NEW YORK (MainStreet)  At the end of June, the federal government released a proposed rule that requires all the health insurance companies to accept all forms of payment at the exchanges formed under the Affordable Care Act (ACA).

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This should come as welcome news to people without a bank account, because as of January 1, 2014, Americans will be required by federal law to have health insurance or pay a penalty.

The problem, according to a recently released study by Vanderbilt University, was the result of regulatory "blind spots."

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Health policy expert John Graves, assistant professor of preventive medicine at Vanderbilt University, who co-authored the study, and his co-authors at the tax preparation firm Jackson Hewitt, which funded the study, found that there was no language in the law that required insurers to accept various forms of payments. Insurers, therefore, could require their customers to pay for premiums from checking accounts, says Graves. Many of the unbanked use pre-paid debit and credit cards.

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The report found that states that are allowing the federal government to run their insurance marketplace have a larger number of unbanked individuals compared with other states. Tennessee is one such state with a large number of unbanked residents. Three of the larger insurance companies there do not allow customers to pay their premiums with pre-paid debit cards. (State-run exchanges can collect premiums themselves.)

The researchers worried that left on their own, health insurance providers would choose to reject credit and debit card payments, because they are more expensive to process. Furthermore, insurance companies may be concerned that the unbanked are more likely to be less healthy, so insurance companies may not be incentivized to change their payment requirements, Graves says.

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Hard hit would have been many of the people the ACA was intended to help -- the uninsured. The researchers found that 29% of uninsured Americans do not have a bank account from which to write a check for premiums. Among the uninsured and unbanked, 27% qualify for the federal tax credit meant to make health insurance more affordable for people in the lower and middle income range.

--Written by S.Z. Berg, author of College on the Cheap, for MainStreet