Some thoughts on Peru’s midterm election

After the Constitutional Tribunal ruled them legal, Peru held extraordinary legislative elections on 26 January. President Vizcarra dissolved Congress on the grounds that Congress had voted no-confidence in his cabinet (although not directly) twice. This was the first use of this provision since Peru’s 1992 constitution was promulgated, and as such it was the first time when legislative and presidential elections were not held concurrently.

However, the election did not merely lack a presidential contest. Almost uniquely, President Vizcarra, despite having been elected as part of former President Pedro Pablo Kuczynski’s party (previously Peruanos por el Kambio, now Contigo), chose not to endorse any party for the elections, merely advising voters to inform themselves. This reluctance was seemingly not due to any concern that Vizcarra’s endorsement would be a weakness for any party: at the time of the election, his approval rating stood at 58%.

Peru’s unicameral Congress is elected using open party-list proportional representation in 26 regions, with a 5% threshold applied at the nationwide level. The average district magnitude of 5 makes this a relatively moderate form of proportional representation, which explains why Keiko Fujimori’s Fuerza Popular was able to win a comfortable majority of 56% of the seats in Congress at the 2016 election despite only winning 36% of the vote.

The results of this election, however, were extraordinarily fragmented. The largest party, Accion Popular, got only 10% of the vote, and nine parties made it above the 5% threshold to enter Congress. More than a quarter of votes went to parties below the threshold, and in four provinces the leading party will receive no representation in Congress.

I will leave it to Peruvian experts, which I most certainly am not, to discuss what this result means for Vizcarra’s ability to pass his agenda. However, the results are interesting for other reasons.

Since the promulgation of the 1992 Constitution, Peru’s party system has remained quite stable (at least in terms of numbers, the identity of the parties has changed quite a lot). It has also remained quite close to the number of parties that the Seat Product Model (Shugart and Taagepera, 2017) would predict.

These elections are thus extremely unusual, and are perhaps indicative of the high importance of presidential elections and presidential endorsements in imposing structure on legislative elections in presidential countries. A fact particularly suggestive of this is the disastrous result for the two leading parties in 2016, both of which were affiliated to presidential candidates. Keiko Fujimori’s Fuerza Popular fell from 36% of the vote and 78 seats to 7% and 15 seats, while Peruvanos por el Kambio/Contigo fell from 16% and 18 seats to 1% and no seats.

Jamaica 2011: As good as PR–or not (updated)

Final results show the PNP won with 53.3% of the votes, to the JLP’s 46.6%. However, even as the final vote total was much closer than the preliminary result upon which this entry was based, the PNP picked up an additional seat. (Note that this gives it exactly two thirds of the seats.)

Thus the result was far from proportional, after all. In fact, it was even more majoritarian than a “typical” FPTP result would be with the given input parameters. The PNP’s Advantage Ratio is 1.25, whereas the Seat-Vote Equation would predict it to have been 1.14.

I am leaving the rest of this as originally crafted. The analysis of other elections stands, but that of 2011 would be altered by this new information. Thanks to Jon, in a comment, for the tip.
Jamaica held its general election on 29 December. Like the other former British territories in the Caribbean, Jamaica elects its parliament by first past the post (plurality) in single-seat districts. Also like other English-speaking Caribbean islands, Jamaica has a parliament that is significantly undersized, given its population. So this makes Jamaica a perfect opportunity to break out our old favorites, the Cube Root Rule of Assembly Size, and the Seat-Vote Equation.

The election result itself saw an alternation in power from the Jamaican Labour Party (JLP) to the Peoples National Party (PNP). Various news reports before the election had said the election was expected to be close. But it was not. The PNP won 41 seats to the JLP’s 22. Thus the JLP was defeated after a single term, which had been its first time in power since its defeat in 1989. (That was a two-term government, although its second term then was tainted by the PNP’s election boycott in 1983.)

The Jamaican case is of some interest to comparative elections specialists because it has an almost perfect two-party system. The two main parties combined for 99.87% of the vote in this election. The PNP won 61.3%.

Only once since 1959 has the third party in a Jamaican election won more than 1% of the vote (NDM, 5.2%, 1997). That makes Jamaica arguably a more “pure” two-party system than its very large neighbor to the north, and probably the biggest country to have a strict two-party system other than that really big one.

