I have been debating the question of differential turnout in Northern Ireland by email with a correspondent, and this has prompted me to actually do some more analysis, cross-referencing the election results with census data.

The results are rather interesting. The 2001 census found 43.8% of the population to have a Catholic background, and 53.1% to have a Protestant or other Christian background. The votes cast in 2010 were 42.0% for Nationalist candidates and 50.5% for Unionist candidates. Strikingly, the Nationalist vote in 2010 was 38.4% of the 2001 Catholic population, and the Unionist vote in 2010 was 38.0% of the 2001 Protestant population, within a whisker of each other. This I take as support for my basic thesis that the variation in turnout across Northern Ireland is more of a geographical effcet than a sectarian one; Catholics and Protestants, and indeed others, are all voting at about the same rate these days.

Drilling down a bit into constituencies, the pattern is very much more complex (table below ranked by the difference between the two last columns):

	Prot ’01	Cath ’01	Oth ’01	U vote ’10	N vote ’10	Oth ’10	U ’10-Pr ’01	N ’10-Ca ’01
EAST BELFAST	87.0%	7.5%	5.4%	59.3%	3.4%	37.2%	-27.7%	-4.1%
SOUTH DOWN	28.7%	69.6%	1.7%	19.4%	77.2%	3.4%	-9.3%	+7.6%
SOUTH BELFAST	52.6%	41.0%	6.4%	41.0%	41.0%	18.0%	-11.7%	0.0%
WEST BELFAST	16.1%	82.8%	1.1%	10.7%	87.5%	1.9%	-5.4%	+4.6%
FOYLE	22.9%	75.7%	1.5%	15.1%	76.6%	8.3%	-7.8%	+0.9%
NORTH BELFAST	51.6%	45.1%	3.3%	47.7%	46.3%	6.0%	-3.9%	+1.2%
MID ULSTER	33.7%	65.3%	1.0%	32.8%	66.3%	1.0%	-1.0%	+1.0%
NEWRY AND ARMAGH	31.8%	67.2%	1.0%	33.4%	65.4%	1.2%	+1.5%	-1.7%
LAGAN VALLEY	81.1%	14.2%	4.7%	79.5%	9.0%	11.4%	-1.5%	-5.2%
FERMANAGH S TYRONE	43.0%	55.6%	1.4%	45.5%	53.2%	1.3%	+2.5%	-2.4%
SOUTH ANTRIM	68.3%	27.4%	4.4%	69.7%	22.6%	7.7%	+1.5%	-4.8%
EAST ANTRIM	75.3%	19.8%	4.9%	75.5%	13.4%	11.1%	+0.2%	-6.5%
STRANGFORD	78.8%	16.7%	4.6%	79.3%	10.2%	10.4%	+0.6%	-6.4%
EAST LONDONDERRY	57.2%	40.1%	2.7%	59.8%	34.7%	5.5%	+2.5%	-5.3%
WEST TYRONE	31.3%	67.8%	1.0%	33.9%	62.4%	3.7%	+2.7%	-5.4%
UPPER BANN	54.7%	42.9%	2.5%	59.5%	37.5%	3.0%	+4.9%	-5.4%
NORTH ANTRIM	70.3%	27.4%	2.4%	75.5%	21.2%	3.2%	+5.3%	-6.1%
NORTH DOWN	81.6%	11.7%	6.7%	88.5%	2.8%	8.7%	+6.9%	-8.9%
NI total	53.1%	43.8%	3.1%	50.5%	42.0%	7.5%	-2.6%	-1.8%

The biggest and least recoverable part of the variation will be the drift in population between 2001 and 2010. The 2001 figures include those aged under 9 who were not able to vote this year, those who have died since 2001 and those who have moved away since 2001, not to mention those who lived there throughout but did not vote in 2010; they do not include those voters who have moved in since 2001. Just from the nature of modern life one can speculate that this may have had the biggest effect in South Belfast, and in other more urban and suburban areas; but we have no way of quantifying it.

However we can be pretty clear about the two seats where the difference is most striking. In East Belfast, assuming (dangerously) that turnout was more or less the same across the communities, over a third of voting Protestants and over half of voting Catholics supported Alliance, resulting in the biggest difference between the Protestant population share and the Unionist vote. And in North Down, again assuming equal turnout, three-quarters of voting Catholics may have supported candidates other than the SDLP and Sinn Fein.

There is some suggestion from the figures of ‘tactical voting’ by Protestants for Nationalist candidates (presumably the SDLP) in South Down, West Belfast, and Foyle (there is also something interesting going on in South Belfast, but due to the mobility of the population it is difficult to be sure of what exactly that is); there is also some suggestion of ‘tactical voting’ by Catholics for Unionist candidates, presumably to prevent a more hard-line candidate getting in, in North Antrim (the likely beneficiary being, of all people, Ian Paisley Jr) and Upper Bann (possibly support for the UUP’s Freddy Mercury lookalike candidate). It’s fairly clear also that Alliance draws a lot of support from Catholics in East Antrim and Strangford.

I don’t think we can really take it much further than that, though of course that will not stop people trying.