[N]o alternative to the individual mandate can cover more than two-thirds as many uninsured as the Affordable Care Act does as passed by Congress and enacted into law. Second, no alternative to the mandate saves much money—even removing the mandate altogether, which cuts the number of uninsured covered by 50 percent to 75 percent but only reduces government spending by 25 percent to 30 percent. Strikingly, broad and aggressive auto-enrollment, which I estimate to cover two-thirds as many uninsured as the mandate, costs just as much because the coverage comes almost exclusively through auto-enrollment into public insurance. Finally, any alternative imposes much higher costs on those buying insurance in the new health insurance exchanges as the healthiest opt out and the less healthy face increased premiums.
The full paper is here.