Millions of Americans are giving their vacation days back to their employer.

More than half (55%) of Americans didn’t take all their vacation days in 2015, up from 42% in 2013, according to a new study released Tuesday by the U.S. Travel Association’s Project Time Off. This is the first time it found a majority of workers are not using all their vacation time. The average worker took 16.2 days of vacation last year, down from 20 days in 1993, resulting in $61.4 billion in forfeited benefits. These workers gave up 658 million unused vacation days and 222 million of those days cannot be rolled over or exchanged for money. (Over 5,600 full-time workers were surveyed, including 1,184 managers.)

This is similar to previous studies on the subject. Employees only use 51% of their eligible paid vacation time, according to a 2014 survey of 2,300 workers who receive paid vacation. What’s more, 61% of Americans work while they’re on vacation, despite complaints from family members; one-in-four report being contacted by a colleague about a work-related matter while taking time off, while one-in-five have been contacted by their boss. And employers, too, are clamping down: Summer Fridays are dead in the water, according to a survey last year of 15,723 Americans carried out by Google Consumer Surveys for vacation site

Most U.S. companies have an “employment-at-will” policy, meaning they can be fired for any reason or no reason at all, unless they have a written contract, they’re in a labor union that has other rules relating to conditions of employment or they’re fired because of some kind of discrimination. “We have almost no job security in the U.S., no legal requirement for severance pay and, with very few exceptions, can be laid off without notice,” John Schmitt, research director for the Washington Center for Equitable Growth, a left-leaning think tank in Washington, D.C. focusing on economic inequality and public policy.

Why don’t they take what’s due? “Fear,” says Scott Dobroski, career trends analyst at Glassdoor. “That’s the underscoring theme.” Some 28% of workers told Glassdoor’s survey that they fear getting behind while they’re sitting on a beach, another 17% actually say they fear losing their job, 19% don’t take all of their days in the hopes that it will give them an edge for a promotion, while 13% are competitive and wanting to outperform colleagues. As workers shoulder a heavier workload post-recession, he says others are afraid of not meeting goals.

The most recent study by GfK Public Affairs and Corporate Communications and the U.S. Travel Association — which obviously has a vested interest in workers using up all their paid vacation time — found a variety of reasons why people don’t take their allocated days. These include fears that they would return to a mountain of work (37%) and that no one else can do the job (30%). Others said it’s harder to take time off when they have a more senior position (28%), while others (22%) said they want to show complete dedication to their job. Some 80% of employees said if they felt fully supported and encouraged by their boss, they would take more time off.

The vacation situation is mixed overseas. Workers in the European Union are legally guaranteed at least 20 paid vacation days a year — and 25 or even 30 days a year in some European countries. However, Chinese workers who have been employed between one and 10 years are entitled to five days, and entitled to two weeks between 10 and 20 years’ service, the China Law Blog notes. Over 72% of Chinese workers have not taken a paid vacation in the last three years, according to a report by Xinhua, the state news agency, cited last year by The New York Times.

But Americans have less job security than many European workers. People not used to taking time off may not understand that paid vacation is actually built into their compensation package. Under the The Fair Labor Standards Act, the U.S. is also one of the few developed countries that doesn’t require employers to provide paid time off. Still, 91% of full-time U.S. workers receive paid vacation, according to the Center for Economic and Policy Research, but only 49% of low-wage workers — those in the bottom fourth of earners — get paid vacation.

And Americans don’t just get the short straw when it comes to the law regulating paid vacation days. The U.S. is one of the few industrialized nations that does not require employers to provide paid family leave for new moms and dads, even though companies are mandated to offer 12 weeks unpaid leave for new mothers. Nearly two-thirds of full-time employees (63%) who are parents did not take paid parental leave in the U.S. and over three-quarters of women (77%) indicate their spouse/partner is not eligible for paid parental leave, according to a recent survey by EY, a global provider of financial services.