This article is a preview from the Spring 2016 edition of New Humanist. You can find out more and subscribe here.

It’s safe to say that drug researchers really, really don’t like the UK’s new Psychoactive Substances Act. Commonly referred to as the “legal highs” ban, it prohibits “new psychoactive substances”, products chemically designed to mimic banned drugs. The Act has provoked months of heated debate about which evidence the government did, and did not, take notice of. Why did it not consult the Advisory Council on the Misuse of Drugs (ACMD) until after the bill was written? Did ministers listen to the ACMD’s long list of misgivings?

It is rare to see so many researchers charging unprompted into the heat of public policy debate. A clinical psychologist told Guardian readers the Act was “a work of monumental ignorance that has taken drug legislation beyond the point of farce into the realm of surreal fantasy”. The New Scientist headline put it more simply: “You’re not hallucinating, MPs really did pass crazy bad drug law”.

Beyond the headlines, there’s a good deal of substance – of the intellectual kind – in this public row. Drugs experts have pointed to the uselessness of, even harm caused by, the vague definition of “psychoactive”. The ACMD’s list of concerns flagged up the fact that “psychoactivity cannot be definitively proven” and that broad definitions left “the potential for unintended consequences”. Experts noted that compared to, say, alcohol, there isn’t much evidence of harm caused by these substances. Independent researchers have looked at the advice given by the ACMD and brought in other research findings to question or add to their conclusions. Government has argued back: in order to prevent harm, it has to make a law to prevent high-street shops and websites openly selling compounds designed to produce similar effects to traditional illicit drugs. A number of deaths have been linked to their use. The government can’t wait for the evidence and anyway drugs are bad, full stop. And therefore we have a wider debate going on too, about facts versus values.

I’m going to say it. I think the Psychoactive Substances Act is actually an excellent example of evidence in policy. We can get to grips with the arguments and make our minds up precisely because the government and researchers are having it out in the open. I’m not defending the way the ACMD’s advice was handled, but the government’s motivation was clear, as was the evidence available to them.

The ideal, as far as I’m concerned, is that government defaults to evidence-led decisions – they should always be looking to evaluate whether a policy is likely to work or not. But of course there is a gulf between how researchers and policymakers look at an issue. That’s as it should be.

Take the Married Couple’s Allowance – the tax break for married couples introduced in 2013. If the government launches something like that, essentially saying, “We like married people, marriage is good,” then as a voter the message is pretty simple – you either agree or not. You don’t need evidence to enter the discussion. But if they say, “To combat the problem of social breakdown, we’re going to introduce this, because it will increase marriage, which prevents social breakdown”, that’s a whole different ballgame. You’d have to ask: what’s the government looked at to come to those conclusions? How have they evaluated the evidence? How are they going to evaluate the policy, to see if it works?

In other words, we have to look at the government’s chain of reasoning. On the “legal highs” ban, the government is saying we have to make law to close what is essentially a loophole allowing the sale of illicit drugs. And we don’t like drugs, so we’ve got to ban them. What’s surprising – shameful, frankly – is how hard it is to do even that in so many areas of policy. Government departments don’t keep a formal, public record of the research or advice they’ve commissioned or drawn upon, which means that evidence is often simply not available to the public. That also means government research is easily suppressed, delayed or left unpublished. Right now, we have no real way of knowing which parts of government are using evidence well and which are using it badly.

It is not desirable, or possible, to live in a world where politicians make decisions based on research alone. The debate around the Psychoactive Substances Act has exposed something of a culture clash between those who think research should be the only basis of policy and those who see it more broadly. Despite the outrage, we know what evidence has been used in the policy and why. If this were true for other bills too, we could at least start a grown-up conversation about the way our lives are governed.