The evidence for confirmation bias stretches back to the experiments of Albert Hastorf and Hadley Cantril. The psychologists based their findings on an American football game between stark rivals: Princeton and Dartmouth University.In the psychologists’ turn of phrase, the game was full of “rough play”.

Quite an understatement: the Princeton quarterback broke his nose and the Dartmouth quarterback broke a leg.

In the bitter aftermath the academics showed footage of the game to 324 Princeton and Dartmouth students and asked them to count the number of fouls committed by each side.

The results were conclusive. Students were twice as likely to see the opposition commit a foul as they were to spot a foul by their own team. The match was seen through a prism of loyalty.

The experiment is not an historical anomaly. In 2015, during the UK general election, Jenny Riddell and I surveyed 1,004 nationally representative voters about their views on raising VAT by a penny to fund 10,000 extra nurses. The results were then split by political affiliation. The twist was that half the respondents were told it was a Conservative policy and half were told it was an initiative by Labour.

When Labour supporters thought the policy came from their party there was strong support: 14% completely agreed. However, support plummeted to 3%, less than a quarter of the original level, when it was described as a Conservative proposal.

Similarly, among Tories the policy was four times more popular when it was seen to come from their party. The scale of the effect means that policy is far less influential than existing party affiliation.

The experiments prove that it’s hard to overturn negative opinions. Rejecters of your brand are difficult to convince because they interpret your message through a lens of negativity.

As the legendary stock market investor, Charlie Munger, said: “The human mind is a lot like the human egg, in that the human egg has a shut-off device. One sperm gets in, and it shuts down so that the next one can’t get in. The human mind has a big tendency of the same sort.”

Identify who it’s best to target

On a finite budget, focus your money where it makes the biggest impact. That means avoiding rejecters. I term this process “marketing triage”, after the medical procedure devised by Dominique Jean Larrey, chief surgeon in Napoleon’s army. He ordered his surgeons to split incoming patients into three categories (hence the word triage, from the French, ‘to sort out’):

1. Those likely to live regardless of the care they receive.
2. Those unlikely to live regardless of the care they receive.
3. And finally, those for whom immediate care might make a difference.

In a similar vein, marketers should apply a threefold categorisation of their own:

1. Those likely to buy regardless of communications.
2. Those unlikely to buy regardless of communications.
3. And those for whom communications might make a difference.

Marketing efforts should ruthlessly focus on the final category. This sounds blindingly obvious, but from my own work with brands I’ve rarely seen it applied.

The reason to avoid rejecters is sound though – confirmation bias means it requires inordinate efforts to convince them.

Similarly, ignore heavy buyers. Their purchase frequency means they are routinely exposed to the brand’s packaging, website and store. These touchpoints therefore become disproportionately important in shaping perceptions.

Additionally, Byron Sharp’s work shows that heavy buyers offer limited headroom. If you’re already buying a can of Coca-Cola a day, how much more can you polish off?

Finally, these customers will be far more interested in your ads than average, so they’ll overhear campaigns aimed at other people anyway. By focusing on the lukewarm, spend is targeted to where it works best. This means that more brands will be able to afford a constant presence.

Identify when to talk to rejecters

Of course, there are rare occasions when you can’t avoid rejecters. Perhaps they’re such a large or vocal group that ignoring them jeopardises your future. In this scenario, the solution is to target them when they’re distracted.

Counter-intuitive? Maybe, but the supporting evidence originates from one of the 20th century’s greatest psychologists: Leon Festinger.

In 1964, Festinger and Nathan Maccoby, academics at Stanford University, recruited members of college fraternities. They played those students an audio argument about why fraternities were morally wrong. The recording was played in two different scenarios: students either heard it on its own or they watched a silent film at the same time.

After the students had heard the recording, the Stanford psychologists questioned them as to how far their views had shifted. Those who had heard the argument at the same time as the silent film were more likely to have changed their opinion.

The psychologists’ hypothesis was that the brain is adept at generating counter-arguments that maintain its existing opinions, but when the brain is distracted that ability is hampered. We’re more easily persuaded when focusing on more than one thing at a time.

The lesson is clear: target rejecters when they’re partially distracted. Luckily, these moments are becoming more common. As Herbert Simon, who won the Nobel Prize for Economics in 1978, pointed out, we live in an information rich age and:

We’re more easily persuaded when focusing on more than one thing at a time…

What information consumes is rather obvious: it consumes the attention of its recipients. Hence a wealth of information creates a poverty of attention.

It’s your task to pinpoint these moments of inattention. When targeting rejecters, brands should prioritise media such as radio, which tend to be consumed while people are doing something else.

Even with a medium like TV, which is often the sole focus of attention, media planners can identify the programmes or times when the audience is likely to be second screening.

According to Nielsen’s global AdReaction study, among people with access to multiple devices, an average of 35% of TV viewing time was spent simultaneously watching TV and using another device. The best programmes to reach second screeners are either those with a social nature or low involvement, such as daytime TV.

One of the most cherished beliefs of media planners, that attention is crucial, may not be right in all circumstances. This is particularly interesting when you bear in mind the hefty premium for attentive moments.

For example, cinema ads, perhaps the medium that gets the highest attention, trades at five times the cost of TV. By targeting distraction you benefit twice. First, you’re more likely to overturn misconceptions and second you pay less for the pleasure.

How to win over rejecters

As the brain can generate counter-arguments prolifically when directly challenged, it’s best to avoid this. Instead use subtler cues. That way the conscious mind doesn’t realise it’s being persuaded and confirmation bias isn’t activated.

The British Airways ads illustrate the pervasive power of subtle cues – ever since 1989 they have featured the Flower Duet from Delibes’ opera Lakmé as the soundtrack. If British Airways had directly claimed they were luxurious, Festinger’s theory suggests consumers would scour their memories for counter-arguments.

However, since this isn’t explicitly stated, the brain’s capability isn’t activated. Instead, the beautiful strains of the Flower Duet insinuate luxury by association.

Advertisers, therefore, have two options. The recommended route is to apply the process of marketing triage and avoid targeting rejecters and heavy buyers.

A riskier alternative is to convert this resistant audience by reaching them at moments of distraction or with oblique and incidental detail. Whether that turns out to be brave or foolhardy depends on the execution.

Another reason why entrenched opinions are so hard to shift is that most people are overly confident in their abilities.

Since people tend to think they’re cleverer than average there’s a tendency to ignore contrary opinions from other people. The bias of overconfidence is the subject of another chapter.