The Tragedy of the Commons

First, though, let me quote an explanation of what the tragedy of the commons is.

The tragedy of the commons develops in this way. Picture a pasture open to all. It is to be expected that each herdsman will try to keep as many cattle as possible on the commons. Such an arrangement may work reasonably satisfactorily for centuries because tribal wars, poaching, and disease keep the numbers of both man and beast well below the carrying capacity of the land. Finally, however, comes the day of reckoning, that is, the day when the long-desired goal of social stability becomes a reality. At this point, the inherent logic of the commons remorselessly generates tragedy.

As a rational being, each herdsman seeks to maximize his gain. Explicitly or implicitly, more or less consciously, he asks, “What is the utility to me of adding one more animal to my herd?” This utility has one negative and one positive component.

1. The positive component is a function of the increment of one animal. Since the herdsman receives all the proceeds from the sale of the additional animal, the positive utility is nearly +1.
2. The negative component is a function of the additional overgrazing created by one more animal. Since, however, the effects of overgrazing are shared by all the herdsmen, the negative utility for any particular decision-making herdsman is only a fraction of −1.

Adding together the component partial utilities, the rational herdsman concludes that the only sensible course for him to pursue is to add another animal to his herd. And another … . But this is the conclusion reached by each and every rational herdsman sharing a commons. Therein is the tragedy. Each man is locked into a system that compels him to increase his herd without limit—in a world that is limited. Ruin is the destination toward which all men rush, each pursuing his own best interest in a society that believes in the freedom of the commons. Freedom in a commons brings ruin to all.

The above quotation is from Garrett Hardin's article The Tragedy of the Commons. In spite of its name, the article isn't the original source of the idea, but it's a good reference. (A reference I picked up from Irrationality Is the Square Root of All Evil.)

It's quite a short article, actually, well worth reading, so you should just go read it now. I found a copy online, which you can get to by following the link above. Here's one of the points you're missing out on if you don't read it.

The argument has here been stated in the context of the population problem, but it applies equally well to any instance in which society appeals to an individual exploiting a commons to restrain himself for the general good—by means of his conscience. To make such an appeal is to set up a selective system that works toward the elimination of conscience from the race.

So, as I said, I tend to confuse the prisoner's dilemma with the tragedy of the commons. It's true that both are situations in which one can gain some benefit by acting non-cooperatively, i.e., by defecting, but there are differences.

First, the harm inflicted on others by a defection is distributed differently: in the prisoner's dilemma, the whole harm falls on a single person, while in the tragedy of the commons it's divided among lots of other people.

Second, the tragedy of the commons involves the idea of capacity. The commons can support some fixed number of cattle; until that number is reached, defecting doesn't really cause much harm; as a result, people get used to defecting. Actually, “defecting” isn't the right word, here—people get used to performing a particular action, an action which under other circumstances becomes non-cooperative. Or, as Hardin put it,

… the morality of an act is a function of the state of the system at the time it is performed.

The word I was looking for at the start of this essay was “conflate”.