By Allison Linn, TODAY

(NBC) - The Memorial Day weekend typically marks the start of the summer vacation season –  which for about three-fourths of American workers will mean the possibility of some paid time off.

The United States is the only highly developed nation that doesn't require employers to offer paid vacation time, according to a new report from the Center for Economic and Policy Research, a left-leaning economic think tank.

The report examined vacation policies in 21 developed countries, including the United States. The researchers found that every country they examined, except the United States, had laws requiring employers to offer somewhere between 10 and 30 paid vacation days a year.

France's laws granted employees the most paid vacation, at 30 days per year. But several countries actually guarantee workers more total paid time off when you add in mandated paid holidays.

Japan and Canada were at the lower end of the list, with laws requiring employers to offer 10 paid vacation days apiece.

The researchers found that 77 percent of U.S. workers do get paid vacation time, although their employers are not required to offer it. The workers who do receive paid vacation time get an average of 13 days a year, according to the analysis.

Full-time U.S. workers were much more likely than part-time workers to get paid vacation time, and higher wage workers also were considerably more likely than lower wage workers to have that benefit, according to the report.

In addition, people who worked for large employers were more likely than those who worked for small businesses to get paid time off.

The CEPR report argues that the data is evidence that the U.S. lags other countries when it comes to offering workers time off, and is especially tough on low-wage and part-time workers.

John Schmitt, a senior economist with CEPR and one of the report's authors, said that even when U.S. workers have paid time off available, they don't always take it. He argued that many U.S. workers may feel like they don't have enough job security to risk being out of the office for a long period of time even if they technically have that time.

"I think what that means is that people get nervous about both asking for time off and taking time off," he said.

But conservative thinkers counter that vacation is something that employers and employees can work out on their own.

"I don't know of any compelling reason why the government has to decide for people in what way they want to get paid," said Andrew Biggs, a resident scholar the the conservative-leaning American Enterprise Institute.

Schmitt said the data on U.S. paid vacation is little changed from the last time he did this same analysis six years ago. That's despite the weak economy and extremely tough job market of the past few years.

He said it may be that employers didn't want to reduce vacation benefits – which tend to be less costly than other benefits, such as health care – at a time when they were laying off some staff and asking other workers to take on more work.

"I think it's something that's difficult to cut back on," he said.

For its analysis, the CEPR used data from the United Nations and the other countries themselves. Schmitt said CEPR then used two sets of U.S. government data to create an apples to apples comparison between the U.S. other countries.

The analysis excluded some very small European countries, as well as countries whose economies aren't quite as developed as the United States.