The market reforms include the law's ban on discrimination for preexisting health conditions and its requirement that plans cover certain essential health benefits.
According to the study, only Connecticut took action on all seven of the Affordable Care Act's market reforms. California addressed all but one, but did not require compliance from all health insurance plans, or in all markets.
Arkansas, Maine, Maryland, New York, Oregon, Rhode Island, Utah, Vermont and Washington were the other states that took some action.
Read more from the study here.