We didn't always know that smoking was bad for you. It used to be viewed as perfectly healthy. Over the years, the scientific evidence from many studies accumulated, and the consensus came around to the view that smoking does indeed cause cancer.
The tobacco industry fought this view. They conducted their own studies, which of course showed no link between smoking and cancer.
When you have studies that point in opposite directions like this, how do you know what to believe? Do we simply reject the studies done by the tobacco industry because we know they have a stake in the game?
We look at the studies they did. We try to replicate the results. We treat them as we would any scientific study, and let them stand or fall on their merits, not on the basis of who is funding them.
Fortunately, when we do this, we find that the studies that show the link between smoking and cancer are stronger by far than the studies showing no link. We update our beliefs on the basis of the evidence we have available.
Could GMO's be like smoking? Could it be that we find out down the line that they are very very bad, but for now Monsanto and others are preventing us from seeing the truth?
Maybe, but it's very unlikely.
Why so unlikely?
There have been studies done that make claims that fall on both sides of the issue, and what we see is the exact opposite of what we saw in the case of smoking above. The strongest studies show no evidence of harm caused by GMO's and the studies which show harm are weak and poorly designed. This could change in the future, but as of now we have no indication that it will change, and the best evidence out there says we have nothing to worry about.
Great, but why is this important?
Recently, Green Peace activists trashed a field of a special genetically modified form of rice. It was meant to be given to children in areas where vitamin C deficiency is a large cause of death. The rice had been modified to supply vitamin C. See where I'm going with this one. Because certain groups are out to demonize a product that is most likely harmless, children in developing countries aren't getting the nutrients they need to survive. The bottom line is that science denial causes real harm in the world. Even assuming that GMOs are harmful, I would rather my kid live to see adulthood than die of malnutrition.
Here's a link to a meta-study regarding the safety of GMOs compared with traditional methods:
Here's a link to the infamous Seralini rat study claiming that GMOs cause tumors: