A study done in part by Leemore Dafny, a health economist now with the Harvard Business School, also illuminates the competition-premium connection. She and co-authors found that premiums in the first year of the marketplaces were 5.4 percent higher just because one national insurer opted out. Another study, published in Health Affairs, found that premiums fall by 3.5 percent with the addition of another insurer.

“Marketplaces will only succeed if enough insurers participate, and many are running away from what they perceive as a high-risk, low-reward market opportunity,” she said.

All of this — insurer withdrawals and sharply escalating premiums — was avoidable and is fixable. We know how to draw insurers into markets, keep them there, and limit premium growth. We can do so by subsidizing plans more and by limiting their risk of loss. We’ve done both before.

In the early 2000s, Medicare+Choice — then the name of what is now the Medicare Advantage program, which offers private plan alternatives to traditional Medicare — was struggling. The proportion of Medicare beneficiaries with access to a Medicare+Choice plan declined from 72 percent in 1999 to 61 percent in 2002. The number of plans offered dropped 50 percent, and enrollment dropped 21 percent. Insurance industry representatives said that the problem was that government subsidy payments to plans were not keeping up with costs.

After payments to plans drastically increased as part of the 2003 Medicare Modernization Act — passed by a Republican Congress and signed by President George W. Bush — insurers flooded the market. This was controversial. Members of Congress from both parties expressed concern that plans were overpaid, wasting taxpayer resources.

By 2007, every Medicare beneficiary had access to at least one plan. The market stabilized, so much so that even as payments to plans were cut by the Affordable Care Act, plan enrollment continued to grow. Today, about one in three Medicare beneficiaries is enrolled in a private plan — a record high. Increasing the subsidization of Obamacare plans might have the same effect — reducing costs to consumers and drawing more of them, and insurers, into the market.

The Medicare Modernization Act also established Medicare’s prescription drug program, Part D, which offers another lesson. It’s also run entirely through private plans. They’re cushioned against large losses by a risk corridor program. This helps plans stay in the market if they miscalculated the mix of patients they’d attract, and it allows them to keep premiums lower than they might need to if they had to hedge against the full brunt of potential losses.