Monday, April 18, 2016

What's the Expected Marginal Impact of Voting?

I've always felt conflicted about voting. On one hand, I like the idea of participating in democracy. On the other hand, there is almost no chance of an election being so close that my voteone out of millionswill break a tie and have a marginal impact.

In the past, I have argued it is so unlikely my vote will matter that voting is not worthwhile, even if I altruistically account for the vast number of people whom elections affect. That is, I've argued that the expected outcome of my votethe chance it will swing the election, times the impact that would have on each person, times the number of people the election impactsis next to nothing. The expected causal effect of voting is so small, I claimed, that it would be more altruistic for me to write a nice letter to my grandmother than to vote.

Last week I finally thought through it carefully... and it turns out I was wrong.

I now think the expected net impact of one vote in a typical US presidential election is on the same order of magnitude as the impact of the election on one eligible voter. So if you care about the way your vote will affect the rest of the country and the world (and you think you know what effect it will have!), voting may be a very valuable use of your time.

In this post, I'll explain my old argument against voting, show why it was wrong, andwith minimal amounts of mathballpark a very rough estimate of the expected marginal impact of a vote. (If you're not interested in the fallacious argument, just skip ahead.)

Disclaimers

1. This whole post works within the framework of a plurality (whoever gets the most votes wins) election between two parties. The electoral college is somewhat more complicated, with aggregation of votes happening at more local levels. For the purpose of the ballpark arguments in this post, I don't think these details matter, but I'm happy to discuss in the comments if anyone disagrees.
2. I'm also ignoring the possibility that the election will be tied after I vote, and I'm ignoring the fact that very close elections are decided by complicated politcal processes I don't understand. Again, I don't think these matter to first order, but I'm happy to discuss in the comments.
3. There are lots of arguments for (and against) voting, and their omission does not represent a stance on any of them. I am simply focusing on causal impact.
4. None of the reasoning or math in this post is particularly sophisticated. I write it not because it is interesting but because it is important! And because I owe it to everyone to whom I've made the wrong argument.

My old, fallacious argument against voting

My thinking went like this: If we anonymize all of the $N$ ballots in an election and then consider them one by one, we can think of each as an identical random variable $X_i$. This random variable can take the value $-1$ (Democrat), $0$ (didn't vote), or $1$ (Republican). Since I don't know who voter $i$ is, and since he or she might make a mistake or forget to vote, $X_i$ could equal any of $-1, 0,$ or $1$, so it has some variance $\sigma^2$. Further, there is some bias to the votesi.e. people on average slightly prefer one candidate to the otherso $X_i$ has some non-zero expected value $b$.

What is the chance that I swing the election? It's the same as the chance that the election was tied before I voted. The probability of a tie is just
$$P(\sum X_i = 0) = P(\sum X_i \leq 0) - P(\sum X_i \leq -1) \\ = P(\sum X_i - E(X_i) \leq -b \cdot N) - P(\sum X_i - E(X_i) \leq -b \cdot N - 1) \\ = P(\frac{1}{\sigma \sqrt{N}} \sum X_i - E(X_i) \leq \frac{-b \sqrt{N}}{\sigma}) - P(\frac{1}{\sigma \sqrt{N}} \sum X_i - E(X_i) \leq \frac{-b \sqrt{N} - 1/\sqrt{N}}{\sigma})$$
Now, applying the central limit theoremwhich tells us that the average of many independent, identically distributed, mean-zero random variables converges to a normal distribution distributed "very tightly" around zerowe can say that the probability of a tie is
$$\approx P(Z \leq \frac{-b \sqrt{N}}{\sigma}) - P(Z \leq \frac{-b \sqrt{N} - 1/\sqrt{N}}{\sigma})) \\ \frac{1}{\sigma \sqrt{N}} \cdot \phi(-b \sqrt{N} / \sigma) = \frac{1}{ \sigma \sqrt{2 \pi N}} e^{-\frac{b^2 N}{2 \sigma^2} }$$
This is absurdly small for large $N$ (many voters). But more importantly, even when we compute my expected impact, multiplying the chance of swinging the election by the number of voters $N$ and the impact $I$ of the election on each voter, it is still basically zero. For example, if we suppose that the election has a whopping $\$$100,000 effect on each voter, that there are only one million voters, and that voters are biased toward the Democrats by only half a percent (so that b=0.01), the expected impact of my vote is$$(number \ of \ voters)(impact \ per \ voter)(probability \ of \ impact) \\ = N \cdot I \cdot \frac{1}{\sigma \sqrt{ 2 \pi N}} e^{-\frac{b^2 N}{2 \sigma^2} } = \frac{I}{\sqrt{2 \pi} \sigma} \sqrt{N} e^{-\frac{b^2 N}{2 \sigma^2} } \approx \frac{10^5}{\sqrt{2 \pi} \cdot 1} \sqrt{10^6} e^{-\frac{10^{-4} \cdot 10^6}{2 \cdot 1} } \\ = \$0.0000000000000077

Wait, really?

How can this be? No US presidential election has ever come within one vote. Is it really reasonable to think this might happen?

These questions are tempting, but ultimately misguided. We've never seen a tie before, andsince there is only a $1/N \approx 0.0000003\%$ chance of it happening in each presidential electionwe shouldn't expect that we ever will. But on the off-chance that there is a tie, each vote will have a marginal impact whose magnitude is as large as the off-chance is small. Since our brains are bad at understanding both tiny probabilities and huge impacts, and since this problem requires us to weigh the two against each other, we shouldn't really expect this to be intuitive.

Loose ends

So far, I've left all estimates in terms of $I$, which I've called the average impact of the election on a voter. By this, I mean the expected difference in outcomes for an average person if your preferred candidate is selected instead of the other one.

It's important to be aware that $I$ may be negative; you might chose a candidate who will actually do a lot of harm to other voters (not to mention the rest of the world!). If you are really humble, you might think that you have no better idea of what is good for people than does anyone else, in which case your $I$ is about zero, and you'll need to find some other reasons to vote.

However, you might also think that you are better informed or better educated than other voters, or that your values are "better" than theirs in some moral sense. In this case, $I$ could be quite large, since different presidents have significantly different priorities. I'll guess that I'd put my own $I$ in the range of thousands or tens of thousands of dollars (though I cringe at the idea of trying to monetize outcomes across such a wide swath of topics, as well as at having to put a number on something about which I'm so uncertain). This is huge, considering that it will take me at most a few hours.

tl;dr

In case you weren't going to already, you should really voteand you should make an informed decision about whom to support. It is unfathomably unlikely that you will swing the election, but if you do, you will impact an unfathomably large number of people.

Thanks

To Margaret, for having a conversation about voting that finally prompted me to formalize these arguments. To Jake, for some helpful edits and comments.