By the numbers: Premiums are 50% higher this year in areas with just one insurer than in areas with two insurers.

• “The presence of a monopolist insurer was the strongest, and most precise, predictor of 2018 premiums,” the study says.
• Other factors commonly associated with higher premiums — like hospital concentration and the health of the people who live there — showed significantly smaller effects.

How it works: Jessica Van Parys, the Hunter College economics professor who conducted the study, suggests that insurers underpriced their ACA offerings in the first few years to capture market share, then raised their prices over the years.

• Costs and regulatory uncertainty largely kept new competitors from entering those monopolized markets.