I spend a good deal of time Lag theory, much more than the text and I also cover how it applies to Monetary Policy as well as just Fiscal Policy as the text would have you think. It is controversial because it suggests that in only cases where business cycles are severe that the government should intervene. The question is why do they tend to do so anyway? I, and many others, believe that it is something to do with political agendas, that politicians sometimes do what is best for themselves rather than the people who elect them. Citing one source should do it as you get to the bottom of this