"The United States has transformed into the planet's undisputed worry champion," Clark adds.

Things only seem to be getting worse, unfortunately. "Surveys show that stress levels here have progressively increased over the past four decades," says Paul J. Rosch, MD, Chairman of the Board of The American Institute of Stress. New research indicates that anxiety will continue to grow with modernity: Millennials and Generation Xers are more nervous than their elders and less capable of handling the pressure in their lives, much of which comes from worries related to money and work. The screws are tightening for our kids, too: A 2011 study from UCLA found that first-year college students are more tense than ever before. The pressure starts well before they graduate from high school, of course: "American teens, and perhaps even pre-teens now, with Ivy League-obsessed parents, experience sleep deprivation, lack of downtime, and stress due to round-the-clock efforts to create impressive resumes for college admissions," says Carrie Barron, M.D., a New York psychiatrist and co-author of The Creativity Cure: A Do-It Yourself Prescription for Happiness. "Too many hours slumping over screens and study tasks leads to depression and anxiety."

For adults, jobs are the leading source of stress, says Rosch, who points out that work-related anxiety has multiplied in recent years—both for the unemployed and the employed. So many companies have down-sized and so many industries have shrunk that employees who manage to hold on to their jobs are expected to work longer hours; they have more and more to do, and less time to do it to their satisfaction.

But it's not just the recession—which is affecting countries around the globe—that's to blame for America's nervous temperament. Even if the economy were in great shape—as it was in 2004, when we spent $2.1 billion on anti-anxiety meds, almost double the amount we spent in 1997—we'd still be chewing our nails. Here's why.

Reimagining American meritocracy

Despite the fact that most Americans believe our country is still The Land of Opportunity, the greatest meritocracy in the world, the United States is actually a terrible place for fortune-seekers. Chris Hayes, author of the new book Twilight of the Elites: America After Meritocracy, notes that when citizens of different countries are polled about their perception of how easy it is to start off poor and work their way up to wealth, "the U.S. is near or at the top in terms of people who say 'yes.' And yet it is also near the bottom in terms of actual social mobility."

In other words, as Hayes argues in his book, America isn't truly a meritocracy. Sure, the Civil Rights movement, feminism, and equal opportunity laws have helped to remove many of the barriers to success—but people at the top tend to stay at the top, from clique to clique, and generation after generation. "Those who climb up the ladder will always find a way to pull it up after them, or to selectively lower it down to allow their friends, allies, and kin to scramble up," Hayes writes.