In the 1992 general election, the opinion polls gave John Major’s Conservatives between 38% and 39% of the vote, about 1% behind the Labour Party. In the final results, the Conservatives had a lead of 7.6% over Labour and won their fourth successive general election. Psephologists tried to explain this with what they call “shy Tories”. People who are too ashamed to tell the pollsters that they intend to vote Conservative. This is obviously ridiculous. Yet after the recent election The Guardian newspaper once again trotted out this silly concept. The discrepancy between the polls and the results has nothing to do with shyness.

Human beings are social animals, we succeeded to become the most successful life form on earth because we are tribal. This is not something that we each learn in our youth, it is something that is programmed into us by evolution. We can’t help the way that we behave because it is fundamental to being a human being. One of the behavioural traits that makes us social is loyalty. Loyalty to the leader of our tribe, loyalty to other members of our tribe. And when it comes to elections this loyalty manifests itself as voting to favour the incumbent.

In a British general election loyalty to the incumbent has two forms. Firstly to the sitting MP, secondly to the Prime Minister in office. He is our leader and we are programmed to be loyal to him.

You can see the power of incumbency in many different elections. John Major’s 1992 success was just one of many instances. That Gordon Brown’s Labour party didn’t receive the trashing they deserved in 2010 was another example. And the fact that American presidents, no matter how bad, nearly always get a second term is yet another example.

So how does incumbency make the polls so wrong? There are two mechanisms. The first is the floating, or undecided, voter. When confronted with the ballot paper their tribal instincts kick in and they vote for their existing leader. There were a disproportionately high number of floating voters this time, so the polls were more inaccurate than usual. The second is the propensity to actually make the effort to vote. If the leader is of your political persuasion you are far more likely to go and do it because of deep seated tribal loyalty. If the leader is not of your political persuasion you are more likely to stay in and watch TV. The turnout at this last election was just 66.1%. So a third of people just didn’t bother.

One final example of the power of incumbency is the Scottish referendum. The polls were showing a yes victory, yet the yes campaign lost because people, when confronted with a ballot paper, voted for the status quo.