In that case, how can this metric be used as a measure of Gerrymandering ? How do the proponents of this metric deal with that issue ?
Handwaving. They assume that over the course of several election cycles, the vote would go differently. Either the incumbent would get more than 51% of the vote or the challenger would sometimes win. That's because that's what normally happens. Incumbents normally win unless the challenger has some kind of advantage that year.
There's also a theoretical problem with really unbalanced districts requiring gerrymandering. For example, if Salt Lake City is roughly even in voters and the rest of Utah is overwhelmingly Republican, then the wasted Republicans could outnumber the wasted Democrats in a 4-0 split. Because excess votes in winning by too much is as bad as losing votes in the efficiency gap.
The primary problem is that gerrymandering is not a measurable thing. Start with the fact that gerrymandering is not just one thing. It is a collection of different behaviors, united by how people feel about them. Things that can be called gerrymandering:
- Giving the district an odd shape (e.g. like Gerry's salamander).
- Splitting up a municipality or county unnecessarily.
- Putting too many voters of a group in one district (packing).
- Putting not enough voters of a group in one district (cracking).
The problem is that these things can be contradictory. The most even shape might be to split an entity across two districts. If we keep the city together and want to keep the rest of the county in one district, we tend to have to shape it like a doughnut (with a hole in the middle). In one district, it might take 60% minority by population to ensure 55% by registration and 50% by actual voter. But in another district, they may call that same proportion packing.
For example, let's look at the 12th congressional district in North Carolina in 2016. This district was 44.6% black, less than half. Yet the argument was that this district packed too many black voters into one district. So the courts mandated that they crack the district and distribute its voters among other districts.
Gerrymandering is a charge that politicians use to complain about districts that are not favorable to them. Republicans point out that the districts drawn in Illinois and Maryland are unfair to them. Democrats counter that districts drawn in North Carolina and Ohio are unfair to them. Libertarians and other third parties point out that every geographic districting plan is unfair to them.
The genius of the efficiency gap is that unlike distance from proportionality, it favors Democrats without favoring third parties or Republicans in Democratic states. So it reinforces the two party hegemony. They don't even bother giving examples including third party candidates.
The first thing to notice is that the efficiency gap doesn't diagnose any states as having anti-Republican gerrymanders. This is because the threshold they chose doesn't see anti-Republican gerrymanders in states like Illinois and Maryland.
The second thing is that it doesn't show states that have a Republican minority. For example, Massachusetts is roughly 25% Republican, but Republicans have no seats in the state.
The third thing is that independents and third parties get no help from the efficiency gap. For example, independents make up about 25% of Massachusetts and also have no seats in the House of Representatives. And in states like California and Texas, there are enough Libertarians (and possibly Greens in California) to make up at least one seat.
As a statistic, the efficiency gap currently helps Democrats a lot. It might help Democrats less after an election like 2008, but right now it would shift districts towards the Democrats in some states. It does so without providing any help for third parties or independents. As such, in my opinion, it distracts from real reforms and props up the two party system. It perpetuates many of the inequalities of the current system and encourages the courts to get involved in each and every districting.
There isn't a lot of data showing them doing their work. The original paper does not include real calculations on real data, merely on examples. And most news articles don't include more than a mention of the final results. But since posting this, I have found two articles that at least tell us what states this method identifies as being the most gerrymandered. In 2012, that was Florida, Michigan, North Carolina, Ohio, Pennsylvania, Texas, and Virginia.
In 2016, that was North Carolina, Pennsylvania, Michigan, New York, and Texas. Note how three states dropped off the list in those four years: Florida, Ohio, and Virginia. Florida redistricted, but Ohio and Virginia did not. One state was added: New York. The total number of seats affected (outside of the threshold) was seven in 2016 in the five states.
New York is an interesting case. It is actually rather absurd to view it as a Republican gerrymander. In 2011 (when redistricting was being done), New York had a Democratic governor and one Democratic-controlled legislative chamber. It had one legislative chamber (the Senate) that was split evenly between the parties. As a result, Republicans could block a plan but couldn't force one. So rather than being a pro-Republican gerrymander, it is better thought of as a bipartisan plan.
Efficiency gap analysis identifies New York as a pro-Republican gerrymander. Why? Primarily because Democrats do really well in some districts of New York City while Republicans are only moderately dominant in any district. Another reason is size. 6% of 27 is two seats. Finally, the efficiency gap is biased. If we look at New York in 2016, we would see that Republicans were actually underrepresented. They won 33% of the seats but took 36% of the vote.
That statistic is even worse in California. There we find that Republicans won fourteen seats (26%) despite winning 36% of the vote (nineteen seats). So a proportionality standard would give a pro-Democratic gap of five. But the efficiency gap is zero seats.
Meanwhile, a proportional system like Single Transferable Vote (STV) could eliminate the very concept of redistricting. Each state would be a separate bucket but within the state, any voter could vote for any candidate. Voters would have their choice of candidates. Candidates would not be able to choose their voters, as every geographic district system allows. Candidates would only be able to choose how and where they marketed themselves to voters.
Is a candidate the local candidate? Is the candidate the ideological candidate, who best matches the voter? Is the candidate some kind of compromise? Voters could choose how to pick among those options.
If the problem is wasted votes, then let's not minimize the difference in wasted votes between two arbitrary groups. Let's minimize them at a larger scale. With STV, the total wasted vote is less than the number of voters for one district (because everyone else gets one of the candidates on their list). With the efficiency gap, the wasted vote is always 50%. The only difference is whose vote is wasted.
This would still leave some decisions to states. Do they use traditional STV? Or one of the Condorcet-compliant alternatives? Or something else (that may not have been developed yet)? I'm not crazy about List forms of proportionality (they reduce the advantage of good candidates), but some places like them. States would have room for choice and experimentation within the constraints.
That would allow true freedom of association, unlike the efficiency gap, where politicians still choose how their voters associate. They only face more constraints.