by Brandon Keim / Wired.com

In short, telling people how much money carpooling will save them won’t convince them to recycle, but telling them how transcendent carpooling is because it will save the planet will.

Earnest, well-meaning environmental messages are supposed to be ineffective relics of a bygone age, when bumper stickers still worked and treehuggers hadn’t realized that self-interest speaks louder than Mother Earth ever could.

But don’t put that Save the Whales t-shirt on eBay just yet. In experiments published August 12 in Nature Climate Change, psychologists found that telling people about carpooling’s money-saving benefits seemingly makes them less likely to recycle.

In short, appeals to self-interest backfired, accidentally encouraging people to behave selfishly in other areas.

Constantly encouraged to care about nature because it’ll save money, people could forget it’s possible to just care.

“These results reveal the potential for self-interested concerns to inhibit pro-environmental behavior,” wrote the researchers, who were led by Laurel Evans and Greg Maio of the United Kingdom’s Cardiff University.

In the study, 80 Cardiff University undergraduate students were each asked to read statements on a computer about carpooling. Some learned that it saved money, others that it’s good for the environment. A control group learned neither.

The students also filled out paper questionnaires about unrelated topics, which they were told to dispose at the session’s end. Unbeknownst to them, this was the experiment’s purpose, and a microcosm of environmental tensions: Would they use a recycling basket inconveniently located under another table, or a general waste bin at arm’s length?

Among those students “primed,” as psychologists say, with a message of carpooling’s self-transcending benefits, 89 percent recycled. Of the group that learned about the cost saving, just 50 percent recycled, as did 49 percent of the control group.

In a second version of the experiment, some students were told that carpooling is environmentally friendly and that it saves money. Their recycling rates were still just 50 percent, while 83 percent of those told only about environmental benefits recycled. Forty percent of a control group and just 15 percent of the cost-savings group recycled.