When California voters vote next month, they'll be choosing more than elected leaders. Love it or hate it, California allows voters to make law directly through ballot propositions. This year, one of those would require labels on some foods that contain genetically engineered ingredients, or GMOs.

Proposition 37  has attracted tens of millions of dollars, much of it from big corporations including Monsanto, DuPont and Pepsi — all of those opposing the labels. Ads from the "No on 37" side call the proposed legislation "a complex, badly written labeling proposition that makes no sense."

Ads from the "Yes on 37" group say the proposition "gives us the right to know if there are genetically engineered ingredients in our foods, with clear information on package labels."

And that's just the beginning of the back-and-forth between the two sides. They disagree about pretty much everything: if there's any reason to worry about GMO foods, how the law will work, what the labels really tell us, and what it might cost shoppers.

The cost estimates between the two sides vary pretty dramatically. "We estimate that the cost to the average California household would be between $350 and $400 per year," says Kevin Dietly with Northbridge Environmental Management Consultants. He was hired by the No on 37 people to evaluate the impact of the proposition on our wallets.

In coming up with his number, he made one big assumption: that companies won't want to put labels on their products. Labels like "Partially Produced with Genetic Engineering" or "May be Partially Produced with Genetic Engineering."

Dietly thinks food producers will swap out genetically engineered ingredients for more expensive organic and non-GMO ingredients. Consider, he says, mandatory GMO labeling laws in Europe: "We certainly don't know what will happen, but looking at what has happened in Europe and other places, companies have worked to avoid using the labels, primarily because the labels don't convey much useful information and be frankly frightening to people."

Emory University professor Joanna Shepherd-Bailey isn't so sure. "For very important reasons, America is very different than the European market," she says. "Specifically, American consumers have been far more inclined to eat genetically engineered foods than Europeans for decades really."

Shepherd-Bailey wrote the cost estimate the pro-labeling side cites. She thinks companies will put the labels on their foods and get on with it.

Her best cost guess: about $2 per California family. And that's on the high side, she says, "that's basically assuming that food producers passed on all of the costs of redesigning labels."

So why a label? Most reputable science and many national and global health groups say genetically engineered foods on the market are safe to eat.

The World Health Organization: "GM foods currently available on the international market have passed risk assessments and are not likely to present risks for human health. In addition, no effects on human health have been shown as a result of the consumption of such foods by the general population in the countries where they have been approved." 

The Safety of Genetically Engineered Foods: Approaches to Assessing Unintended Health Effects publication from the National Academies Press: "To date, no adverse health effects attributed to genetic engineering have been documented in the human population." 

The American Medical Association: "Bioengineered foods have been consumed for close to 20 years, and during that time, no overt consequences on human health have been reported and/or substantiated in the peer-reviewed literature. However, a small potential for adverse events exists, due mainly to horizontal gene transfer, allergenicity, and toxicity." 

So why a label? The pro-labeling side says it's simple; people have a right to know what's in their food.

But the millions being spent on this measure have a lot to do with how exactly the label will change the behavior of people and companies. Many experts think the people who don't want to eat GMOs are likely to spend a lot of time label spotting. Other people just aren't going to care.

But what about the people who take time to read the label and don't really know what it means?

"They might view it as a warning," says Jill McCluskey, a professor at Washington State University. The FDA has signed off on GMOs. But, McCluskey says, a mandatory label might send a different signal.

"If the government now requires consumers to be informed that their products have GM ingredients," she says, "they may choose to avoid them."

Industry analyst Gary Karp at Technomic thinks the pressure on food manufacturers to rework and reformulate foods could be pretty intense if Proposition 37 passes. Even, he says, if the group that wants to avoid GMOs is smaller than the group that doesn't. "If 75 percent or 95 percent of people really don't care, then they don't care one way or the other," he says, "and so the small minority really tend to make an impact."

If you care, you really care.

"What I'll call the vocal minority tends to be very passionate about what they believe," Karp says, "and they make a disproportionate amount of press and what I'll call, not derogatorily, but I'll just call noise."

That could make it hard for food producers to figure out how to respond. Karp say they want to meet the needs of their customers, but he says, "what they like to do though is to know that it's a large group of their consumers as opposed to a vocal group of their consumers."

Most of the people I talked to expect that if Proposition 37 passes, at least some food companies will change what's in their foods. Possibly for everyone, not just California.