"If you find reading and writing hard, you've been working hard all day at two jobs, you've got a young child, are you actually going to go to class? It's challenging," Perin said.
Some economists say that large skills gap in the United States could matter even more in the future. America's economic competitors like China and India are simply larger than competitors of the past like Japan, Carnevale said. Even while America's top 10 percent of students can compete globally, Carnevale said, that doesn't cut it. China and India did not participate in this assessment.
"The skills in the middle are required and we're not producing them," Carnevale said.
Respondents were selected as part of a nationally represented sample. The test was primarily taken at home using a computer, but some respondents used a printed test booklet.
Among the other findings:
—Japan, Finland, Canada, Netherlands, Australia, Sweden, Norway, Flanders-Belgium, Czech Republic, Slovak Republic, and Korea all scored significantly higher than the United States in all three areas on the test.
—The average scores in literacy range from 250 in Italy to 296 in Japan. The U.S. average score was 270. (500 was the highest score in all three areas.) Average scores in 12 countries were higher than the average U.S. score.
—The average scores in math range from 246 in Spain to 288 in Japan. The U.S. average score was 253, below 18 other countries.
—The average scores on problem solving in technology-rich environments scale for adult ranged from 275 in Poland to 294 in Japan. The U.S. average score was 277, below 14 other countries.
Follow Kimberly Hefling at http://www.twitter.com/khefling
Lori Hinnant contributed to this report from Paris.