President-elect Trump was well known before he became a reality TV star. But what elevated him to the higher reaches of the “famous” category was “The Apprentice,” in which he launched a fleeting national craze of tersely stating, “You’re fired.”

Apprenticing made for engaging television. But is it a good idea in the truly real world? Is it something Trump should encourage as president? A new paper argues that what sounds like a fine idea could be very hard, perhaps even dangerous, to foster.

In “Apprenticeships: Useful Alternative, Tough to Implement,” University of San Diego Law Professor Gail Heriot lays out a compelling case for apprenticeships, explaining that many useful, in-demand skills are best learned hands-on. An apprentice watches the master of a craft, sees what can go wrong, how to fix it, and under that expert’s tutelage does the thing himself until he becomes a master. For much of American history, this was a major way people acquired marketable skills. But as time went on, schools, colleges and universities took over what apprenticeships once did.

While universities and high schools can, and sometimes do, teach useful skills, they often don’t. Colleges and universities especially tend to be dominated by faculty interested in more academic (one might even say esoteric) pursuits, and are often too bureaucratic to move with changing market demands even if they want to. Also, there may be too little demand for some very specialized skills to make it worthwhile to set up programs to teach them.

Apprenticeships could fill the gaps, and provide welcome alternatives for people not inclined toward classroom learning. But there’s a problem: Employers run a big risk of taking on an apprentice when the contributions he can make are less valuable than what he is paid, then having the apprentice bolt to another employer when he has marketable skills.

You might think it would be easy to prevent this from happening: They have apprenticeships in Germany, and whatever happened to contracts? But as Heriot makes clear, stopping a “runaway apprentice” is remarkably difficult. Indeed, as she discusses, the mechanisms likely needed to produce strong apprenticeships, perhaps including powerful national unions or new government programs, could make apprenticeships a net loser.

Having an apprentice seems like a great idea on reality television. In actual reality, it is likely to be a far messier thing.