This is a long study I wrote in 2008, which addresses the costs and benefits of state benefit mandates. At the time, a movement to mandate coverage of a controversial therapy for autism was sweeping through the states.
In it, I concluded that an additional mandated benefit increased the number of uninsured residents of a state by about one quarter of one percent. However, I was very tentative in this conclusion. This is why you don't find me in the conservative chorus calling for selling health insurance "across state lines" (whatever that means) to escape mandated benefits.
I had not posted it here previously because I could not find it on the Internet. Well, we have it now! The link is here.