In rural Baker County, Ga., where there is only one insurer, a 50-year-old shopping for a silver plan would pay at least $644.05 before federal subsidies. (Plans range in price and levels of coverage from bronze to platinum, with silver a middle option.) A 50-year-old in Atlanta, where there are four carriers, could pay $320.06 for a comparable plan. Federal subsidies could significantly reduce monthly premiums for people with low incomes.

Counties with one carrier are mostly concentrated in the South. Nearly all of the counties in Mississippi and Alabama, for example, are served by just one insurer, according to The Times’s analysis. Other states with scarce competition include Maine, West Virginia, North Carolina and Alaska.

“The consumer wants some level of choice,” said Alexander K. Feldvebel, the deputy insurance commissioner for New Hampshire, where one carrier, Anthem Blue Cross, owned by WellPoint, now offers plans. “You don’t have that when you have a single carrier offering all the products.”

The Times examined carriers and prices on the federal exchanges for the second-cheapest silver plan, the level on which subsidies are based, available to a 50-year-old. Comparable data for state-run plans was unavailable.

The Obama administration, while not disputing the findings, responded to the analysis in a statement that the marketplaces “allow insurers to compete for customers based on price and quality.” It added that the tax-credit subsidies that will lower monthly payments for many consumers had also “brought more companies to the market, resulting in increased options for consumers and lower-than-expected premiums.”

Insurance executives say they set their rates without knowing what other insurers were doing.

“No one knew who was going to file,” said Barbara Morales Burke, an executive with BlueCross BlueShield of North Carolina, the only insurer offering coverage in 61 of the state’s 100 counties. “We developed the rates we always do based on actuarial information and reasonable estimates.”

Market Concentration

The Affordable Care Act, which was passed in 2010, was designed to make health insurance available to people who had not been able to afford it or had been denied coverage because of pre-existing conditions. It has transformed the market for individual insurance by creating marketplaces aimed at making it easier for consumers to compare their options. The law also sought to level the playing field for new insurers.