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?