In IS-LM models there is always something in the background shifting the IS curve. What is it?
In my view that 'something' is Keynes' animal spirits that we should add to our models as a new fundamental.
In my work, I close my models by adding an equation that I call a 'belief function'. The belief function is an effective way of operationalizing the Old Keynesian assumption of ‘animal spirits’. It is a forecasting rule that explains how people use current information to predict the future. That rule replaces the classical assumption that the quantity of labor demanded is always equal to the quantity of labor supplied.Here is a link to the blog I wrote on that topic last year.