The extent of our knowledge
2009-02-02 · Original WordPress address
One of the reasons I think modern macro is successful is that it cleanly separates what we know from what we don't know. Making assumptions about how people make decisions and assumptions about how those decisions interact, modern macro models give expected behavior. In our models we capture all that we don't know in what are called "exogenous shocks". They're "exogenous" because they're outside the model and they're "shocks" because they're exact value is unpredictable even if the economic agents inhabiting the model's world knows their distribution.
Given the behavior of exogenous shocks and the predicted behavior of the model's agents to those shocks, we can take the output of the model and compare it to real data. If a model replicates real data then its assumed this is due to the assumptions about shocks and behavior. Because the shocks represent things we don't know, we'd like to have the most simply shocks possible and therefore have the structure of the model explain as much of the data as possible.
Similarly, we can compare models by seeing which explains the most patterns in the data (or at least the patterns we care about) using the same exogenous shocks. If one model explains more data, we say that one tells us more about whatever is being studied. The shocks, however, continue to reflect our ignorance.
I like this paper by Eggertsson (h/t EotAW) because it shows how important the assumptions on exogenous shocks can be. Cole and Ohanian have several papers (findings summarized here) that show New Deal policies (by which they and Eggertsson mean industrial and union policies increasing monopoly power) were contractionary. This is actually the standard view that follows from microeconomic theory.
Eggertsson shows that after you include some standard modern macro model features (e.g. sticky prices, monopolistic competition) and you change the assumption on the exogenous shocks, these New Deal policies are, to my surprise, expansionary. The reason is at the zero lower bound for interest rates, without these policies, expectation for inflation doesn't materialize and the economy falls into a deflationary spiral. When the government gives unions and big companies monopoly power, on the other hand, people expect those unions and big companies to use that power to raise prices. In this way, New Deal policies take over traditional monetary policies roles in setting inflation expectations.
But I really, really like the paper because it makes explicit that the reason its results are different from standard results from Cole and Ohanian is because of the assumptions about the shocks. In both sets of research, the shock is to preference for precautionary savings ("animal spirits"). Cole and Ohanian assume the 1929 crash and the bungles of Hoover were the exogenous shock and when Roosevelt took office that shock began to dissipate. Eggertsson, though, assumes the shock persisted through the great depression. Which is the right assumption, nobody knows.
The paper has very nicely separated what we know from what we don't know. This makes task for future research very clear... what were the nature of those shocks to animal spirits?
To me, this strand of modern macro literature is encouraging. Everyone's assumptions are laid out on the table, theories are making heavy contact with data and progress can easily be identified. Its almost like this is science.