I did a little experiment the other day. I stood outside a Whole Foods Market in Washington, D.C., with two cartons of large brown eggs. One carton had the words “Non-GMO Project Verified” on it, with a little orange butterfly. It also said cage-free. The other carton had a different label; a green and white circle with the words “USDA Organic.” One other crucial difference: the organic carton cost 50 cents more.

I asked shoppers which carton they would buy.

“They both sound good,” says Anna Hansen, sounding indecisive. “If it’s non-GMO, great. If it’s USDA organic, great. I don’t know!” Then she pointed at the non-GMO carton. “This one’s a little cheaper, I guess I’d go with this one.”

Most of the shoppers I met made the same choice, and they’re joined by millions of shoppers across the country. “We’ve seen exponential growth since our label first launched in 2010,” says Megan Westgate, Executive Director of the Non-GMO Project, which is responsible for that label on the eggs. “We’re currently at about $16 billion in annual sales of products that have the butterfly on them. Just two years ago, we were at $7 billion.”

Organic food sales are growing, too, but not as rapidly. And it’s creating some soul-searching among organic companies, some of which actually launched the Non-GMO Project because they wanted to have their products tested for the presence of GMOs. The official organic rules, while they prohibit the use of genetic engineering, do not require organic food companies to test their ingredients for the presence of GMOs.

“There’s a concern, for sure, that consumers are getting ripped off, or that they’re not getting what they think they’re getting,” says Dag Falck, the organic program manager at Nature’s Path Organic Foods.