I’ve lingered at the fringes of the debate over genetically modified foods since the ’90s, hoping that some solid fact would filter out and show me clearly who was in the right. But that hasn’t happened. Every shred of information, it seems, is contested, and all this turbulence keeps the water muddy.

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Now the debate is coming to a head again. Britain is reconsidering its restrictive position. Here in the U.S., bills to require the labeling of GM foods were introduced to the legislatures in 28 states this year. Now that I’m writing on food for Grist, I can’t keep waiting on the sidelines for someone else to clear this up. I’m going to have to figure it out for myself.

A project like this requires humility. Many people — including, I’m sure, many of you — may have greater expertise in this area than I do. If so, let me know where you think I should be pointing the searchlight. Or, if you’re like me, and just want to get reliable information from someone who’s not bent on convincing you one way or the other, well, come along for the ride.

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My goal here is to get past the rhetoric, fully understand the science, and take the high ground in this debate — in the same way that greens have taken the high ground in talking about climate. It’s hard to make the case that we should trust science and act to stem global warming, while at the same time we are scoffing at the statements [PDF] of *snort* scientists on genetic modification.

Now that doesn’t mean we have to stop thinking, and simply accept everything that the voice of authority lays in front of us. I’m going to look at the science critically, and take into account the efforts of agricultural corporations to cant the evidence. When Mark Lynas made his speech saying that he’d changed his mind about genetic engineering, I was unconvinced, because he didn’t dig into the evidence (he provides a little more of this, though not much, in his book). Lynas did, however, make one important point: There are parallels between opposition to GM crops and other embarrassingly unscientific conspiracy theories. If there are grounds to oppose genetic engineering, they will have to be carefully considered grounds, supported by science.

Of course people who are concerned about genetic engineering don’t have a monopoly on error and overstatement. As the journal Nature put it in a special issue in on transgenic crops:

People are positively swimming in information about GM technologies. Much of it is wrong — on both sides of the debate. But a lot of this incorrect information is sophisticated, backed by legitimate-sounding research and written with certitude. (With GM crops, a good gauge of a statement’s fallacy is the conviction with which it is delivered.)

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Over the next few weeks, I’ll be writing a series of pieces, attempting to highlight legitimate concerns and identify the arguments that should be taken out back and … retired. In the courtroom, a judge will often work with both sides to determine a set of facts that all can agree upon, before moving on to argue about how the law should apply to those facts. I’d like to do something similar here: sort out established facts, and gain a sense for what the bulk of the science indicates.

I’m going to start with the most politicized issue: Is there any evidence that genetically modified food is directly harmful to people who eat it? There’s a one-word answer to this: no.

If you aren’t prepared to take my word for it (especially that particular word), things get a bit more complicated. The most persuasive evidence is that millions of people have been eating genetically modified foods for the past 20 years without any obvious ill effects. If anyone exhibited acute symptoms after eating GM food, we would have seen it.

At the same time, the absence of evidence of harm does not prove safety. If the effects were subtle and chronic, and showed up in only a small subset of the population, it’s possible that we could have missed something. And we don’t know what to look for.

That’s the point Margaret Mellon made when I called her at the Union of Concerned Scientists, in Washington, D.C. Mellon has been critical of U.S. policies on genetically engineered crops.

“People need to understand how hard it is to use the scientific method to address the issue of, ‘Is genetic engineering safe?’” she said.

The problem: It’s not a yes-or-no question.

“It does not appear,” Mellon said, “that there’s any risk that applies across the board to all genetically engineered food and to all people. Each plant is different, each gene insertion is different, each person’s response is different.”

In other words, every GM food could be wonderfully healthy until one particular gene insertion causes things to go awry in just such a way that it messes with the immune system of one particular person. How do you deal with this?

“You need to make a list of all the things that might be potential problems and analyze each of these risks in a wide variety of genetically engineered products,” Mellon said.

Dozens of scientific advisory panels have done this sort of brainstorming. The World Health Organization [PDF], for example, reached the fairly common conclusion that the problems in genetically engineered foods are fundamentally the same as the dangers that arise naturally in plant breeding. Each relies on mutations randomly mixing up the genome. Each sometimes provides unexpected outcomes — try to make corn disease-resistant, end up with too many toxins in the kernels. In both GM and conventional breeding, scientists rely on screening to weed out the bad cobs.

However, researchers generally acknowledge that there’s something a little different about genetic engineering. The United Kingdom’s 2003 Genetic Modification Science Review [PDF], led by David King, puts it this way: “By virtue of the different processes involved, there will be some sources of uncertainty and potential gaps in knowledge that are more salient with respect to GM food production techniques.”

If you have no idea what that means, that’s because it’s incredibly vague. Sure, King is saying, there’s something unusual about transferring a firefly gene into a tomato — that kind of thing doesn’t happen very often in nature. (Although it does happen, amazingly — scientists have found examples of genes moving between different species.) But we don’t know what that difference implies. The report goes on to say that the science so far suggests that those implications have amounted to nothing so far. All the same, this unique technique does create “uncertainty and potential gaps in knowledge.”

The quest for greater certainty on genetic engineering leaves you chasing shadows: When you’re dealing with gaps in knowledge, rather than hard data, it’s hard to tell what’s an outlandish hypothetical, and what’s the legitimate danger. Anything, of course, is possible, but we shouldn’t be paralyzed by unknown risks, or we’ll end up huddled in our basements wearing tinfoil hats. Exhibit A:

There’s no way to completely eliminate risk. The real question is, have we thought through the realistic potential for problems, and put regulatory safety nets out to protect ourselves?

Trying to answer that opens another can of worms. Critics like Mellon say that, right now, the producers of GM crops aren’t required to do any testing at all. GM boosters say that regulations are so onerous they stifle innovation. Clearly, someone is wrong here. I’ll take that up in my next post.

More in this series: