Roughly 7.5 million American work part-time even though they want full-time jobs.

As the economy has gotten stronger, more part-timers have been sliding back into full-time jobs – but the shift back to full-time has been much slower than the flow into part-time work when the economy went bad.

So while the number of these involuntary part-timers is way down from the worst of the recession, it is still 3 million more people than before the painful downturn started in late 2007.

Economists have been puzzling over the why? That is, why has the number of part-timers stayed so high?

This week, a pair of Atlanta Fed economists had a go at the issue.

Senior economist John Robertson and researcher Ellyn Terry wondered if the trouble part-timers were having was because some particular subset of part-timers was running into obstacles getting back to full-time.

If, say, women were getting back to full-time, but men weren’t. That would explain it.

It’s not just an academic question, of course. For starters, there is the financial pain for households where someone can’t find full-time work: If you are stuck with a too-small paycheck you may struggle making the rent or the mortgage, paying off loans and other bills – not to mention saving money for a rainy day.

Of course, if you are the prime paycheck in the house, that is even worse.

If the shortfall is modest, maybe you eat a lot of mac and cheese. Worst-case scenario, you could end up delinquent on loans or even evicted.

Add up millions of people in the situation, all the households with less money to pour back into the economy, and you have an economic drag that holds back growth.

So the Fed duo ran the numbers.

They had data for manufacturing, for service jobs. They had data for men and for women, for workers with some college, for those with a high school diploma or less.

They discovered very similar patterns, they wrote in the Atlanta Fed Macroblog.

“It doesn’t seem that much of this can be attributed to any particular demographic or industry characteristic of the worker,” they wrote.

That brings the economists back to a kind of Big Picture explanation: the problem is not in the job-seekers, their age, gender or line of work. Not even in some discrimination against some of them.

The problem is in the strength of the job market itself, Robertson and Terry write: “The phenomenon is pretty widespread, suggesting that the problem is a general shortage of full-time jobs rather than a change in the characteristics of workers looking for full-time jobs.”

Simply put, when there are more full-time jobs, there will be more full-time workers – and fewer part-timers who just don’t wanna be.