So, how did the system perform, in terms of the proportionality of translating votes into seats? We might expect a party winning over 60% of the vote in a first-past-the-post system to be significantly over-represented. The expectation is all the greater given the small size of the parliament, for the country’s population. With a population of around 2.7 million (just over a million voters), the Cube Root Rule would lead us to expect an assembly of more than double its actual size of 63. ((To be fair, they did increase their assembly size. It was only 60 seats from 1976 to 2007!)) Smaller assemblies mean less proportionality, other things constant. They tend to produce very high disproportionality under FPTP.

Yet the PNP’s 41 seats represent 65.1% of the total, hardly at all greater than its 61.3% of the vote.

The Seat-Vote Equation suggests that a “normal” case of about one million voters, 63 seats, and the top two parties at 61% and 38% of the votes would result in a leading party winning 84% of the seats. That would have been 53 seats, to 10 for the JLP.

In the 2011 election, then, Jamaica’s electoral system produced an almost completely proportional result.

This is not a systemic tendency, or if it is, it is a very new one. In fact, the Advantage Ratio (percent votes divided by percent seats) for the largest party in Jamaica had never been below 1.10 before this election (when it dropped to 1.06). Something has been going on in Jamaican elections recently: Every election that was contested by both major parties since 1959 had seen an Advantage Ratio of at least 1.16. Every contested election from 1976 through 1997 saw this ratio be at least 1.33, peaking at 1.50 in 1997, when the PNP won a third consecutive term. Then suddenly it dropped to 1.12 in 2002, when the PNP won a fourth term, in a very close election (50.14% to 49.77%). ((And, in case you are wondering, as I was, I checked: there is only a small relationship in FPTP systems between the top two parties’ difference in votes and the largest party’s advantage ratio. The effect is statistically significant, but the coefficient is around only .007. In any case, the falling ratio in close elections in 2002 and 2007 is consistent with the modeled relationship, but the greater fall in 2011 is most certainly not.))

From looking at the data on seat allocation, I can’t tell what has changed. But I can certainly tell that something has. For the third time in a row, the result has been unusually proportional for a FPTP system–and, in 2011, quite proportional for any electoral system.

The election was called early, as one was not due until the fall of 2012. The Prime Minister, Andrew Holness, in September replaced Bruce Golding (yes, another case of inter-electoral change of PM through “intra-party” mechanisms). Apparently, Holness felt he needed to go to the people for a new mandate. Apparently, it did not work out so well.

As an aside, how often do countries (especially in the Western world) hold elections in the final week of December? I imagine it must be very unusual.

As a further aside, in how many other countries is the more right-wing of the major parties called “Labour”? Or does the more left-wing party have “National” in its name? ((Yes, of course, it also has “People’s”, which is pretty much the only way I can remember which is which.))

Data cited in this entry are from my own research files.

Canadian provincial elections this week

Update: In a comment (#7), I compare the result to the seat-vote equation estimate.

Three Canadian provinces have elections this week. Voting has already been completed in Prince Edward Island (PEI) and Manitoba, and is taking place today in Ontario, the largest province. Each elections shows–or is likely to show–the vagaries of FPTP.

(Newfoundland & Labrador votes next week, 11 October)

First, the election in PEI produced a lopsided majority–again. The incumbent Liberal party returned to office with 22 of the 27 seats, on a slightly reduced vote percentage (51.4% compared to 52.9% in 2007). This was a loss of one seat, with the Conservatives winning 5 (+1). For the second straight election, the Greens supplanted the NDP as the (distant) third party, with 4.3% (up from 3%).

The province has a history of lopsided results (as I have shown in graphs); the 2003 Liberal victory marked an alternation from a Conservative government, which itself had 23 seats. In the election before that, the Conservatives had 26 of the 27 seats. In 1996, the last time no party won a majority of the vote, the Conservatives, with 47.4% could manage “only” 18 seats (a 2/3 majority).

The seat-vote equation, which estimates seats under FPTP systems, based on jurisdiction-wide votes for the top three parties, the size of the assembly, and the number of voters, says that a party with around 51% of the votes, where the second party has around 40%, “should” be expected to win around 65% of the seats, rather than the 85% it won in this election. ((Four seats in PEI were decided by fewer than 100 votes, and some of these might swing on recounts. Each major party has won two of these seats, based on current results.))

One key reason why PEI has such lopsided results is that its assembly is about half the size that the cube root rule says it “should be,” for its electorate. With around 80,000 voters turning out in recent elections, an assembly of 55 seats would be more appropriate than 27. The undersized assembly is why the seat-vote equation sees as “normal” for FPTP even a a party with just over 50% of the votes potentially getting almost two thirds of the seats. The geographic distribution of the vote in PEI, and its tendency towards big island-wide vote swings, only exacerbate an inherent tendency for big seat bonuses for the largest party.

