In February, Chicago joined Washington, D.C., Los Angeles, San Francisco and Seattle in implementing a tax on disposable carry-out bags. According to a study led by Professor Tatiana A. Homonoff of New York University, the city is already seeing results.

Homonoff’s study was commissioned by the Mayor’s Office in Chicago — and the NYU professor paired with research organization ideas42 and the University of Chicago’s Energy and Environment Lab. It does have one serious, and obvious, caveat: The 7-cent tax was implemented on Feb. 1 and today is April 24. That’s not a lot of time. However, the findings are still striking — perhaps even more so for their immediacy.

Prior to implementation, customers shopping in the study’s sample stores in Chicago used an average of just over two disposable bags per trip, with over 80 percent of customers using at least one disposable bag. After the tax was implemented, the average number of disposable bags used per shopping trip decreased by roughly one bag per trip—over a 40 percent decrease. Additionally, less than 50 percent of customers in Chicago used any disposable bags after the tax was implemented—a decrease of more than 30 percentage points.

This is not Chicago’s first attempt at limiting disposable bags. In 2015, the city “banned chain stores from providing disposable plastic bags at checkout,” according to the report. However, that ban excluded certain kinds of disposable bags but left others unregulated. So last November, the Chicago City Council “repealed the ban on plastic bags and replaced it with a 7-cent tax on all paper and plastic checkout bags,” starting on February 1.

One of the main ideas behind the ban is one that’s commonly cited with theories about changing behavior that impacts the environment: The stick is more powerful than the carrot.

“The differential impact of the tax on disposable bag use and a reward for reusable bag use is consistent with the concept of loss aversion, i.e., individuals experience losses more strongly than they do gains of the same amount,” the report states.

Over the last few years, a number of cities (and the state of California) have banned plastic carry-out bags. The state of Hawaii also has a ban on the books, but it’s been criticized for its loopholes. In some cases, those bans have been reversed. In others, states have crafted legislation to ban plastic bag bans.

So far this year, the fight over what shoppers should use to haul their groceries continues. In February, a New York City law to create a 5-cent tax on plastic bags at grocery stores was blocked by the governor. Elsewhere in New York state, though, several counties already have bag bans, and Madison County is currently considering one, according to the AP.

A number of Iowa cities have been recently exploring bag bans as well, according to the Des Moines Register. But in March, the state’s governor signed a law “that prevents any Iowa city or county from enacting a ban on the lightweight plastic bags,” the paper reports. “The measure took effect immediately, killing all local efforts to enact bag bans or any taxes or fees to curb their use.”

Nevada, however, is considering a statewide tax on plastic bags, according to the Las Vegas Review-Journal.