Charging pennies for a plastic bag is an effective way to reduce waste, but not just because of the cost.

It may have served you for a quick trip home from the grocery story, but a single plastic bag can take over 1,000 years to decompose. Many bags don’t even make it into the trash—becoming a source of litter and pollution around the world. To fight this source of trash, places like D.C. and Ireland have imposed fees or taxes on the use of plastic shopping bags. The New York City council is currently considering a 10 cent charge per disposable bag.

These small fees work to reduce waste—but not just because of the cost. New research finds that fees also promote awareness about the environmental impact of our choices.

Two studies led by Adrianna Jakovcevic look at how fees impacted shoppers’ choices in Buenos Aires, Argentina. In the first study the researchers observed how many people used grocery bags before and after the bag fees of about 2.5 U.S. cents for small bags and 4 cents U.S. for large bags had been implemented. They found that more people did start bringing their own reusable shopping bags after the fee was implemented.

The second study digs deeper: the researchers tried to assess why shoppers made the switch. They also wanted to know if people who supported the policy of taxing bags were more likely to actually switch to reusable bags, or if the cost was the main motivation. So, they gathered data through interviews outside grocery stores in middle- to low-income neighborhoods. The findings were recently published in the Journal of Environmental Psychology.

It turns out that even people who don’t support the fee were carrying their own bags. “Differently from our expectations, the association between policy support and bag use behavior was not statistically significant,” write the authors. “While 54 percent of policy supporters carried their own bags to shopping, a high percentage of opponents (49 percent) did so as well.” People who disagreed with the policy but still carried their own bags said they did so to avoid the fee.

Those who did support the policy and carried their own bag said they did so to help the environment. “It seems that the charge also reminded at least some consumers of their pro-environmental attitudes, making it more likely that they act upon their intrinsic motivation to protect the environment,” the authors write. In other words, the charge may have interrupted most shoppers' habitual routines, and instead had them make a conscious choice about their use of disposable bags. Asking consumers to explicitly approve or decline an item suddenly made them think more about that item and its potential impact.

These are encouraging findings. They not only show that fees decrease the use of disposable bags among both supporters and opponents of the policy, but also suggest a route for raising awareness about the negative consequences of disposable products.