Over no 20-year period since 1925, a span that includes both the stock market crash of 1929 and the global financial crisis of 2008, have [U.S.] equities failed to beat inflation. That cannot be said of any other asset class:… The same pattern is confirmed in Barclays’ home market of the UK, for which its data stretches back to 1899. The last 125 years have been much less kind to Britain’s economy and financial markets than they have been to the US, and so there was one 20-year period over which stocks lagged inflation… <bloomberg.com/opinion/a…>