So prudence is in order when tinkering with such an interconnected system and when making confident predictions about the effects of reform, for good or ill. Will younger adults, who account for about half the population of uninsured non-elderly adults, sign up for coverage — or will they pay the fine instead? How will that decision affect premium levels and the adequacy of federal subsidies?

Will the expansion of coverage create a shortage of health-care providers and result in higher prices, or will, for example, higher Medicaid payments for primary-care doctors stem an exodus of doctors from the program? Will employers add coverage because workers facing the mandate to obtain insurance will press for it, or will they drop it because it will be cheaper to pay the penalty and let employees fend for themselves?

Will increased coverage of preventive care save money because diseases will be caught earlier — or will the added cost of widespread screening exceed the economic benefits? The Congressional Budget Office has concluded that, “for most preventive services, expanded utilization leads to higher, not lower, medical spending overall.”

The legislation is a risk worth taking.