Uninsurance rates in Massachusetts and the United States, 2000-2010. (SOURCE: BLUE CROSS FOUNDATION OF MASSACHUSETTS)

Last night, Rick Santorum delivered one of the most aggressive attacks on Mitt Romney’s Massachusetts health reform that we’ve seen so far in the Republican debates. He attacked Romney’s plan as “top-down, government-run health care” that was incredibly similar to the federal health-reform law. “Those are not the contrasts we need to beat President Obama,” Santorum charged, later adding “Your mandate is no different than President Obama’s mandate.”

There’s pretty widespread agreement across the political spectrum that Romneycare and Obamacare are nearly identical in structure so, on that front, we didn’t hear much that was new from the Pennsylvania senator. But he did raise one less-common policy challenge to Massachusetts health reform: the idea of free-riding.

In Massachusetts, and in the federal health-reform law, the fine for not purchasing insurance tends to be much lower than the cost of purchasing a plan. So it makes sense that you would see a lot of free riders, individuals who don’t purchase coverage, pay the fine and then rely on the emergency room if they fall ill. Santorum claimed that “free-ridership has gone up fivefold in Massachusetts because people are ready to pay a cheaper fine ... rather than pay for high-cost insurance.”

Here’s one reason we don’t hear much about free-riding in discussions of Massachusetts health reform: It’s barely happening. About 0.6 percent of Bay State adults under 65 paid a fine for not carrying health insurance in 2009, the most recent year for which data are available from the Massachusetts Department of Revenue. That was back when 96 percent of the population had health insurance, a number that has since risen (as Romney pointed out last night) to 98 percent. The number of free riders would presumably drop as coverage increases, although we’re still waiting for data on 2010 numbers.

Why don’t many Massachusetts residents free-ride? It could, after all, be the more rational, cheaper choice. Part of it could have to do with the state’s financial well-being: It has the sixth-highest median income in the country, which could make purchasing health insurance more feasible. A lot of Massachusetts advocates, meanwhile, will attribute the high compliance with a “culture of coverage” that grew out of the health-reform effort. Unlike the federal health-reform effort, Massachusetts’ law is popular — another reason that Bay State residents are willing to comply with it.