Of course, the Island could also get less distorted results with even a modestly proportional mixed-member system, such as the one resoundingly turned down in a referendum in 2005.

In Manitoba‘s election, the incumbent NDP was returned to office with 37 of the 57 seats (64.9%) on just 46% of the votes. The NDP had won 36 seats in 2007 on 48% of the votes. So the party’s votes declined, but it seats increased. The second-place Conservatives substantially increased their votes, from 37.9% to 43.7%, yet saw their seats remain steady on 19. Such are the vagaries of FPTP. Liberals saw their votes fall from 12.4% to 7.5%, and dropped from 2 seats to 1.

The seat-vote equation would expect such a close race between the top two parties to have resulted in a seat split of about 30-27, instead of the actual 37-19. ((Given the greater gap in votes between the top two, we would expect the 2007 election to have split the seats 37-20; in other words that election turned out almost exactly as expected.))

Manitoba has no record of particularly odd results, although in both 1990 and 1995 the second largest party won many more seats than it “should have” won. This is a pattern that can result in a plurality reversal (higher seat total for the second largest party in votes), if the election is close enough. In both of those elections, the Conservatives won narrow seat majorities on less than 43% of the votes, while the second-place NDP in 1995 had 40% of the seats despite only 33% of the votes. ((In 1990, it had only 28.8% of the votes, yet 35% of the seats.)) Evidently, in several recent elections the NDP’s geographic distribution of its votes has been such that it can translate them into many more seats than expected, whether it is the largest or runner-up party. I point this out simply because this week’s election was quite close in votes (46%-44%) yet produced an unexpectedly large seat bonus for the NDP. A plurality reversal may have been barely more than a couple of percentage points of the provincial vote from happening.

In today’s Ontario election, we see real three-party competition, with the third largest party, the NDP, polling at around a quarter of the votes. The incumbent Liberal party won 71 seats in the 2007 election, or 66.4% on just 42.2% of the vote. For most of this year, it was expected to lose, possibly by a wide margin, to the Conservatives. Yet as the official campaign got underway, the Liberals and NDP made gains in polls. For a while the Liberals and Conservatives looked headed for a near tie in seats, with neither winning a majority, and a potential plurality reversal. Now the Liberals could retain a majority of seats, depending on how some key ridings (districts) turn out.

The ThreeHundredEight final projection sees the Liberals winning 58 seats (54.2%) on 36.6% of the vote (to 33.3% for Conservatives). No party in Ontario ((at least since 1967, which is the first year in my data.)) has won a majority of seats on less than 40% of the votes since the NDP won 74 of a then 130-seat parliament on 37.6% of the vote in 1990–the only time the NDP has been the governing party. For the record, the seat-vote equation agrees that this projected vote split would produce a majority (about 56 seats); what it does not expect is the mere 29 seats the Liberals are expected to win, according to the ThreeHundredEight projection. The seat-vote equation expects such a close second place to be good for 44 or 45 seats, which would leave only 7 for the NDP. That the NDP could be projected to win 20 seats by ThreeHundredEight–which takes into account district-level information unlike the seat-vote equation ((As I often point out, the seat-vote equation is not a projection tool. It is only meant to see how close an actual result deviates from what a “typical” FPTP election would produce, for a given jurisdiction-wide votes breakdown, and number of voters and seats)) –only shows how much the existing FPTP electoral system favors the NDP. Their huge manufactured majority in 1990 shows this pro-NDP bias is not new. ((Of course, potentially winning in this election nearly three times the number of seats as could be expected in a “normal” FPTP system offers minimal benefit when some other party has won a manufactured majority. Clearly the NDP today–although not back in 1990!–would benefit from a proportional system that would promote minority or coalition governments in which such a strong (in votes) third party could have real policy influence.))

Ontario’s three-party competition suggests it would be well served by a proportional system, such as the mixed-member system proposed by a citizens assembly, but turned down in a referendum the same day as the provincial parliamentary election in 2007.

Finally, both Manitoba and Ontario, like PEI, have undersized assemblies. For their population sizes, the cube root rule expects around 100 seats in Manitoba (instead of 57) and 200 in Ontario (instead of 107). Small assembly sizes only exacerbate the chances of anomalous results, although if one wanted seats distributions more reflective of votes distributions, a proportional electoral system would do the trick without needing to increase assembly size.

