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

The average U.S. employee who receives paid vacation has only taken about half (54%) of those days n the past 12 months, a new survey of over 2,200 workers by careers website Glassdoor found. This is relatively consistent with how much vacation time employees reported taking in 2014 (51%), when Glassdoor first conducted this survey. If an average worker who receives two weeks vacation leaves five days on the table, they’re effectively giving hundreds of dollars back to the company.

This is similar to previous studies on the subject. Last year, more than half (54%) of Americans didn’t take all their vacation days, up from 42% in 2013, according to a separate study released by the U.S. Travel Association’s Project Time Off. 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.)

Why don’t they take what’s due? “Fear,” says Scott Dobroski, career trends analyst at Glassdoor. “That’s the underscoring theme.” They fear getting behind on their work (34%), believe no one else at their company can do the work while they’re out (30%), they are completely dedicated to their company (22%), and they feel they can never be disconnected (21%). As workers shoulder a heavier work-load post-recession, he says others are afraid of not meeting goals.

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.

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The other study released last year 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 — also found that people were fearful 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.

But there was one thing that most people agreed upon: 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. The amount of vacation time American employees receive, however, depends on the generosity of the employer.