Rothman and colleagues also found that national neuroscience organizations were somewhat more likely to disclose the Lilly funding, with a 36 percent acknowledgment rates, compared with just 16% among local chapters.
"This lack of transparency is disappointing," the researchers wrote.
The Affordable Care Act requires companies to report gifts and payments to physicians, but there is no similar provision for advocacy groups. Rothman and colleagues recommended that the law be revised to mandate such disclosure.
"Legislators, regulators, and the public would more easily be able to follow the money and evaluate possible biases and conflicts of interest in HAO advocacy," they suggested.
The researchers noted that their study was limited by its focus on a single company's self-reported payments over a six-month period and on information posted on websites. They conceded that groups could have disclosed the funding in other ways, such as printed brochures that the authors did not examine.