The simplest, rules-based way to take advantage of lower stock prices is portfolio rebalancing. An investor who holds a portfolio split 50-50 or 60-40 between shares and bonds sells the bonds that have gone up in price, as interest rates fall, to buy shares that have fallen in price, and are now cheaper. She does this once a month or once a quarter to keep the weights constant. A bolder group of investors keeps cash in reserve so they can take advantage of bargain prices when the markets have turned away from risky shares. “There is a point when I say ‘this is getting interesting’,” says one investor. The threshold for “interesting” is a fall of at least 20%. For deep-value investors, it might be 40%.