Responding to a growing concern over ethical animal treatment, retailers Whole Foods, Safeway and Supervalu are moving to more clearly define "humane treatment" in the animal products they sell.

Grocery giant Safeway has announced it will increase sales of cage-free eggs—from 6% to 12%—over the next 2 years.

The decision follows a study published last month in the Journal of Agricultural & Food Industrial Organization, which reveals that California’s Proposition 2—legislation passed in 2008 that outlaws cages for hen-laying eggs by January 2015—had “a significant effect on consumer preferences for eggs, increasing demand for cage-free and organic eggs by 180% and 20%, respectively.”

The study shows that “the very act of putting an issue like Prop 2 on the ballot affects consumers’ preferences—likely because consumers are largely unaware of, and have incorrect beliefs about, modern agricultural practices,” concludes author Jayson L. Lusk, PhD, a professor of agricultural economics at Oklahoma State University.

Dr. Lusk found that, despite higher prices, demand for cage-free and organic eggs increased 180% and 20%, respectively, in response to news stories about Prop. 2, even as overall egg demand remained the same.

“California egg producers have an opportunity to thrive by meeting this demand and abandoning cruel cages,” says Jennifer Fearing, who managed the YES! on Prop 2 campaign for the Humane Society of the United States.

Safeway’s 1,712 North American stores include the Vons, Pavilions, Dominick’s, Genuardi’s, Carrs and Randalls grocery chains.