For more on the seat-vote equation and estimating the seats in first-past-the-post systems, see:

Matthew S. Shugart, “Inherent and Contingent Factors in Reform Initiation in Plurality Systems,” in To Keep or Change First Past the Post, ed. By André Blais. Oxford: Oxford University Press, 2008.

Past election data and estimates of seats come from the data set originally prepared in conjunction with the chapter, and updated since.

Error on year of NDP majority in original entry corrected.


Really, the Conservatives are under-represented, the Liberals over

What if we had a FPTP parliamentary system in which there were three national parties, and their vote percentages in any given election were:


We would have to call that fairly typical FPTP stuff. Not your ideal Duvergerian pattern, to be sure, but nothing remarkable in the real world of FPTP elections. Now let’s suppose their seat percentages were:


Pretty unremarkable, too, right?

Yes and no. On the one hand, this is what we should expect with FPTP: the two biggest parties with higher percentages of seats than votes, and the third party with significantly lower seats than votes.

Of the 211 FPTP elections in my database, there are 23 in which the largest party won from 38% to 42% of the vote (regardless of other parties’ percentages and excluding four plurality reversals). Of those 23 elections,* what’s the average seat percentage for the largest party? 54.35%. (The median is 52.63%, and the range is 36.15% to 69.09%.) So a large party winning around 40% of the votes and 54% of the seats is totally unremarkable.

Yet in another sense, the largest party in this Canadian election, the Conservatives, is under-represented–relative to a norm of FPTP expectations. Here I am speaking of the expectation set by the seat-vote equation,** which takes a distribution of the top three parties (plus “others”) and computes a “normal” output of seats for a given voting population and assembly size. Here is what the seat-vote equation thinks the seat distribution should look like, given the actual vote percentages:

    185 (60.1)
    100 (32.5)
    22 (7.1)

We’ll call that 1 “other” seat the Green winner, given that the Greens indeed did win their first elected seat. The seat-vote equation does not do well with regional parties. Fortunately for the equation, the regional party in this election almost disappeared (4 seats for the BQ, down from 50).

So the Liberals did quite a bit better than can be expected for the national third party. As a result, the Conservatives are under-represented, relative to FPTP “norm,” with 18 fewer seats than the equation’s estimate.

For all those who think the Liberals’ run as a viable party is over, be cautious. The British experience tells us that a Liberal party can survive for a good long time between the big parties of left and right. The party’s over-shooting of the seat-vote equation estimate underscores the extent to which it retains an efficient regional distribution on which it could build to win back seats in the future. In percentage terms, it is about where the British Liberal Democrats are in seats. This is a big shift, to be sure, but it is premature to write the party off, or to assume it will merge with the NDP.

Perhaps the bigger question is whether the NDP can survive as a major national left-wing party; first it will have to reconcile its now dominant Quebec wing with the NDP constituencies in the rest of the country. If it can’t, the Liberals will resume relevance, whether or not they surge back to “major party” status again anytime soon.

For all those advocates of proportional representation in Canada, this election is bad news. The first past the post system functioned about as expected, notwithstanding the under-inflation of the governing party’s plurality.

* The elections are: BC 1963, BC 1972, BC 1991, CA 1963, CA 1965, CA 1972, CA 1993, CA 1997, CA 2000 (the last majority government in Canada before this election), MB 1986, MB 1988, NS 1999, NS 2006, ON 1977, QC 1976, SK 1975, UK 1975, UK 1992, UK 2001, IN 1967, IN 1977, IN 1989.

** For details, click the words, seat-vote equation in the “Planted in” line above. There was an entry on election day applying the equation to the EKOS final projection, and many previous entries applying it to various past elections.

The seat-vote equation and Canada 2011

I have hesitated until now to run the seat-vote equation on the polls for Canada’s current election, because the campaign has been so unpredictable and regional and riding-level factors are likely to be decisive. Then again, maybe this is Canada’s most nationalized election in two decades or so…

So I ran it, based on the EKOS final numbers:

    CPC 34.0
    NDP 31.6
    LPC 20.8
    BQ 6.4
    GP 5.9

(Most other vote projections do not differ much from this.)

