Correcting and decrying the perceived wrongness of others is the chief political preoccupation. David dismisses Ed, Andy elbows Harriet, Jeremy and Nigel condemn everyone else and on it goes. People who say they dislike this denounce others as pursuing yah boo, negative politics and these people are, in turn, are impeached for failing to provide proper opposition.

If the goal is to persuade people to change their minds, then attack and rebuttal has a mixed record. On one extreme, SNP attacks on Scottish Labour clearly moved voters dramatically at the last election. But on the other hand, think of all the criticism that a George Osborne or a Harriet Harman has faced in their careers: it hasn’t seemed to have really held them back from their political projects.

One explanation may be found in the idea of “backfire” or “boomerang” effects that students of persuasion often talk about. Being told that Protestants are conservative can make Catholics more liberal. Saying that vaccines have no side effects appear, at least sometimes, to reduce the chances of someone getting a vaccination. No smoking signs can increase the desire to smoke. Informing American conservatives about the dire consequences of climate change on France has been found to reduce support for addressing carbon emissions.

Once we hold an opinion, it seems that we will try and defend it. Suppose you tell me all the reasons why you hate the West Wing. I’ll take the time to mentally dismiss each one of your criticisms: why shouldn’t every major political issue be resolved with a 90 second speech and some rousing music? I’ll begin to delve into my memory for my favourite episodes. By the end of the conversation, I’ll have applied myself to building the case that the West Wing is brilliant and my own counter-arguments may stick with me for much longer than your criticism.

So there is a heroic psychological assumption behind the commonplace political idea that when one politician angrily denounces another, this will help win over those voters who are watching. According to at least one major review of the evidence, there are a similar number of examples of negative campaigning backfiring as there are for it working. Whatever the equivalent statistics would be for the UK, it shows how developing a criticism of your opponents that actually wins you votes is an immensely complex and subtle a task.

However, much of politics has nothing to do with winning elections. People singing songs from the football terraces aren’t expecting the other team’s supporters to suddenly change their minds. When we think we are punishing rule-breakers, the reward centres of our brain are activated. Loudly insulting the other side is the easiest way for mediocre politicians to get covered in their more supportive media. It is a way of overlooking internal disagreements and demonstrating passion and it avoids the risky and time consuming process of coming up with a detailed policy proposal.

Why is this tendency towards ineffectively denouncing the other side so dangerous? Because the backfire effect from one person’s shouting can hurt a much wider cause. When things are complicated and life is busy, it’s often a time saving rule of thumb to know who you disagree with. In a cynical age, it seems a lot easier to use the compass-point that your worst opponent is always wrong, than it is to trust that your favourite hero is always right.

I’m more pro-European for every Nigel Farage interview I’ve watched and more pro-immigration for every Daily Mail article I’ve read. But we have to have the imagination to think that this must work the other way too. If the left and centre left has done one thing over the last five years, it has been to loudly denounce the Tories. To put it mildly, the net effect – votes gained minus votes lost - does not appear to have been in our favour.