New York’s new ban on plastic bags is, on its face, an effort to reduce consumer waste. It likely also offers an important lesson on consumer psychology and how we think about money.

As of Sunday, most businesses in the state can no longer offer single-use plastic bags to consumers. Several cities and counties, such as New York City, are also imposing a 5-cent fee for each paper bag a consumer chooses to use.

A similar rule in Los Angeles (which imposed a 10-cent fee) led to a 94% reduction in single-use bag consumption, according to a New York task force report. In Washington, D.C., a 5-cent fee on both paper and plastic bags caused a 50% reduction. Internationally, bag fees have resulted in reductions ranging from 50% to 90%, according to the report.

How can a seemingly trivial fee change consumer behavior so dramatically?

For one, people really like “free” things — so much, in fact, that they tend to overconsume them.

Think about a buffet. Folks tend to overeat because each additional plate is “free.”

And when something is no longer free, consumer reactions are strong. In the case of paper bags, New Yorkers likely will use them less frequently.

“Because of the fact people don’t have to think of the cost for consuming that thing, they consume way too much of it,” said Daniel Egan, managing director of behavioral finance and investing at Betterment, a robo-advisor.

Research conducted by Dan Ariely, a professor of psychology and behavioral economics at Duke University, demonstrates the power of “free” in consumers’ minds.

In the book “Predictably Irrational,” Ariely discusses an experiment involving two types of chocolate candy: Hershey’s Kisses and Lindt Truffles.

In one trial, students were offered a Lindt chocolate for 26 cents, and a Hershey’s Kiss for 1 cent. Buying behavior was split evenly, with 40% opting for each. However, behavior shifted greatly when the price of each chocolate was reduced by 1 cent — 90% of students opted for the Hershey’s Kiss, which was now free, even though the relative price between the two chocolates remained the same.

“Fees are generally annoying, particularly if they are on items that were normally free,” said Warren Cormier, a behavioral economist.

Nearly 100 cities, towns, and municipalities in the U.S. had banned single-use plastic bags as of March 2017, and fees existed in roughly a third of them, according to the New York task force report.

Read the rest of Greg Iacurci’s article at CNBC.