Why do economists keep advocating impossible policies?
Policy activists on both the fiscal and monetary side are united by one common thread. The policies they propose are impossible to credibly implement.
Start with the fiscal side. Christina Romer in the NY Times again calls for an increase in our debt now for "stimulus" combined with a long term reduction in our debt. Mark Thoma and many others have made similar calls.
Dr. Romer is an excellent economist with a fantastic research record, but her policy proposal is impossible! In our political system, we cannot make any type of future long term commitment to do anything.
We cannot bind future politicians. Long run plans will only come to pass if they are in the interest of the politicians who are in office at each point in time over the course of the plan. In econo-speak, the policy must be time-consistent if it is ever going to be followed.
Amazingly we find the exact same problem on the monetary side. We are told by the Sumnerians that targeting a path for nominal GDP or the price level would help avoid situations like our current one.
But for these policies to work, when shocks hit the economy people have to believe that the Fed will do things in the future that they know the Fed doesn't like to do!
The Fed cannot bind either (A) future Feds, or (B) future politicians, who after all actually created and run the Fed.
The popular economics discussion of policy alternatives seemingly takes place in a world where Finn Kydland never won the Nobel Prize.
I will allow that there are papers that take commitment issues seriously in monetary policy. Many of them are well discussed in this excellent post. But, reading the post will show just how thorny those issues are and just how hard it actually is to implement an "optimal" policy over time.