Remember back to before NZ’s November General Election and the scrutiny of National’s leader and future Prime Minister John Key? He was labeled as ‘Labour-lite’ and centrist; his party was careful not to wage war on some of Helen Clark’s more popular policies; the differences between the two main parties were small, negligible even.

And for those who follow US politics – you must have been struck by the partisan divide in both society and politics between the Democrats and Republicans. It is, in the words of a columnist, ‘The 50/50 Nation‘. Heck, the two camps even have their own separate news channels!

Over in Germany, people ask themselves: What is the SPD doing in opposition? (oops, sorry, the election isn’t for 2 weeks. I meant, Grand Coalition.) They could form a government with the Greens and the Left Party tomorrow!

Discussion of all these issues, and some others which I have speculated upon here previously, can be informed by a powerful, but simple, political science model (originating in economics) called the Median Voter Theorem.

Briefly, the theorem is a model of majority decision making which states that the median voter’s preferred policy will beat any alternative in a vote; in a stronger form it also says that the median voter will almost always get his/her most preferred policy. The rationale behind it is too much for this blog and is explained briefly and clearly here, but the consequences are far-reaching.

Median_voter_modelIf we apply the model to the Left-Right spectrum above, we can come to the following conclusions:

1) if voters cast their vote for the candidate closest to their ideal policy, then the candidate who stands nearest to the median voter will win. In this case, this would be candidate A, who is closest to point M.

2) if the candidates are free to choose their policies, they will choose positions which will place them closest to the median voter’s at M; so candidates A and B will move in the directions of the arrows towards the middle of the graph and the median voter’s preferences will be fulfilled.

The theory puts the median voter in the lucky position of always getting what they want, but it also has other consequences – many more than will be expanded on here, but to go back to the examples above:

Key vs. Clark in NZ: parties will choose policies which are moderate and middle of the road; no wonder that they look so similar.

Democrats vs. Republicans in the US: if parties continue to act as above for many years they could create blocs, each accounting for approximately half of the population (see the column from Mickey Kraus).

The SPD and the Left Party in Germany: parties will be very careful not to take extreme positions which could alienate voters closer to the median.

So there you have it. A brief but hopefully not-too-incomplete rundown of the Median Voter Theorem in five minutes. A simple, powerful analytical tool for analyzing party politics.