For quite some time, studies have shown American students faring significantly worse than their international peers on basic math and reading tests.

American adults, it turns out, aren't doing much better.

U.S. adults trail their counterparts in other developed nations in literacy, numeracy, and technological problem-solving, according to a new study from the Organization for Economic Cooperation and Development. Despite appearing to be among the most educated countries in the world on paper — and despite the U.S. spending more money as a percentage of GDP on higher education than virtually anyone else — American adults are below average in all three categories when compared with 22 other developed countries, the study shows.

The findings reinforce a growing concern in the U.S. that the nation's workers are falling well behind their foreign peers in the kinds of basic skills needed to excel in the workplace. The results "show our education system hasn't done enough to help Americans compete — or position our country to lead — in a global economy that demands increasingly higher skills," Secretary of Education Arne Duncan said in a statement.

Billed as the most expansive study of its kind, the OECD survey tested 166,000 adults between the ages of 16 and 65 in 23 countries, including the U.S.

American participants rank 16th in literacy, 21st in math, and 14th in tech skills. Meanwhile, Japan and Finland rank first and second in every field, with Sweden, Norway, and the Netherlands typically near the top in every category as well.

A closer look at the data shows just how far behind Americans are.

While nearly half of all adults in the study score at or above the middle proficiency level in math, only 34.4 percent of American adults do. At the same time, 9.1 percent of U.S. adults fall into the lowest proficiency level — almost twice the international average.

The gaps are more pronounced in younger generations. While older Americans fare comparatively well — American adults ages 55 to 65 hit the international average for math proficiency, for example — teens and young adults are the back of the pack when compared to their similarly aged peers.

However, it's not necessarily that America's kids are getting dumber, or that America's education system is precipitously crumbling. Rather, the study says the age-based drop-off is because proficiency "has risen so much faster in so many other countries across successive generations."

In addition, the study underscores how much harder it is for poor or disadvantaged Americans to get ahead.

For instance, in literacy and numeracy, adults with college-educated parents fare much better than those whose parents who never finished high school. And black and Latino adults are three times more likely to have poor skills than whites in America, according to an analysis by Inside Higher Ed.

"We have a real state of crisis," Duncan told NBC at a Tuesday forum. "We have to close what I call the opportunity gap. The gap between the haves and the have-nots is far too wide."