John At The Center

The reason is that these are the only changes that can be used to reliability identify the effects of tax policy. If a tax change is made in response to some event, call it X, that influences the economy, it is hard to disentangle the effects of the tax change from the the direct effects of X. The Romers

http://gregmankiw.blogspot.com/2009/01/importance-of-being-exogenous.html