Are IVs Going the Way of the Atlantic Cod?

It's hard to come up with a good instrumental variable (plausible source of exogenous randomization) so when someone does come up with one (e.g. legal origin) it's tempting to want to use it again and again. Unfortunately, as Randall Morck and Bernard Yeung point out in Economics, History and Causation, IVs with more than one use are deeply problematic. If a variable is a good IV for X then it can't also be a good IV for Y without also controlling for X and vice-versa. What this means is that every new use of an IV casts doubt on every previous use. Or as, Morck and Yeung, memorably write:

A Tragedy of the Commons has led to an overuse of instrumental variables and a depletion of the actual stock of valid instruments for all econometricians. Each time an instrumental variable is shown to work in one study, that result automatically generates a latent variable problem in every other study that has used or will use the same instrumental variable, or another correlated with it, in a similar context. We see no solution to this. Useful instrumental variables are, we fear, going the way of the Atlantic cod.

I am not quite so pessimistic, I don't see this as a fundamentally new problem or one specific to IVs. Nevertheless, I appreciate their advocacy for a wide variety of empirical methods as a solution to the over-fishing problem.

Among the alternative methods Morck and Yeung recommend, are event studies and Granger causality. I couldn't help but laugh at the last. But I wholeheartedly applaud their primary recommendation which is greater use of and respect for narrative history.


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