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In September 2012, the research team led by Gilles-Eric Seralini at the University of Caen published the findings of their feeding trial on rats to test for toxicity of Monsanto’s genetically modified (GM) maize NK603 and/or Roundup herbicide in the online edition of Food and Chemical Toxicology.

Seralini and his colleagues had previously found evidence for toxicity of GM feed in data from Monsanto’s own experiments, which they had obtained through a Freedom of Information demand. Monsanto challenged their conclusions and, to no one’s great surprise the European Food Standards Agency (EFSA) supported Monsanto. So the team decided to run their own experiment, using an unusually large number of animals and over a period of about two years, roughly the life expectancy of the rats, rather than the usual 90 days required in toxicity trials including Monsanto’s.

What Seralini and his colleagues found was that NK603 and Roundup are not only both toxic as expected, but also carcinogenic, which was unexpected. The proportion of treated rats that died during the experiments was much greater than the controls; moreover, in almost all groups a higher proportion developed tumors, and the tumors appeared earlier.

As soon as the paper appeared, the GM lobby swung into action. In particular, the Science Media Centre (SMC), a London-based organization partly funded by industry, quickly obtained quotes from a number of pro-GM scientists and distributed them to the media [4]. According to a report in Times Higher Education, the SMC succeeded in influencing the coverage of the story in the UK press and largely kept it off the television news.

Seralini has rebutted the pro-GM critics point by point on the CRIIGEN website.  The statistician Paul Deheuvels, a professor at the Universite Pierre et Marie Curie in Paris and a member of the French Academie des sciences, has now drawn attention to another serious error in the criticisms: the complaint that Seralini used only 10 rats per group when the OECD guidelines  recommend 50 for investigations on carcinogenesis. Because the experiments did not follow the accepted protocol, their results, they argue, can be safely ignored.

In the first place, this was not a willful disregard of the guidelines. The experiment was designed to test for toxicity, and for that the recommended group size is 10.

But Deheuvels pointed out that the fact Seralini and his colleagues had used smaller groups than recommended makes the results if anything more convincing, not less. That is because using a smaller number of rats actually made it less likely to observe any effect. The fact that an effect was observed despite the small number of animals made the result all the more serious.