If you live in Washington, you know the drill: After bagging your groceries, the checkout machine asks you how many bags you used. And if you used plastic or disposable bags (rather than bags you brought on your own), you have to pay 5 cents per bag. The District passed a law requiring as much in 2009 -- a policy that states like New Jersey and New York are also considering, and that has been adopted around the world from Ireland and Scotland to South Africa.

Some localities have gone farther still -- California and Hawaii have effectively banned plastic bags outright -- but recent research suggests that charges or fees can also be effective (and have the added benefit of being less coercive). Moreover, it suggests that they work, at least in part, through a surprising mechanism. It's not just the relatively minor added cost, on its own, that impels people to stop using plastic bags and to instead bring their own bags with them to the store. Rather, it's the way this small change disrupts habitual behaviors and helps people draw a tighter linkage between the environmental awareness that they already possess, and actions in the world that actually advance that consciousness and their values.

Such is the upshot of a new study on plastic bag charges published in the Journal of Environmental Psychology by a team of Argentinian researchers, led by psychologist Adriana Jakovcevic of Buenos Aires University. Charging a relatively small amount for bags "produces changes in behavior," says Jakovcevic, "and these changes are not only because of the economic value of the incentive -- there are also some other processes at play that involve environmental concerns."

As the researchers note, Buenos Aires provides a perfect opportunity to study the effects of plastic bag charges because in 2012, the vast city's Environmental Protection Agency put in place bag restrictions that in turn led the leading supermarket chain association to institute a bag charge (the equivalent of 2.5 U.S. cents for bags of medium size and 4 cents for large bags) Oct. 9, 2012. For smaller supermarket associations, meanwhile, the same charge went into effect roughly two months later on Dec. 10, 2012. But for Gran Buenos Aires, the larger area that surrounds the Autonomous City of Buenos Aires (CABA), there was no change instituted at all.

This is, the researchers note, a "natural experiment": You have three groups of supermarket shoppers, two of which experienced new bag charges at different times, and one of which did not experience plastic bag charges at all. So the researchers conducted several field studies, observing shoppers leaving supermarkets in low-to-middle income areas in the three different regions at different times -- before any policies went into place, after the first change, and after the second change.

The result was that sure enough, the policies greatly increased the percentage of shoppers who were observed to be carrying their own bag. Here's a figure from their study, showing the effects for the first set of supermarkets to institute the charge (CABA1), the second set (CABA2), and for supermarkets in Gran Buenos Aires that never saw a bag charge instituted (GBA). Consumers were observed at four times: before any policies took place (Time 1), just after the first policy affected CABA1 (Time 2), four weeks after the policy change (Time 3), and then shortly after the second policy affected CABA2 (Time 4):

Clearly, the bag fees worked to dramatically increase the habit of people carrying their own bags. But importantly, in a second study that involved directly interviewing consumers who were observed to leave supermarkets carrying either plastic bags or their own bags, the researchers tried to get at why they had begun to adopt this new behavior, rather than paying the relatively small bag charges that had been instituted.

First, it turned out that a surprising number of people didn't like the bag charge policy, but started carrying their own bags anyway. According to Jakovcevic, it is likely that the small economic cost is the best way of explaining the behavior of these individuals. Past research, however, has called into question whether a purely economic effect like this is a lasting one, with sustained influence on behavior.

But there was another group of Argentinian shoppers -- those who supported the charge and carried their own bags for reasons of environmental concern. They cared much less about economics and much more about green motives. "The people who supported the policy most, they also say they do it for environmental reasons," says Jakovcevic, "and this is a stronger finding because it was an open question, the people could say anything that comes into their mind, and most of them say it was to protect the environment." For these shoppers, says Jakovcevic, the policy provided an opportunity to "rethink why they are using plastic bags or their own bags, and if they care about the environment, this will push them to change their behavior and change it longer over time."

Thus, the paper concludes that the bag charge might have worked "both by the activation of pre-existent pro-environmental attitudes and by a direct effect of the charge on behavior." In addition, it suggests that the new policy worked in another way as well -- by being disruptive of the status quo and changing people's default options and choices. "After the introduction of the plastic bag charge, customers had to explicitly approve or request to obtain a bag and pay for it," notes the study. They couldn't just get one without thinking about it.

So just maybe, a little 5 cent charge can indeed help the environment. You don't have to confiscate all the plastic bags in the world to save the environment (enraging anyone from industry to the Tea Party in the process). You can just give people the slightest push, and let them fix the problem themselves.