An estimated 658 million vacation days went unused in 2015, and America’s managers may be to blame.

More than two-thirds of employees who receive paid time off say they hear either negative chatter, mixed messages or nothing at all from their bosses about using their vacation days, according to a new study from Project Time Off, a research initiative funded in large part by the U.S. Travel Association.

Nearly 60% of American workers left vacation time unused last year, continuing a 50-year decline in the number of paid days off that workers take. On average, workers took 20.3 days off in 1976 compared to 16.2 in 2015, reports “The High Price of Silence,” a study that surveyed 5,641 full-time workers in the U.S., including 1,184 respondents who have managerial responsibilities and 312 executive and senior leaders.

“I think a good boss will encourage their teams to utilize the days when needed,” says Rich Corvalan, vice president of sales at Charles River Apparel (asi/44620). “HR should play a role in that, making sure this is communicated as well.”

In response to the survey, 95% of bosses said vacation time is important to them and those they manage. However, 52% of managers said they think no one is capable of doing their work when they aren’t around, and 55% believe taking a vacation will only result in “a mountain of work” upon their return.

“Unless I am out of the country, I am always linked in to emails and voicemails, on vacation or not,” Corvalan says. “Our business works on demand. If a customer needs you, then you need to be there for the customer.”

The avoidance of taking paid days off appears to trickle down from the top brass, according to executive search firm Korn Ferry. In a global survey of 1,033 executives, 97% said they connect to work while on vacation, and 65% said they didn’t plan to take all of their vacation time in the coming year.

“While managers are incredibly supportive of taking vacation, they are out of touch with employee attitudes concerning time off and how loudly their actions communicate a much different message to workers,” said Katie Denis, senior director of Project Time Off.

The perpetual insistence to be in the office or on call is an inherently American trait, according to “Hours Worked in Europe and the US: New Data, New Answers,” a new report from economists Alexander Bick of Arizona State University, Bettina Bruggemann of McMaster University in Ontario, and Nicola Fuchs-Schundeln of Goethe University Frankfurt.

The report states that the average person in Europe works 19% less – about an hour less each weekday – than the average person in the U.S. The economists suggest a plethora of reasons for the difference, among them stronger labor unions in Europe as well as higher taxes, which reduce the incentive to work additional hours each week. In addition, there’s a wider range of incomes in the U.S., increasing the incentive to work towards a promotion.