Reddit mentions of The Science of Fear: How the Culture of Fear Manipulates Your Brain

At the base level, it's because the basic human function of categorization is how our brain makes sense of the world. People find comfort in assigning rigidity to their beliefs. While that's not always a bad thing (for example, I'm very rigid in my beliefs that (1) nazis are bad and that (2) I should continue to breathe air in order to live), it can often lead to nonsense like you're describing.

People like you're describing often have the base belief that "science tampering with nature is bad". They feel that "natural" always means "better".

This base belief may have originally been reaffirmed by some things we all share as true. For example, the understanding that eating a tomato is generally better than eating candy.

However, these folks didn't allow education and logic to inform the boundaries of their beliefs (for example, arsenic is poisonous, even though it's "natural"- so the "natural always means better" belief cannot be true in all cases). So, it expands into "if science tampering with nature is bad, and GMOs are science tampering with nature, then GMOs are bad."

This problem is magnified by the fact that the work with GMOs is very vague to most laypeople. It can conjure an image of a white lab coat-ed person injecting an evil-looking syringe into a monster-vegetable. It may even relate unconsciously to themes of "evil tampering" by "science" in media. Even examples as silly as the poison apple offered to Snow White stick in our brains, and our brains use those examples to weigh our gut reactions. (in fact, the brain uses something called the Availability Heuristic, which basically boils down to "if I can think of an example that proves my point quickly, then my point must be true!) The brain uses all this info to make the snap judgement that GMOs = BAD/UNSAFE.

In comparison, breeding a vegetable is easily pictured and conjures images that relate to what we understand as safe. Picking a good-looking vegetable, re-seeding it, harvesting again.. the historical aspect also plays a part, since we unconsciously associate our ancestors with having "special knowledge" or a "special relationship" with nature.

In the end, it doesn't meant that the person is evil or stupid. What's happening in their mind is a very normal, typically human process. But when we recognize that these "brain shortcuts" are making us miss out on logical conclusions and evidence that contradicts our beliefs, we have a problem.

One of the best ways to (gently) address this with someone who you think could be capable of facing a change in their beliefs is by looking for common ground and then gently moving from there to things you don't agree on. By finding a new "example rule" for the brain to draw on, it eases the way to change. (For example, if a young child thinks that EVERY square shaped object is a tablet, you need to expose them to many other square shaped objects before directly challenging their idea)

>"I think we agree about a lot of things. For example, we both want to feed good, healthy food to our children! And we both agree that care has to be taken in making sure that our methods of creating food is safe and sustainable. [found common ground]

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>I'd like to find out your opinion on how farmers, over the generations, have influenced food by selecting the best crops to then seed and re-grow. For example, broccoli never actually existed until many, many generations of farmers bred it by combining different plants and re-growing the best results from those plants. Watermelon was like that, too! Am I correct in assuming that we agree that those efforts were VERY worthwhile and resulted in better food for everyone? [expanding common ground]
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>Awesome! Then, let me ask you this: if the choices of which plants to seed was done by a scientist after careful study, that wouldn't be objectionable, would it? A farmer would likely welcome their input! [treading into conflicting belief systems]
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>So, then, if there was a way to look into the actual DNA of a plant and choose which plants to combine at that level, that would be very useful, wouldn't it? Of course, care would need to be taken that the right plants be combined, so many tests would have to be done. But it might be a way for farmers to be able to skip many generations of trial-and-error and find a faster way of doing what they've always done! One example of this is with wheat..." etc.

Of course, many, many people simply aren't ready to face a conflict in their beliefs, especially if they've wrapped their identity around the belief ("tampering with nature is bad, therefore GMOs are bad, therefore I am smart if I avoid GMOs, therefore I am smarter than others who DO support GMOs, therefore I shouldn't trust any evidence that they give me to the contrary"). With those people, don't waste your time argueing.

-TL;DR: It's how the brain works in order to make sense of a ton of info quickly.

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If the psychology side of this interests you, this book was an interesting read on how the brain uses "shortcuts" to influence our choices and feelings, specifically when under the pressure of fear.