The world has changed after the Great Recession, but some of the fundamental tenets of investing still apply and probably always will. Exhibit A is mean reversion, a theory that says that prices fluctuate around an average of historical prices or some other measure of value. True, you can’t count on mean reversion to apply like clockwork, particularly in the short run. In fact, there’s no way to know if mean reversion will apply at all in the years and decades ahead. Par for the course in finance: everything’s always open for debate. But a small library of empirical analysis suggests that we should take mean reversion seriously. In other words, buy low and sell high is a worthwhile investment strategy, even if the details are messy.