Licurg: No, they made tests. A hungarian scientist working in England called Alfred Pusztai did a test, where he had mice fed with only GMO potatoes and mice fed only natural potatoes. After 3 generations, the mice fed with GMOs suffered from sterility and organ failure.
Okay, some obvious questions:
1. Who exactly are the "they" that made the tests?
2. Who funded the tests?
3. In which peer-reviewed science journals were the results published?
4. What happened to the mice who got fed "natural" potatos?
5. Were the mouse populations in the experimental group and in the experimental group comparable, and which precautions were taken to assure that?
6. Which precautions where taken to prevent the organ failures to be just artifacts from the in-breeding? Which kinds post-hoc analysis were performed to assure that the cause of the malfunctions matched the hypothesis of the researchers?
7. Where are the objective results of the test, there ought to be at least some probability tests and a statistically valid comparison against the control group. Against which level of probability has the test been performed?
8. Did the test procedure adhere to a double-blind setup (this is an absolute necessity for tests like this)?
9. Have the results been replicated by other scientists (and if yes, by whom, and where have they published their results)?