Consider these two statements:

1. The model in Lucas (1972), Expectations and the Neutrality of Money, made a path breaking contribution to economic theory. It is comparable in importance to the Solow model and the Dixit-Stiglitz formulation of monopolistic competition.

2. The model in Prescott and Kydland (1982), “Time to Build and Aggregate Fluctuations," has no scientific validity.

Next, consider these two statements:

3. Einstein’s model of the universe based on his theory of General Relativity, made a path breaking contribution to theoretical physics, even though in his first application, Einstein built a model of a steady state universe.

4. Models of a steady state universe have no scientific validity.

In each case, the first statement in the pair is about the contribution of a mathematical model to scientific progress. The second is about the empirical validity of a specific model.

It goes without saying (or at least it should) that in science, one must always be willing to consider new evidence and be open to the possibility that it could induce a shift in posterior beliefs about what is true. So in a Bayesian sense, posteriors never have mass points that assign probability 1 to the truth-value of any statement.

Nevertheless, in everyday conversation, we say that we believe that a statement is true if the posterior odds in its favor are sufficiently high. In this sense, I believe that statements 1, 2, 3, and 4 are true. I realize that other macroeconomists do not agree.

In a previous post, I wrote that it is a sign of a problem in macroeconomics that we see persistent disagreement about whether statements such as 1 and 2 are true.

In this post I want to point to a different indicator of problems in macroeconomics. Set aside the question of whether or not I am right that 1 and 2 are true. Think of some macroeconomist X that you know. Consider these questions:

• Would X agree that there is an objective sense in which statements 1 and 2 can be said to be either true or false?

• Would X agree that a reasonable person could conclude that statements 1 and 2 are both true?

• Would X be able to examine dispassionately the evidence for and against these two statements and evaluate them independently?

A useful indicator of the degree to which macroeconomics has been infected by tribalism might be to put macroeconomists into the role of X and ask for what fraction of them the answers to at least one of the questions A, B, and C would be no.

If we replace statements 1 and 2 with statements 3 and 4, we could construct a comparable measure of tribalism in physics. If we did, I suspect that we would find little tribalism there. I suspect that in comparison, this indicator would reveal that macroeconomists today are very tribal.

The positive question that this assessment prompts is how this tribalism emerged. To me, this is what makes the recent intellectual history of macroeconomics so interesting.