Thursday, February 25, 2016

Primary Game Theory

[Nominations open for the SIGACT Distinguished Service Prize. Deadline: April 1]

The US presidential primaries have not gone as expected as you can see from the crazy shifts in the prediction markets. This year besides the usual democratic/republican split, we have an establishment/non-establishment split in both parties. Back in my day outside candidates like Trump, Cruz and Sanders would have run as independents like Ross Perot and John Anderson.

Despite the split, the establishment candidates focus more on themselves than the establishment. Christie's attack on Rubio in New Hampshire may have handed Trump the election and it certainly didn't save Christie's campaign. Kasich should just drop out now if he cares about keeping the nomination for an establishment candidate--it's just not his year, though maybe he's playing some game theory of his own.

The democratic side does not offer such interesting game theory, since we have a two horse race. Mostly a one horse race because the delegate math doesn't work well for Sanders.

Let's look at the election from the point of view of a hypothetical Georgia voter voting on Super Tuesday next week. Such a voter can choose which primary to vote on in election day.

Clinton will easily win Georgia but as long as Bernie gets at least 15% of the vote (likely), delegates will be allocated proportionally. So a vote in the democratic primary could affect a delegate but less likely to to affect who will be the nominee than on the Republican side. Unless Bernie surprises in South Carolina, the hypothetical voter may opt to vote in the Republican primary instead.

The republican delegate allocation rules most likely mean that the candidates receiving at least 20% of the votes will get a proportional allocation of 31 delegates and the winner in each of the 14 congressional districts gets two delegates while the runner up gets one. Looking at the polls, Trump will easily win the election with Cruz and Rubio hovering about 20%. A single vote could affect 6 delegates (20% of Georgia's at large 31 delegates). A vote for Kasich or Carson would not net Kasich or Carson any delegates but could bolster Trump by pushing Rubio's vote percentage down towards that 20% mark.

This scenario plays out across the Super Tuesday primaries. Trump is favored to win in every state voting that day except Cruz's Texas. If Rubio can get at least 20% of the vote in those states he keeps the race alive and could make up ground in winner-take-all states coming up later. Kasich doesn't draw much voters but enough that by not dropping out he may help close out this election on Tuesday. Game theory indeed.

In an early primary season already full of surprises we may see many more. It would be a lot more fun to watch if the fate of the US and the entire world didn't depend on the outcome.


  1. My democrat friends have the following choices for the primaries:

    Vote TRUMP since HILLARY can beat him.
    Vote TRUMP since he's a closet moderate.
    DO NOT vote TRUMP since he might beat HILLARY.
    DO NOT vote TRUMP since if he loses and goes third party then HILLARY wins.

    Vote CRUZ since Hillary can beat him.
    DO NOT vote CRUZ since he's worse than TRUMP.

    Vote RUBIO since he's the best of a bad lot.
    DO NOT vote RUBIO since he might beat HILLARY

    Vote HILLARY since you actually prefer her to BERNIE.
    Vote BERNIE since you FEEL THE BERN.
    Vote BERNIE since if its TRUMP vs BERNIE then BLOOMBERG runs as third party.

    Why do the Republicans go by their lsat name and the Democrats go by their
    first name?

    The best commentary on the race:

    1. How regrettable it is, that Computational Complexity has chosen to promulgate the callow political weltanschauung of Saturday Night Live, as contrasted with (for example) the mature political rationality of the Brookings Institute.

  2. CORRECTION- The fourth line should be
    VOTE TRUMP since if he loses and goes third party then HILLARY wins.

    Also- while some say that Rubio should drop out and be Trumps VP:

  3. Kasich has highest chance of becoming VP according to prediction markets. He is not running to become Trump's VP.