Over 10 years, there's been a very steep drop in youth employment. In the Philadelphia area, the number of young people in the workforce has fallen from 44 percent in 1999 to 25 percent in 2012.

Drexel University researchers set out to find out why this is happening around the country.

They spoke with employers in workplaces traditionally staffed by teens, says Paul Harrington, director of the Center for Labor Markets and Policy. 

Think retail or grocery chains. Business owners told him teens could do the jobs, and had excellent technical skills, but were not always a good fit in their companies. They might be late or on their phones too much. 

"We're flooding these high school labor market jobs, these entry-level jobs and these high school kids don't have the behavioral proficiencies that these older workers and these college grads who push into the high school labor market have," Harrington said.

Essentially, companies consider a cautious hire a better bet than a promising young candidate, particularly if inexperienced applicants do a poor job of presenting themselves. That's especially so in a time of underemployment, Harrington notes.

"When you ask employers -- and I've done this -- when you ask employers, 'Why do you hire a college grad for this job?' The answer is 'because I can,'" he said.

Harrington says more students are deciding to pursue post-secondary education. But he also worries because studies show that work experience is particularly important for young adults.

He recommends partnerships between businesses and schools to support teens just entering the workforce and their employers.

The study was conducted with the Massachusetts-based Commonwealth Corporation.