Still the go to model for understanding the usefulness of policy.
"But economists who knew basic macroeconomic theory – specifically, the IS-LM model, which was John Hicks's interpretation of John Maynard Keynes, and at least used to be in the toolkit of every practicing macroeconomist – had a very different take. By late 2008 the United States and other advanced nations were up against the zero lower bound; that is, central banks had cut rates as far as they could, yet their economies remained deeply depressed. And under those conditions it was straightforward to see that deficit spending would not, in fact, raise rates, as long as the spending wasn't enough to bring the economy back near full employment. It wasn't that economists had a lot of experience with such situations (although Japan had been in a similar position since the mid-1990s). It was, rather, that economists had special tools, in the form of models, that allowed them to make useful analyses and predictions even in conditions very far from normal experience.
"And those who knew IS-LM and used it – those who understood what a liquidity trap means – got it right, while those with lots of real-world experience were wrong. Morgan Stanley eventually apologized to its investors, as rates not only stayed low but dropped; so, later, did Gross. As I speak, deficits remain near historic highs – and interest rates remain near historic lows.