Can you find a dozen couples in civil partnerships who would benefit from your marriage tax break?
Perhaps a journalist at Mr Cameron's manifesto launch should ask him whether he himself knows anybody in a civil partnership who would be included.
The Conservatives have made a lot of the inclusion of civil partnerships alongside straight marriages. Their exclusion from the tax change would have been illegal but, still, the symbolism is good. David Cameron made a good, positive statement about gay equality at the weekend.
But telling gay couples their marriages are not excluded does not mean they will get the tax break. How far is the inclusion of civil partners theoretical or real?
It is clear that most civil partnered couples will miss out.
What we don't yet know is how much less likely those in gay marriages are to qualify for the tax break than straight married couples, one in three of whom will benefit. But it ought to be possible for the media or campaigners to cast some further light on this by the end of the campaign.
The focus on the relatively small amount of money - £150 per year, or £2.88 per week - has meant fewer people have noticed that the Conservative policy is flying under a false flag.
The first reason that most civil partnered couples will miss out is because most married couples will miss out. The Institute of Fiscal Studies calculate that the Tory policy will recognise one in three marriages: around 4 million of Britain's 12.3 million married couples will receive the tax break, while 8.3 million won't.
The policy was presented as sending a signal that "marriage matters". Instead, it determines that some marriages merit "recognition" more than others.
The second reason is that gay married couples are probably less likely to fit the TradCon model of the married household of a Conservative policy designed to recognise some marriages and not others.
The as yet unspoken thought behind the policy design would appear to be:
We will recognise in the tax system those married households which operate on the 1950s sole breadwinner model.
(Some married couples who fit this TradCon model do then miss out for the ProgCon reason of excluding higher rate taxpayers: the IFS have yet to calculate how many are excluded for these different reasons, but told Next Left that they will try to include the breakdown in their Tory manifesto analysis).
What do we know about civil partnered couples?
There are now somewhere around 40,000 civil partnered couples in the UK. The last set of official figures, to the end of 2008, showed that 33,956 civil partnerships had been contracted, with 7169 in 2008 and 8728 in 2007 after many long-established couples took an early opportunity to take-up their new rights in the first 13 months from December 2005.
If gay married couples were just as likely to benefit from the marriage tax break, around 13,000 civil partnered couples who got the tax break. The real number will be lower than this, though how much lower would be difficult to estimate.
That is because there is limited statistical information on the socio-economic status of those in civil partnerships. What is known is summarised in the National Equality Panel report (pages 240-243) and a research paper for it.
The sample in the labour force survey is small, and contains both cohabitees as well as those in civil partnerships, it finds that there do not seem to be significant differences in pay for gay men, or for women.
Men in same sex couples have similar employment rates to other men in couples: that average covers both partners in male same sex couples, of course, compared to one partner in straight couples.
The Equality Panel reports that "the full-time employment rate for women reporting that they lived in a same sex couple is 67 per cent, compared to 39 per cent for other women". However, controlling for other characteristics (such as qualifications) reduces the difference to women in same sex couples being about 8 per cent more likely to be in work, and being about 13 per cent to be in paid employment more likely than women in same sex couples without children, compared to women with similar social profiles. (However, we can't assume that those in civil partnerships do have similar profiles to other married women: they could, for example, have a different educational or class profile).
We also know that the average of those entering a civil partnership - 42 for men and 41 for women in 2008 - is higher than that for straight married couples, though that has also been rising, to 36 for men and 33.7 for women in 2005.
One argument might be that civil partners would naturally miss out on a marriage tax break with this design to the extent that they are less likely to have children. However, that does not make much sense as a defence of the Conservative policy, since the IFS analysis shows it does not especially target married couples with children: two-thirds of the couples who benefit from it do not have dependent children, while only one in five of all families with children benefit. (One-third of the beneficiaries are pensioner couples, and it may well be the case that gay pensioner couples may be more likely to benefit than other gay couples).
The implications of the tax break policy
Most civil partnered couples won't get the marriage tax break, because it applies to a minority of married couples and is then somewhat less likely to apply to them.
It would have been possible to design a marriage tax rebate or credit without these features of discriminating between different types of married household, at which point the main debate would once again be the principle of whether or not to recognise or discriminate in favour of marriage itself.
How far short of one in three gay couples the policy falls (if it was as likely to cover gay marriages as straight marriages, while excluding most of both) remains to be seen.
I doubt that this could be statistically proven in the next three weeks, but further light could certainly be cast on this by both the media or campaigning groups.
A useful anecdotal sense could be gained by, for example, the gay press reporting on or seeking to hold surveys of their readers in civil partnerships, or campaigning groups like Stonewall gauging what proportion of their civil partnered supporters are included or excluded.
Local and national papers could get a snapshot of this simply by attending registry offices or contacting those about to get hitched, to see whether there was anything like a one-in-three rate, or whether the vast majority of civil partnered couples say they would be excluded. Certainly, any local candidate noting the inclusion of those in civil partnerships could legitimately be asked to provide examples from their own constituency.
So can the Conservatives find the civil partnered couples whose marriages they would recognise?
Meanwhile, if you are in a civil partnership, with one partner paying the basic rate while the other with some tax allowance to spare (and, which could be a further stretch, if you support the Tory party and their marriage recognition policy too) you would do Dave a great favour by calling Conservative Central Office to arrange the photocall.