Central Banks - Is There a Constraint on What They Can Do?

Central banks have inserted themselves into by doing fair amount, but also claiming to be able to do a lot more right so central bankers will never tell you we've run out of tools. Every central bank intervenes in securities markets andna moves in long term interest rates up down, moves share prices up down. So it's almost as if there ere no constraints on what a central bank can do. If there are no constraints, politicians say, why don't you do some of the things we want? And if you're not helping us keep un employment relatively high us be something you can do that you're not willing to tell us?