One of the first things we generally cover in intro microeconomics is the determinants of demand– i.e. the factors that influence how much of a good we are willing and able to purchase. The price of a good is obviously one of these determinants, but so are the prices of what economists call “related goods.” related goods are broken down into two categories:
- Substitutes- roughly speaking, goods that are consumed instead of one another
- Complements- roughly speaking, goods that are consumed together
More precisely, economists define substitutes and complements in terms of the relationship between the price of a related good and the demand for the good in question. By this definition, the demand for a good decreases when the price of a substitute decreases (and vice versa). Conversely, the demand for a good increases when the price of a complement decreases (and vice versa). While this definition isn’t wrong per se, I’m surprised that few (if any) textbooks address how this relationship applies when substitutes and complements enter the world in the first place. After all, it stands to reason that substitutes entering a market decreases demand for an item, and complements entering a market increases demand for an item. (Hence the existence of iTunes and the Apple app Store, for example.) In order to reconcile this with the textbook explanation, I usually have to dance around some story about how a price of an item is technically infinite if a product doesn’t exist, which then implies that a product entering the market at a finite price is a form of a price decrease (which makes the official definitions apply).
Ok, now I’m even boring myself, but I think about this more than is reasonable and therefore wanted to put it on the Internet. Now what was my actual point…oh, right- sometimes it’s not obvious whether goods are substitutes or complements, and I am often reminded of this when I make up exam questions that I think are obvious and then have students students complain when they lose points. (I actually had a student rather convincingly argue that lemons and limes are complements because lemon-lime soda is a thing.) Therefore, it’s often helpful to work backwards from the data to infer whether goods are substitutes or complements. Take Broadway musicals and movies made from them, for example- substitutes or complements? On one hand, they might be substitutes because people don’t want to watch the same story twice. On the other hand, however, they could be complements because the widespread release of the movie could make people more interested in going to New York to see the musical. Even though I didn’t know which way the relationship would go, I wasn’t expecting to see this from the data:
(You can see more on the topic here.) So I am to believe that Chicago and Chicago are complements but The Producers and The Producers are substitutes? (Yes, I realize that the chart shows revenue and not demand specifically, but revenue seems like a reasonable proxy in a way that quantity of tickets does not because revenue accounts for price changes.) The article that this chart comes from gives more detail and, at a rough level, rules out the possibility that the differences are attributable to how long the musical had been out prior to the movie or when during the year the movie came out.
I thought I would point this out not only because it could make for an interesting classroom discussion but also because we tend to make a lot of assumptions about how goods are related when we discuss intellectual property protection, and it’s important to remember that these relationships aren’t necessarily obvious or even consistent, as evidenced above. Or, put more simply, why assume when you can actually go to the data and find out for real?