A new study, published in July by University College London, has apparently revealed that men (but not women) who consume a large amount of sugar are at higher risk of long term mental illness. The scientists who carried out the study used their press release to support the British government’s so-called “sugar tax” — a special levy due to be imposed on certain soft drinks in 2018. One author expressed dismay that a similar policy was not enacted in other European countries, while another added a jab at the “commercial forces” that “exploit” the gullible, weak-willed populace by selling us sugary drinks. The message is loud and clear: sugar is bad, government regulation is good, and now we have the science to prove it.
Although the study itself is available online, I am not qualified to provide a detailed critique of its methodology or conclusions. A cursory inspection suggests that it does at least look like science, and does not contain poetry, “performative autoethnography”, or any of the other gibberish that characterises the worthless vanity papers highlighted by @RealPeerReview on Twitter. In particular, the authors claim to have anticipated and ruled out the most obvious objection: that increased sugar consumption is a consequence, rather than a cause, of mental health issues. Therefore, for the sake of argument, I shall assume that the paper is methodologically sound, and does indeed provide strong evidence for a causal link between sugar consumption and long term depression.
But regardless of the paper’s validity, the ease with which the authors segue from an interesting scientific conclusion to enthusiastic applause for meddlesome, authoritarian government policy is troubling. The study did not prove that there needs to be a sugar tax, it merely suggested the existence of a link between sugar consumption and mental illness. Other factors claimed to improve mental health include physical exercise and social activity, but I have yet to hear of any proposal to introduce mandatory gym membership or a “no-friends tax” (perhaps these are planned for 2019). Furthermore, we already knew that eating too much sugar is unhealthy — this is why the sugar tax was announced in the first place — so the evidence from this paper doesn’t change anything. The fact that both lead authors used their results to promote nanny-state legislation makes one wonder whether they had an agenda other than objective scientific investigation.
The logic behind the sugar tax is that artificially inflating the price of sugary drinks will make people think twice about buying them, thereby encouraging a more healthy lifestyle. To support this policy requires a couple of significant assumptions. Firstly, that this sort of “sin tax” is actually effective at reducing sales of the supposedly sinful product. In the case of sugar, evidence for this effectiveness is slim at best. Other countries have tried similar policies, and while there is some indication that they do produce an immediate drop in consumption, the effect is small (a single figure percentage), possibly temporary, and thus far not linked to any noticeable improvement in public health. As politicians tend to double down on bad ideas, rather than abandon them, the most likely outcome is that the levy will be continually increased — as the taxes on tobacco have been for decades — until the policy becomes almost indistinguishable from outright prohibition.
Then there are the political assumptions: that the government has a right to regulate our behaviour for the good of our own health, and that flat-rate sin taxes, which invariably hit the poor much harder than the rich, are a morally justifiable policy. These are not propositions for which any notion of “evidence” can be discussed, because they are not factual in nature. They are purely political judgements, about the role and responsibilities of government and its relationship to the individual. You may believe that a tax on soft drinks is a good idea, or you may not. I certainly don’t. But one way or the other, these are subjective, personal views based on our own particular principles. They have absolutely nothing to do with science, even if a scientist uses a scientific paper as a platform to promote them.
A famous problem in philosophy, articulated by David Hume in the 18th century, concerns the “is-ought gap”. This is the separation between two fundamentally different kinds of statements: those which simply describe the world, such as “the sun is hot”, and those which express a prescriptive moral viewpoint, such as “people ought not to steal”. What Hume realised is that there is no easy way to cross the gap. It is logically impossible to reach a conclusion about how people ought to behave purely from factual statements about how the world is. But this is exactly what the authors of the UCL paper are doing. From a newly-discovered scientific fact — sugar consumption may increase the risk of depression (in men) — they immediately declare that the government should be imposing new taxes and trying to control our diet.
None of this is intended to suggest that anyone should be censored. Scientists, regardless of their funding source, are free citizens who should be able to speak their mind like everyone else. Nor would it be appropriate to accuse the authors of the UCL study of being government mouthpieces; there is no evidence to suggest that their statements are anything other than genuinely-held personal opinions. Indeed, the soft drinks levy was actually demanded (and originally rejected) in a public petition that attracted well over a hundred thousand signatures.
But we should be quite clear: this is a political decision, made by ministers who clearly believe that the relationship between citizen and state should be one of paternalistic authority. An idea such as this needs to be debated and defended in the public sphere, not dressed up as some kind of objective scientific truth that the rest of us should accept without question.