I’ve got a thing for models. And COVID has meant a lot of cool little models of disease transmission have been coming across my desk. This has been fun for me. But it’s also an intellectual minefield. Models help us tell stories and think through versions of the world that haven’t happened, but could. And they leave us feeling confident that we understand the world we’re operating in.
But it’s worth remembering a key inescapable fact: you always have to use your best judgment. There are no straightforward conclusions you can get for free without taking a risk of being wrong. A model showing that masks are worth it misses knock on effects. That doesn’t mean the model is useless, just that it only captures one part of the world.
Take the humble supply and demand model. We take a couple of lines, add in some other conditions (e.g. taxes, transaction costs, price controls, etc.), do a little algebra, and voila! You’ve got yourself a conclusion: subsidizing a good will result in people buying more (despite the private benefits of those extra units being less than the private costs). If you find some reasonable estimates of the elasticity of supply and demand for a product you can figure out how much impact a subsidy would have. Ceteris paribus.
All models rely on the ceteris paribus assumption in some form. If a model didn’t hold something constant it wouldn’t be a model anymore, it would just be a copy of reality.
In the case of supply and demand we’re rolling pretty much all the interesting things into that all-else-held-equal assumption. Language, history, legal structure, current events, politics, technology, and all the infinite possible interactions between things. Subsidizing face masks in 2019 would have seemed like a mistake, holding constant the state of affairs in 2019. Sure, we could have figured out that there was some sort of positive spill-over for masks even without a pandemic. But we could have also identified any number of other threats competing for scarce resources.
My advice to students: maintain humility. (My advice to non-students: maintain a student mindset.) Economics provides an incredibly powerful set of tools, but it doesn’t make you a god. There’s no getting around the fact that you’ve got to simplify reality to understand it and there’s no fool-proof formula for identifying things that make sense to hold constant in a constantly changing world.