Disclaimer and background: The seat-vote equation is NOT a seat predictor. This is not a “projection”; you can find those elsewhere. The seat-vote equation simply tells us what the main parties’ seat totals “should have been” for a given votes distribution, based on “mechanical” features of the electoral system–how many districts there are in relation to the number of voters. It offers no insight into district-level factors. It has missed some past Canadian elections badly; in fact, I assembled the database specifically to see which elections were so out of line with how FPTP works that electoral reform might be put on the agenda. There have been many of those over the years in Canadian provinces, although at the national level Canada’s FPTP has not been prone to “anomalous” results, but rather has tended to be relatively proportional compared to other FPTP systems. (The seat-vote equation performed either admirably or terribly in the UK 2010, depending on your criteria.)*

With that disclaimer and background out of the way, what does it say the seats “should be” if we use the above votes?

    CPC 146
    NDP 123
    LPC 38
    others 1

Of course, the BQ is not going to win only one seat, and the Greens just might won one, as well. I said it was not a projection!

The seat-vote equation does not like parties that win seats despite having quite small national vote shares. It is right about the Greens getting 0 or 1 seat on their ~6%, but not about the BQ, despite the latter also being on only 6%. Regional concentration, or its absence, matters in FPTP.

Nonetheless, and for whatever it might be worth, the estimates for the Conservatives, NDP, and Liberals are well within the range of the EKOS seat projections. To be precise, the CPC and NDP numbers are near the upper end of the EKOS projections, and at least one of them will need to be nearer the lower end (130, 103, and 36, respectively, at EKOS) to make room for 10-20 BQ seats.

But, yes, a third straight Conservative plurality–possibly reduced from what it was in the dissolved parliament–and an NDP total around 100-125 really could happen. And if those were the top two parties’ seat totals, it would mean that Canada 2011, far from being any sort of anomalous FPTP election, would be in line with what the seat-vote equation says “should be” the outcome, given these expected votes.

* For more on the seat-vote equation, just click those words in the “Planted in” line above. I have been writing about the equation and various elections, especially Canadian federal and provincial elections, since 2006. The first entry in the series provides the most detail about the equation’s application. If you want the full explanation, please see:

Matthew S. Shugart, “Inherent and Contingent Factors in Reform Initiation in Plurality Systems,” in To Keep or Change First Past the Post, ed. By André Blais. Oxford: Oxford University Press, 2008.

Anomaly Watch: Trinidad and Tobago votes

Campaigning is in the final stages in advance of the Trinidad and Tobago general election of Monday, 24 May. The race is expected to be tight. This is a “snap” election called by PM Patrick Manning, leader of the Peoples National Party (PNM). Will he be sorry for having called it early?

In my work on “systemic failure” and reform in FPTP systems,* I concluded by drawing up a “watch list” of jurisdictions where recent results suggested the electoral system was inherently prone to producing anomalies, based on deviations of actual outcomes from what the Seat-Vote Equation would expect. T&T was on my Watch List. In the case of T&T, the inherent tendency towards unexpected outcomes derives from a frequent over-representation of the second-largest party, relative to expectations based on “normal” performance of FPTP systems.

For instance, in 1995 and 2001, the top two parties tied in seats due to the second party performing considerably better in seats that would be normally expected. In 1995 the PNM was the largest party but it won a lower percentage of seats (47.2%) than of votes (48.8%); in 2001 the United National Congress (UNC) was first in votes by a respectable margin (49.9% to 46.5%) yet each party won half the seats. Either of these elections could have resulted in a spurious majority (or “wrong winner”).

This will be the country’s fifth election since 2000. The 2001 election had been called very early: in 2000 the UNC had won a very narrow majority of both votes and seats (51.7% and 52.7%, respectively). It fell to 49.9% of votes and half the seats in 2001, and then another election was called in 2002. This one produced alternation to the PNM, with majorities of both seats and votes (55.6% 50.9%, respectively). The party was reelected in 2007, and despite a fall in its votes (to 45.9%) its seats increased (to 63.4%). A third party, the Congress of the People (COP), won over 22% of the vote but no seats.

The underlying problem in T&T stems from two common sources of poor FPTP performance: small assembly size and regionalism. The assembly size was stuck on 36 for many elections (at least as far back as 1966). That is very small for a country with now over 650,000 votes cast in the last two elections (and around a million eligible). By the Cube Root Rule, a country this size should have an assembly of 100-125 members. This problem was “addressed” in 2007 when the assembly was finally increased–all the way to 41.

The nature of regionalism can be seen by looking at the maps from recent elections at Psephos. As is common under FPTP, each party has strongholds and only a few seats change hands at any given election. The UNC dominates most of the center and southeast of Trinidad, whereas the PNM wins nearly every seat in Port of Spain and on Tobago. The partisan division mirrors the division between citizens of Indian or African descent, with the governing PNM relying on the latter group.

