What's the proper basis for copyright law?

Sometimes I feel that I spend an inor­di­nate amount of time attack­ing copy­right, as if I wished to elim­i­nate it. I do not. But I do feel the bal­ance is off. But how should we find the proper balance?

If the real pur­pose of copy­right law is to “pro­mote the progress,” then why not make sure it’s doing so? In other words, why not have actual evidence-​​based copy­right law? There’s a lot of his­tor­i­cal evi­dence that can be looked at, and dif­fer­ent ideas around copy­right law can be empir­i­cally tested. If it doesn’t pro­mote the progress, get rid of it. If it does, then shouldn’t that make almost every­one bet­ter off?

via Could Evidence-​​Based Copyright Law Ever Be Put In Place? | Techdirt.

Of course, how to col­lect, mea­sure, and eval­u­ate this evi­dence is not sim­ple. Law & Economics pro­vides one pow­er­ful path, and tends to sup­port chang­ing cur­rent copy­right law. Even if evi­dence is con­tro­ver­sial, at least it gives us a shared foun­da­tion to dis­cuss appro­pri­ate copy­right approaches. So why is it so hard to find stud­ies that pro­vide such actual evi­dence, as opposed to sup­po­si­tion and imag­i­na­tion?