The performance of state Obamacare markets is heavily influenced by the size and makeup of coverage regions in the state.

While millions have signed up for health insurance under Obamacare since it launched in 2014, coverage across individual states has been uneven, with some signing up as many as 87 percent of those uninsured, while others signed up as few as 13 percent of those eligible. In new research which examines state by state differences in health insurance market boundaries, Michael Dickstein finds that when smaller counties were bundled with larger neighbors, insurers were more willing to enter the market, which is associated with lower premiums. They also find that those regions which have a mix of very high and low levels of urbanity were less attractive to insurers, and had higher premiums.