In this election, the UNC and COP have joined forces as the core components of a five-party pre-election coalition known as the People’s Partnership. It might seem that such a coalescence of the opposition would make a dramatic difference in the votes-seats conversion to the opposition’s advantage, but it may not. A quick and not-very-systematic perusal of the district-by-district results in 2007 shows only a few districts where the PNM won with less than 50% and where the combined UNC-COP vote would have meant PNM defeat. Most PNM districts were in fact won with majorities, whereas it was the UNC that often won with less than 50%. Still, if the race really is close, even a relative few seats could tip the result. A few seats could result in an over-representation of the Peoples Partnership even if it second in votes–and could even contribute to a spurious majority.

About the campaign, the Jamaica Observer (second link above) notes:

Music in the nation famed for calypso has played a key role in campaigning.

One PNM video shows red-clad crowds dancing at rallies in front of a smiling Manning, with slogans such as “free education” sliding across the screen to a catchy tune.

On the other side, a People’s Partnership campaign song contains the lyrics: “Allegations here, allegations there,” and shows pictures of flashy high-rise buildings alongside hospitals without beds.

“I can’t vote for that!” rings out the chorus.

Trinidad and Tobago would be better served by some form of proportional representation and has earned its place on the Watch List.

* “Inherent and Contingent Factors in Reform Initiation in Plurality Systems,” in the edited volume by Andre Blais, To Keep or Change First Past the Post.

How far off the seat-vote equation was the UK 2010 result?

How under-represented was the Conservative Party on 6 May? Oh sure, I know that the party was over-represented, relative to its vote share. But that’s what FPTP is supposed to do. In fact, it is supposed to do so sufficiently to give a “decisive” result. At least that’s what David Cameron said throughout the campaign in defense of the current electoral system. So, relative to the expectation of a substantial boost from FPTP, how under-represented was the largest party in the recent UK election?

By running the seat-vote equation on the actual voting result, we can get an idea of the answer to this question. The Conservatives won 307 seats, for 47.2%, on 36.1% of the vote. Labour came second with 258 seats, for 39.7%, on 29% of the vote. For these top two vote percentages, the seat-vote equation says the largest party “should have won” 51.4% of the seats and the second “should have won” 31.5%. (The Liberal Democrats presumably “should have” won the greater part of the remaining 17%, rather than the mere 9% that they have to show for their 23% of the vote.*)

For the largest party, obviously the deviation between an expectation of 51.4% and an actual result of 47.2% is minor, aside from the rather important detail of these percentages straddling the magic 50% (plus 1) marker.

The outcome of the election continues in a striking way the over-representation of Labour. Note that their 29% of the vote could have been expected to result in just over 30% of the seats, but instead they are close to 40%. The bias of the system in favor of Labour, whereby that party wins more seats than the Tories for any given vote share, is well known. It is likely not, however, a product solely of the current district boundaries, as Cameron and other Conservatives are fond of saying. Districting plans come and go, but this bias has been in place for some time.

We can see the differential treatment of the parties by looking at the advantage ratios (%seats/%votes). In this election, Labour had A=1.37, which is the best result for a second-place party in the UK in my data-set (which goes back to 1959). For the Conservatives, A=1.31. While this is a relatively low A for the largest party, in the UK context it is not low–for a first party branded as “Conservative.” Even when the Conservatives were winning substantial seat majorities from 1979 through 1992, their A surpassed 1.25 only in 1983 (1.44) and 1987 (1.37), while in the “Thatcher landslide” of 1979 it was only 1.22. (In 1992 it was 1.23.) Labour, on the other hand, enjoyed advantage ratios of 1.47 or greater in each of the three recent elections when it was the largest party.

These figures suggest that the Conservatives might have a hard time finding a FPTP districting plan** that would really work for them, unless they can again be confident of surpassing 38% or so of the vote. Meanwhile, Labour is benefiting rather handsomely from FPTP, though the 2010 outcome in particular suggests that the bulk of that advantage is coming at the expense of Britain’s rather large third party instead of the Conservatives.

* Various fourth and lower-ranked parties won around 4% of the seats, owing to concentration of the relative few votes won by any one of them, despite combining for 11.9% of the vote. We can discount them for present purposes and just call them “others.” (Which is not to say that some of them might not prove relevant in the coming parliament, of course.)

** That is, without major gerrymandering on a scale not practiced in the UK, unlike the USA.