To understand the bag tax’s success, Homonoff points to a phenomenon in behavioral economics known as loss aversion. The painful experience of loss is more effective at changing habits than a positive gain. During her surveys, customers often said they wouldn’t bring a reusable bag if they could save a nickel but would bring a bag if it meant avoiding a 5-cent charge.

“You do see this asymmetry when we’re talking about a bonus versus a tax,” she said.

Interestingly, that small but painful fee proved effective on customers across all demographics and income levels, she added.

“It’s not to say that it only works when it’s a big financial burden, then you would expect the drop in low-income neighborhoods but not high-income neighborhoods,” she said. “This tells me that it’s not just about the financial burden of this, that maybe there’s something behavioral.”

Knowing that the modest fee can work at different income levels could also help policymakers understand how to calculate the most palatable tax for consumers. In Seattle, the plastic bag industry goaded voters to reject a 20-cent plastic bag fee. The city ultimately replaced the fee with a hybrid solution that banned plastic bags and charged 5 cents for larger paper bags. In the four years after the policy went into effect, the city of Seattle reported a 50 percent reduction of plastic bags going to city dumps. However, the same report noted that about half of convenience stores and grocery stores were out of compliance.

The takeaway, Homonoff says, is that while you might get a bigger change if you have a large tax, you can change consumer habits with a small tax as well.

Parker agreed that even a modest tax can force people to think twice about the environment.

“These tiny slaps on the wrist are effective at making people care,” she said. “The data points to money. If they feel like they’re losing money, the loss aversion is so much more powerful when changing human behavior.”

The city had initially projected the fee would garner $9.2 million in revenue. Instead, the tax collected just $5.6 million in the first year. (Unlike some municipalities that dedicate revenue from the bag fee to environmental cleanups, Chicago retailers collect 2 cents while the city retains the other 5 for its general operating budget.)

While revenue climbed to $6.4 million in 2018, the city’s fiscal year 2020 budget overview projects that number will decline to $5.9 million in 2019 and will stabilize the following year. That drop doesn’t bother the city’s budget experts, who argue the tax wasn’t really about the revenue in the first place.

“It really was trying to encourage people to bring reusable bags when shopping,” said Susie Park, the city’s budget director. “The decline in revenue is potentially a good thing, because it shows a change in behavior. So we’re hoping that continues.”

Evidence of the fee’s success is anecdotal in other cities, such as Washington, D.C., that have implemented one. In Washington, the Anacostia River has served as the poster child for the city’s 5-cent fee. The waterway is the intended recipient of the tax’s revenue and despite evidence that some of the funds earmarked for cleaning the river have floated down other revenue streams, local politicians say there is less plastic clogging the river.

Even as Chicago acknowledges its bag tax hasn’t beefed up the city’s coffers, its performance hasn’t dissuaded the state government from eyeing a similar fee as a potential revenue source. In February, Illinois Governor J.B. Pritzker proposed a 5-cent tax on plastic checkout bags across the state, a fee his administration expects would garner between $19 million and $23 million. State Rep. Ann Williams, a Democrat whose district encompasses parts of northern Chicago neighborhoods, introduced a bag tax bill in the Illinois House of Representatives earlier this year. Williams noticed a dramatic change in behavior in her home district after the tax, but can’t say the same for the state capital, where there’s no disposable bag policy. Her new habits, she said, travel with her.

“I’m throwing stuff in my purse,” she said. “... I absolutely try and there’s that guilt factor.”