The Obama administration for years has been pleading with states to expand their Medicaid programs and offer health coverage to low-income people. Now it has a further argument in its favor: Expansion of Medicaid could lower insurance prices for everyone else.

A new study published by in-house researchers at the Department of Health and Human Services compared places that have expanded their Medicaid programs as part of Obamacare with neighboring places that have not. They found that, in 2015, insurance in the marketplace for middle-income people cost less in the places that had expanded Medicaid.

By comparing counties across state borders, and adjusting for several differences between them, the researchers calculated that expanding Medicaid meant marketplace premiums that were 7 percent lower.

States that choose to expand Medicaid can offer government coverage for everyone earning below 133 percent of the federal poverty level, about $16,000 a year for a single person. People earning more can buy insurance in the new Obamacare marketplaces.