GENETICALLY modified maize causes cancer: that was the gist of a study, among the most controversial in recent memory, published in September 2012 in the journal, Food and Chemical Toxicology. Well, actually, it doesn’t. The journal has just retracted the article. It would be too much to say that GM foods have therefore been proven safe. But no other study has so far found significant health risks in mammals as a result of eating GM foods. The article in question was by Gilles-Eric Séralini of the University of Caen and colleagues. It describes what happened to rats fed with NK103 maize, a variety that is resistant to a herbicide called Roundup, as a result of genetic modification by Monsanto, an American plant-science firm. Because the crop has resistance, farmers can spray their fields with Roundup, killing the weeds but leaving the maize unscathed. In Dr Séralini’s experiment, rats fed with the modified maize were more likely to develop tumours than those which had not. Female rats were especially badly affected: their death rates were two or three times higher than those of control groups. Rats fed with diluted Roundup also suffered health damage. The article was explosive. The French prime minister said that, if its results were confirmed, his government would press for a European-wide ban on NK103 maize. Russia suspended imports of the corn. Kenya banned all GM crops. The article came out two months before a referendum in California that would have required the labelling of all GM foods. It played a role in the vote, though in the event the proposition was defeated. The paper had all the more impact because it contradicted previous studies. A paper in 2007 by Japan’s Department of Environmental Health and Toxicology had reported “no apparent adverse effect in rats” from NK103 maize and Roundup. That finding had been confirmed by a review of all the evidence by a team at the University of Nottingham School of Biosciences in 2010.

But Dr Séralini’s paper was also explosive in the sense of being controversial. Indeed, it stirred up controversy before it was even published, because the authors insisted that journalists who were given advance copies could not seek independent comment on the contents before publication, and would face a large fine if they did so. This was an extremely unusual and widely criticised requirement which had the effect of ensuring that third-party criticism of the paper did not appear during the important early days when a huge amount of public attention was focused on the findings. That may help explain the panicky reactions in France, Russia and Kenya.

When independent criticism did appear it was hostile. Scientists complained that there was not enough detail about how much food the rats were fed and what the control groups got. The authors also offered no explanation of how GM foods might produce tumours. Even if GM food were unsafe, it would still be unclear what the biological connection would be with cancer.

Particular attention focused on the kind and number of the rats used in the experiments. Dr Séralini had used a variety called Sprague-Dawley laboratory rats and followed them for two years—roughly their natural lifespan. But such rats tend to develop cancers towards the end of their lives without any peculiarities of diet. So it is not clear how much the tumours were the result of feeding them GM maize and how much was spontaneous. These problems are widely recognised and because of them, scientific guidelines say a minimum of 65 rats should be used in each group in a carcinogeneity study. But because Dr Séralini’s experiment had not begun as a cancer study, he had used only ten rats in each group.

Scientific institutions queued up to criticise the findings. The German Federal Institute for Risk Assessment said that “the conclusions drawn by the authors are not supported by the available data”. Regulatory agencies in Canada, Belgium, Denmark, New Zealand and Brazil criticised its shortcomings. No fewer than six French National Academies issued an unusual joint statement calling the paper a “scientific non-event”. And the European Food Safety Authority (which had recommended the maize variety be approved in the first place and to which the European Commission had referred the controversial study) said “the study as reported by Séralini et al is of insufficient scientific quality for safety assessments.” On November 28th, Food and Chemical Toxicology bowed to the storm of criticism and retracted the paper after Dr Séralini refused to withdraw it.

That is unlikely to be end of the matter. Scientific opinion runs strongly against the conclusion that GM foods are harmful—but not universally. A group called the European Network of Scientists for Social and Environmental Responsibility backed Dr Séralini. And anti-GM activists are unabashed. In August a group in the Philippines destroyed a field-study of so-called “Golden Rice”. This is a rice which has been genetically modified to carry beta-carotene, which contains vitamin A, deficiencies of which contribute to the premature deaths of millions of children each year. Neither the prospect of huge public-health benefits in poor countries, nor the absence of evidence of health damage in scientific studies is dulling the edge of the environmental campaign against all GM foods.