One obvious reason is fear, or at least caution. In the depths of the financial crisis in 2008, when the safety of the banks seemed in doubt, short-term Treasury bills offered negative yields and investors were happy to take them. (Holding physical cash is impractical, given the sums involved.) Now, with some uncertainty about what might happen to banks were Greece to leave the euro, investors may decide it is worth accepting a negative yield of 0.16% on two-year German bonds. “In effect you’re paying a 16-basis-point custody fee for keeping your money safe,” says David Lloyd of M&G, a fund-management group.