Deflation terrifies economists because once it starts, they have no idea what to do about it. When demand in an economy shrinks, companies cut jobs, and with fewer employed demand shrinks even more. The deflationary spiral is self-reinforcing. Central banks can cut interest rates to near zero and slosh money around like drunken lottery winners, but once hope flickers and dies, there is nothing they can do to persuade anyone to invest in the economy.