Here’s Robin Hanson making a point I always tell my labor students: The best education in the world is already free of charge. Just go to the best university in the world and start attending classes. Stay as long as you want, and study everything that interests you. No one will ever “card” you. The only problem is that, no matter how much you learn, there won’t be any record you were ever there.

Robin actually did this:

As a researcher at NASA Ames Lab in the late 1980s, I found it easy to sit in on classes at nearby Stanford. I sat in on many classes in many departments, participating often in class discussions. I never applied for admission, or paid tuition, but no one ever complained. One professor even wrote me a letter of recommendation based on my work for his class.

So anyone can learn at the very best schools for free, if they are willing to forego the credential. This free ride would probably stop if more than a few people took advantage of it. But in fact almost no one is actually interested in just learning, without the credential.

And thus, we have another deep puzzle that the signaling model of education can explain, and the human capital model can’t.