Boris Island: Shag, Marry, Avoid?

The Airports Commission is eventually going to opt for Gatwick. Prediction. How did I come to this conclusion? And how representative is the commission as an institution?

Well, we can understand the decision-making process here with a simple model. There are various actors involved, who have preferences. To illuminate this, let’s play a game of Shag, Marry, Avoid. We could call these “Tolerate, Demand, Veto” instead, but I think it’s much more fun this way. Basically, any actor has a choice they would prefer, one they would accept failing that, and one that they would reject entirely.

Once we’ve specified the actors, and coded them as Shag/Marry/Avoid for each airport option, we can then look at a rule to model the decision process. This is the table I came up with.

Under my scoring, first-past-the-post would return the Rt. Hon. Heathrow Airport MP on the strength of its 8 marriage proposals. However, this isn’t a parliamentary election, and second choices and veto power are both important. Under an alternative vote-like system, I get 8 votes to marry Heathrow and 8 for all other options. Typically, AV requires 50%+1 for a first round win, so we reallocate the shags – i.e. the second preferences. This gives us Heathrow 11, Gatwick 13, and (Boris Island 3, Stansted 3, Somewhere Else 2) drop out. If we then subtract the avoids, i.e vetoes, we get Heathrow 7, Gatwick 12 and (Stansted 3, SE 2, Boris Island -8).

Another way of doing this would be something like approval voting, where voters cross off all the candidates they reject, and the least rejected candidates win. This gets at the minimum consensus aspect of the whole thing. SE and Stansted both have 0 vetoes, Gatwick 1, Heathrow 4, and Boris Island 11.

This of course raises the problem with approval voting – unless you do something else, it’s possible to have a winner nobody positively wants. In real institutions that use it, this is often dealt with by requiring a given number of electors to second you before you get to take part.

If we require more than one “marry”, Stansted and Boris Island are both eliminated, leaving Somewhere Else, Heathrow, and Gatwick. SE has no vetoes, Gatwick 1, and Heathrow 4. SE wins, but if there has to be a winner, then it’s Gatwick. Alternatively, as I’ve included lobbies for the North and Midlands in the scoring, we might read that as being “Gatwick, plus expansion outside the South-East”.

Yet a third option would be a balance of opinion. We might count a Marry as one point, a Shag as 0.5 points, and an Avoid as -1, implicitly coding a Shrug as 0. In that case you get net scores of Heathrow 5.5, Gatwick 8, Boris Island -9, Somewhere Else 2, Stansted 1.5.

The upshot, then. All three rules tried converge on the same result. Hardly anyone actually wants Boris Island and a lot of people hate it. Quite a lot of people support Heathrow, but a significant group of actors also hate it. Gatwick is everyone’s second choice – quite an advertising slogan, no? – and only one group wants to veto it.

And further, the airports commission does seem to be surprisingly democratic, at least in that you can simulate it as a plebiscitary process with the same inputs and it gives the same outputs.

2 Comments on "Boris Island: Shag, Marry, Avoid?"

  1. > This of course raises the problem with approval voting – unless you do something else, it’s possible to have a winner nobody positively wants.

    This is possible with *any* voting system. Say your options are Bad and Terrible. However, extensive computer simulations by Princeton math PhD Warren Smith, showed that Approval Voting performed remarkably well, as measured by average voter satisfaction averaged over hundreds of thousands of random elections.

    Regarding Instant Runoff Voting (which is unfortunately called “Alternative Vote” in the UK), it came in LAST PLACE of the five commonly proposed alternative voting systems Smith tested.

    Approval Voting is a far simpler and more democratic voting system.

    Clay Shentrup
    Co-founder, The Center for Election Science


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