Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Maximal Lottery-Lotteries, published by Scott Garrabrant on October 17, 2022 on LessWrong.
In the last post we saw how by introducing lotteries, we could resolve one of the major conflicts in voting theory, between Condorcet and consistency (while also satisfying participation and clone independence). Now, we are going to make it better. How? By adding even more lotteries!
As far as I know, unlike the previous posts, the content of this post is a new proposal by me. Everything in this post should be treated as conjecture, rather than proven.
The Proposal
Fix a finite set C of candidates, and an electorate V∈Δ(C[0,1]).
First, recall the following definition of maximal lotteries. Given lotteries A,B∈Δ(C), we say that A dominates B if Pa∼A,b∼B,v∼V(v(a)>v(b))≥Pa∼A,b∼B,v∼V(v(b)>v(a)). A maximal lottery is an M∈Δ(C) such that for all other L∈Δ(C), M dominates L.
Now for maximal lottery-lotteries. Given lottery-lotteries A,B∈Δ(Δ(C)), we say that A dominates B if PA∼A,B∼B,v∼V(v(A)>v(B))≥PA∼A,B∼B,v∼V(v(B)>v(A)). A maximal lottery-lottery is an M∈Δ(Δ(C)) such that for all other L∈Δ(Δ(C)), M dominates L.
Note that when we say v(A), since A is not a candidate, but a distribution of candidates, we mean the expected value of v(a) when a is sampled from A, and similarly for v(B). Unlike in maximal lotteries, to determine whether v(A)>v(B), we need the full information of v as a utility function (up to affine transformation), not just the preference ordering over candidates.
M is a distribution on distributions on candidates, but we need to output a distribution on candidates. Thus, we imagine sampling a distribution at random from M, and then sampling a candidate at random from that distribution. The output of the maximal lottery-lotteries voting system is the distribution that assigns each candidate the probability that they would be sampled from the above procedure.
Lottery Independence
To argue for maximal lottery-lotteries, I introduce a new criterion: lottery independence.
Given an initial set of candidates, C, we could imagine fixing any distribution over candidates P∈Δ(C), and introducing a new lottery candidate, representing P. Voters vote for the lottery candidate based on their expected utility from the lottery. Whenever the lottery candidate wins, we sample a candidate according to P.
A voting system satisfies lottery independence if introducing lottery candidates does not change the probability that any candidate is elected.
This is not a criterion that voting theorists would normally talk about, because in order to satisfy it, you both have to allow the voting system to be non-deterministic, and you have to collect utility data from the voters.
Lottery independence can be thought of as a strengthening of clone independence. Instead of introducing a clone of a single candidate, we introduce a lottery, which is like a randomized clone which clones a candidate at random after the election is over.
Maximal lottery-lotteries satisfies lottery independence. Indeed, one way to think of maximal lottery-lotteries, is first we close the set of candidates under all possible lotteries, and then we run maximal lotteries on the resulting set of candidates.
Maximal lottery-lotteries also satisfies consistency and participation, since they are just maximal lotteries over a larger set of candidates.
Lottery Condorcet Criterion
Since Condorcet, consistency, and clone independence uniquely specify Maximal lotteries, if maximal lottery-lotteries is different, it must sacrifice one of these properties. It sacrifices the Condorcet criterion.
Note that in most contexts, this is where I would stop reading. My normal first question when I see a new voting system is, "Is it Condorcet?" If the answer is no, my next follow-up questions are "How often does it correctly find the correct (Condorcet)...