Via Reason.tv:

Physicians are required to obtain far more education and clinical experience than are nurse practitioners. But there’s little to no evidence showing that, when it comes to primary care, all of that extra education makes any difference in the health outcomes of patients. A 2012 Health Affairs survey of the medical literature found no difference in patient health between groups treated by doctors and by nurse practitioners. The survey did find a slightly higher satisfaction rate among patients of nurse practitioners.

So if outcomes are similar, and patients are satisfied, why are states such as California hesitant to let more nurses open their own practices?