(corrected and re-posted)
The figures that follow are simple local regression (lowess) plots of the share of the vote for a given party and the share for a given electoral system in New Zealand’s general election and referendum of November, 2011.2 The data points themselves are suppressed because with around 6,000 polling places, the graphs would be overly cluttered. (For reasons indicated in a comment referenced below, the analysis is based on 5,375 polling places.)
Before I go any farther with this, I want to shout out a big THANK YOU to Vasi, who extracted the party-vote data from the many separate files posted at the Electoral Commission website.
Figure 1 shows the relationship between the support for each of New Zealand’s four largest parties–National, Labour, Green, and New Zealand First–and support for keeping the current MMP system. What stands out most is how linear the decline in support for MMP is as National support increases. Relative to Labour strength, support for MMP reaches a point where it levels off, but that is mainly because even where Labour receives around 50% support, the vote for MMP is already approaching 80%. Strong Green support is correlated with strong MMP support. No surprise there. There is a rather more limited relationship to votes for New Zealand First.
Figure 2 shows support for the two alternatives that were most likely to be favored by voters who wanted to change from MMP: Supplementary Member (SM, which is what I would call MMM) and First Past the Post (FP(T)P). Clearly, Labour strongholds were duly skeptical of SM, while support for both systems is associated with National’s strong polling places. There is actually a steady 30% or so support (on average) for FPTP in places where Labour wins more than about a third of the vote. I am not sure what to make of this; maybe strong Labour areas include many voters who long for the old days of Labour majorities under the former electoral system.
Figure 3 looks at the two large parties and support for the two options in Part B of the referendum that entail ranked-choice ballots: Preferential Vote (PV, which is usually known as the Alternative Vote) and the Single Transferable Vote (STV). Note that the lowess curves never rise above about 12% for either system. The one thing that stands out for me here is that there is actually some growing support for PV/AV as Labour strength increases. That makes sense, given that Labour could expect to benefit from many Green voters’ transfers. The sharp decline for PV/AV as National support increases is the mirror image of Labour’s trend–and is actually stronger, perhaps because National has so few potential allies who would give it second (or third, etc.) preferences. Still, we can’t make too much out of it, given that support for this system is so low overall. Not surprisingly, support for STV, which is nowhere high, decreases with support for either major party.
Figure 4 shows a rather interesting divergence of support for the two ranked-choice systems according to the strength of three smaller parties, Greens, New Zealand First, and United Future. Finally, we find out who wanted STV: Green voters! Well, actually, even they don’t want it very much, as places where their party vote reaches 40% or more still vote less than 20% for STV as their preferred alternative. Nonetheless, a trend for more STV support in locations with many Green voters certainly makes sense, inasmuch as STV is the only other proportional system on offer. It is a clear second best for supporters of a party like the Greens, which has generally dispersed support, but also (as the graph clearly shows) some pockets where it is locally quite strong. Local strength might enable it to combine with other parties and elect MPs in a few districts, while elsewhere its votes would transfer to Labour and help it elect an additional MP or two.
There is not much trend for the other parties, or for the Alternative Vote (PV), although places where there is a significant minority of United Future voters clearly are not places where voters see much benefit in the Alternative Vote.
Fun with Stata! And thanks again, Vasi, for your assistance!
See comment #6 for some information on the data.