Why won't people face the evidence on legalising cannabis?

A new study by the Treasury has revealed that legalising cannabis would have huge economic benefits for the UK. The move would raise hundreds of millions of pounds in taxes and ease the burden on the criminal justice system.

This comes on the back of growing evidence that cannabis – like many recreational drugs – is often no more dangerous than alcohol or smoking cigarettes. Even where studies identify links between cannabis use at a young age and mental illness, such dangers would be better addressed in a regulated market with proper health warnings and checks on the age of buyers.

But at the level of government, reasonable debate often doesn’t seem to come into it. In 2009 Professor David Nutt was sacked as a government adviser for comments he had made in an academic journal about the dangers of ecstasy. Nutt wrote that taking the drug is no more dangerous than horse-riding. Jacqui Smith, Home Secretary at the time, ordered Nutt to apologise.

The problem wasn’t that Nutt wasn’t telling the truth – he was, statistically speaking – but that his remarks were out of step with government policy.

But this should come as no surprise. In recent years a raft of studies has shown how much we overestimate the rational mind and our ability to analyse information objectively. In his book ‘Thinking, Fast and Slow’, Daniel Kahneman explains how the brain manages the vast amount of information it is forced to process by adapting it to preconceived ideas of how the world works. Information that produces "cognitive dissonance" – stuff that doesn’t make instinctive sense – is filtered out and rejected. Information that produces "cognitive ease" – stuff that confirms your preconceptions – is accepted and applied. Kahneman even observed that cognitive ease made the subjects’ pupils dilate. Proof that I’m not the only one who loves being right.

Even if people are asked to reflect on a piece of information, they still go out of their way to achieve that blissful cognitive ease. In a 2013 study, Yale Law professor Dan Kahan tried to understand why people reach false conclusions. Kahan provided test subjects with fictional statistics on the impact of gun control on crime rates. Some of the data suggested that gun control reduced crime rates; other data showed it had no positive impact.

But because Kahan presented the numbers in such a way that it wasn’t immediately obvious what the data really indicated, subjects – even highly educated ones – failed to reach the correct conclusions based on the evidence in front of them. Instead of taking the time to do some basic sums, many worked entirely on their own prejudices. Regardless of which way the evidence pointed, Liberals would conclude that gun control works; Conservatives would conclude that it didn’t.

But we are also as credulous as we are stubborn. Writing in the New York Times, Noreena Hertz, a professor of economics at UCL, describes an Emory University experiment from 2009:

“A group of adults was asked to make a decision while contemplating an expert’s claims, in this case, a financial expert. A functional M.R.I. scanner gauged their brain activity as they did so. The results were extraordinary: when confronted with the expert, it was as if the independent decision-making parts of many subjects’ brains pretty much switched off. They simply ceded their power to decide to the expert.”

For the financial expert, substitute our politicians. If you’re a hard-line euro-sceptic, Nigel Farage is your expert. If you’re in favour of legalising certain drugs, it’s probably Nick Clegg. In other words, we can exchange facts and figures all we want. But don’t expect people to actually listen: we’ve already made up our mind.