Those findings dovetailed with data I tracked down from Intern Bridge, a widely cited consulting firm that specializes in college recruiting. The firm runs a huge annual survey of intern salaries, and I asked them to pull some unpublished numbers from their 2012 poll. Their findings showed that college students were about twice as likely to receive a job offer at the conclusion of a paid internship than at the end of an unpaid internship.

Intern Bridge's figures require a few disclaimers. In 2012, the firm surveyed more than 11,000 college students who were sophomores or higher during the Fall term. It's possible that many of these students received job offers later in the year. And if the firm only surveyed seniors, the job offer rates would likely be higher across the board.

Nonetheless, that 2:1 ratio seems in keeping with NACE's findings. Even if unpaid internships do occasionally turn into permanent job opportunities, it's relatively rare.

It's not entirely clear why unpaid interns fare so poorly on the job market. Many companies do treat their paid internship programs as important talent pipelines, which boosts hire rates for students lucky enough to land in them. But that doesn't explain why unpaid interns appear to barely outcompete students who skip internships altogether.

Could the issue have to do with which types of majors tend to take paid internships, and which tend to settle for unpaid work? Apparently not. As shown in this graph of hiring rates from a recent NACE presentation, unpaid interns fared roughly the same or worse on the job market compared to non-interns across a variety of fields, including business, communications, engineering, English, and political science.

Maybe unpaid interns just aren't as bright as the students who manage to score paid gigs? Again, not so. According to Intern Bridge's internal data, paid and unpaid interns had about the same distribution of GPA's.

So we're left with a bit of a mystery. Though a few receive long-term offers from their employers, unpaid interns generally don't outperform non-interns in the job search. Their collective lack of success doesn't seem to depend much on major or smarts. It might be the case that unpaid internships are just concentrated in industries with weak job markets (think magazine journalism). However, that isn't obvious from Intern Bridge's published figures. It's also possible that there are inherent differences between the kinds of students who take unpaid internships and their peers that would show up in a more refined data analysis. But again, we don't know.

Meanwhile, we also still can't say for certain if unpaid internships* are useful for students who have already graduated from school, but can't find full-time work. Intuitively, it would make sense that putting something on your resume is better than casting around unemployed. But given the results we've seen among college students, I wouldn't jump to any conclusions.

In the end, thanks to a spate of lawsuits and a landmark court ruling last week, it's possible that unpaid internships are headed for the dustbin of labor history. That might not be much of a loss.

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*It might also be time to stop calling post-collegiate internships "internships." As Intern Bridge Vice President Robert Shindell said to me, whether or not they're paid, they really are just generally temp jobs with a fancy title.