Of all the state-level initiatives on the ballot this Tuesday, few are generating as much vitriol — or advertising — as California's Proposition 37. If it passes, the law would require some genetically modified foods sold in grocery stores to be labeled as such.

Soon this sort of label is going to take a lot more work (Paul Sakuma/AP)

Here's the text of the law (pdf). Those in favor, including watchdog groups and organic food companies, argue that Californians have a right to know what's in their food. Those opposed, including various food and biotechnology firms, say the law could lead to higher prices at the grocery store and hurt small businesses. More than $44 million has been spent on the "no" campaign, with giant agribusinesses such as Monsanto and Dupont donating heavily.

So what's all the fuss about? Here's a basic guide to the fight over genetically modified foods and labeling:

How do you genetically modify food? Farmers have been selectively breeding crops for tens of thousands of years in order to produce desirable genetic traits. But that's not what's at issue here. Since the 1990s, scientists have been able to manipulate the genomes of crops directly. That might involve things like taking a few well-characterized genes from a different species (say, bacteria) and transplanting them into a crop (say, corn) to produce certain desired traits. This is quite different from traditional plant breeding, and it's what is causing all the controversy.

Why would anyone manipulate genes like that? For a variety of reasons. Some crops are modified to be resistant to herbicides — such as Monsanto's Roundup Ready soybeans — so that it's easier to spray fields with weed-killer. Bt corn is modified with a bacterial gene in order to secrete a poison that kills pests. That, by contrast, can reduce the need for chemical pesticides. Golden rice has been modified to help alleviate Vitamin A deficiency in places such as the Philippines.

So there's no one single type of genetically modified food? Right. Simply saying that a food is "genetically modified" doesn't tell you very much. Genetic modification is a tool that can be used for many purposes. In practice, says Jonathan Foley of the University of Minnesota, biotech companies like Monsanto do tend to focus their research efforts on making crops more profitable, rather than making foods more nutritious or boosting yields to solve world hunger. But some researchers, such as U.C. Davis' Pamela Ronald, are interested in harnessing genetic techniques for sustainability purposes. 

How prevalent are GM foods? More than 88 percent of corn and soy planted in the United States is genetically modified in some way. Most of that ends up as animal feed or ethanol or corn syrup, which in turn gets into lots of foods. Cotton, sugar beets and canola are also common genetically modified crops. By some estimates, 40 to 70 percent of foods in California grocery stores contain some GM ingredients.

Are GM foods safe, health-wise? So far, there's been little scientific evidence that they're harmful. At this point, billions of people around the world have been eating GM foods for decades. And numerous scientific bodies have concluded that genetically modified crops pose no more of a health risk than conventional crops. As the American Association for the Advancement of Science put it in a recent statement: "[T]he science is quite clear: crop improvement by the modern molecular techniques of biotechnology is safe."

Some scientists, however, do insist that much more research needs to be done before GM foods can be so definitively considered safe. In a dissent to the AAAS statement, 21 researchers argued that U.S. testing of genetically modified foods is still voluntary, and that things like increased herbicide use — which can occur with things like Roundup Ready crops — may have health effects we don't yet know about. So, they say, why not label and let consumers make up their own minds?

Are genetically modified crops good or bad for the environment? So far, most government agencies are unconvinced that GM foods pose an environmental threat. Here's the National Research Council in 2010: "Generally, GE crops have had fewer adverse effects on the environment than non-GE crops produced conventionally." In some cases, GM crops can be environmentally beneficial. Cotton that's engineered to be pest resistant can allow farmers to use fewer chemical pesticides.

That said, there are some legitimate concerns. For instance, farmers planting herbicide-resistant GM crops tend to use a limited range of herbicides on their fields, which in turn has given rise to a variety of herbicide-resistant "super weeds." That, in turn, leads to the search for new herbicides — what critics have called a "chemical treadmill." And there's the risk that genetically engineered traits could escape into nature. But there are risks to conventional crops, too, and the National Research Council found GM crops, on balance, less harmful.

So what would California's law actually do? It would require a subset of genetically modified foods sold in supermarkets to be labeled as such. Retailers would have to provide extensive documentation for foods that purport to be GM-free. Farmers and food manufacturers might also have to provide records in some cases.

There are plenty of loopholes: The law would exempt many foods, such as meat from animals that are fed GM crops or alcohol. (That's not a minor loophole: Most U.S.-grown soy is genetically modified and used as animal feed.) Restaurants are also exempt. What's more, many biotech companies are now experimenting with new techniques (pdf), such as engineering specific proteins, that do not meet the standard definition of "genetically modified foods." These, too, could elude California's law.

What have past GM-labeling laws accomplished? Studies of labeling laws in Netherlands and China found they did not appear to affect consumer behavior. One possible reason is that labeling laws make GM-free foods more expensive to produce — after all, retailers and manufacturers have to provide extensive documentation to prove food is not genetically modified. So, according to a paper (pdf) by the International Food Policy Research Institute, mandatory labeling laws aren't likely to affect the supply of GM-free foods much.

So what's the best argument against GM labeling laws? What can they hurt? Critics of Proposition 37 have argued that the law is poorly crafted and would unfairly burden small businesses and retailers. Here's Kevin Drum: "This initiative, as with so many initiatives, is sloppily written; it can't be changed after it's passed; and it imposes expensive state labeling burdens on interstate commerce." Drum also points out that California went through a similar experience with toxic-chemical labeling laws. "In the end, so many warning signs got posted that they became essentially useless." 

Meanwhile, U.C. Berkeley's David Zilberman worries that labeling laws will "create a stigma effect" that will hinder future research into using GM foods to improve nutrition or tackle climate change. Remember, genetic modification is a tool that can in theory be used for a variety of purposes — not just bolstering Monsanto's bottom line. See this essay by Pamela Ronald for how plant genetics could play a key role in sustainable agriculture. Note, though, that this is currently a very tiny slice of actual, existing research into GM foods. 

What's the best argument for GM labeling laws? It's true that there are a lot of scientifically unsupported claims about the harms of GM foods. Yet the more thoughtful proponents argue that labeling laws could still force some much-needed transparency on an industry that tends to be dominated by just a few large corporations like Monsanto and Dupont. See Tom Philpott at Mother Jones, who argues that agribusinesses have managed to fend off oversight for decades. That, he says, is why it's worth supporting even a flawed law.

What's the best argument that this entire debate has become far too polarized? Most experts I've talked to tend to have strong views on this topic one way or the other. One exception was University of Minnesota's Jonathan Foley, who has been doing a lot of vital work on how we can feed the world as the population grows and planet warms. "A lot of the fears about GMOs in the public debate are overblown," Foley says. "But a lot of the promised benefits have been oversold too." His suggestion? Far more education about genetically modified foods, as well as more public investment into research — particularly in those areas that large companies are neglecting. Those small steps, at least, are harder to argue with.