The Lucas Critique

The critique itself

Robert Lucas Jr., “Econometric Policy Evaluation: A Critique” (1976). Nobel 1995, substantially for this.

The setting: 1960s macroeconomics had large statistical models fitted to historical data, used to answer questions like “what happens to unemployment if we accept 2% more inflation?” The Phillips curve seemed to give a stable menu.

Lucas’s argument:

1. Those estimated relationships summarize how people behaved given the policy regime they expected.
2. People form expectations about policy and act on them.
3. Change the regime and expectations change, so behavior changes, so the estimated relationship changes.
4. Therefore using the old model to evaluate the new policy is invalid — the model’s parameters are not policy-invariant.

The 1970s obliged by demonstrating it: exploit the Phillips curve deliberately, workers build inflation into wage demands, and you get stagflation — high inflation and high unemployment, which the old curve said was impossible.

The methodological consequence was enormous. It pushed macro toward “microfounded” models built on preferences and technology — things assumed to be deeper than policy — which is where DSGE modelling came from.

The objection Cowen is making

The paper under discussion is Acemoglu, Autor, Beirne & Scott, Baby Busts and Growth Booms (NBER w35401, July 2026), which finds that falling birth rates are associated with higher GDP growth per working-age adult and higher wage growth, because technology responds to scarce young labor by becoming labor-saving. Identification comes partly from World War II deaths — historical variation in the size of younger cohorts.

Cowen’s move:

“there’s a lot of young labor missing after World War II, but right before us is this period of great population plenty, and to some extent, it’s expected. No one thinks there’ll be a 1.3 total fertility rate, whereas today, you have all these countries, some of them quite poor—Brazil, I think, is at 1.36 or so—and that just seems to be ongoing… Isn’t there a Lucas critique objection to your lack of fear of the fertility crisis?”

Unpacked, three distinct differences between the historical episodes and now:

Each breaks the extrapolation in its own way. If the shortage is global there is no elsewhere to import labor or capital from, so general-equilibrium effects differ. If it is permanent and anticipated, investment and family behavior adjust in advance in ways the historical data never contained. That is a textbook Lucas critique: the estimated response to labor scarcity was conditional on a regime that is ending.

Acemoglu’s answer — he concedes it

“Absolutely, 100 percent. Yes, that’s a very good point. The last paragraph of that paper says that.”

He then lists the cases himself — China at a scale never seen, South Korea, Brazil, Turkey, Mexico, India — and grants that “when the whole world goes through it… perhaps the general equilibrium, general world equilibrium effects are going to be different.”

His one counter is that another thing is changing simultaneously: people are living longer and healthier, so they can “accumulate more and more relevant human capital throughout their lives.” Note that this is not a rebuttal — it is a second unmodelled change, which is arguably a second Lucas problem rather than an answer to the first.

This is the most intellectually honest exchange in the interview. Cowen finds the weak point of a paper Acemoglu just co-authored, and Acemoglu agrees without hedging and points at where he’d already written the caveat down.