By Sebastien Malo

NEW YORK (Thomson Reuters Foundation) - Three top U.S. human rights groups scored at the bottom among think tanks as to their openness about the sources of their funding, a watchdog group said on Wednesday.

Human Rights Watch, the United States Institute of Peace (USIP) and the Open Society Foundations (OSF) were graded among 200 advocacy groups and think tanks worldwide, about half of which were deemed "opaque" about their funding, said Transparify, a Georgian-based non-profit.

Disclosing the origins of funds gives think tanks credibility and shields them from potentially slanted, lobby-funded or agenda-driven research, the group said.

The results have improved from 2014, when four out of five think tanks were rated "opaque," the group said.

"The number of organizations who still consider it acceptable to take money from hidden hands behind closed doors is rapidly dwindling. They are running out of excuses," said Hans Gutbrod, who runs Transparify.

The yearly ranking grades groups from "highly transparent" to "highly opaque" as to how much their funding and donor information is disclosed online.

U.S.-based human rights groups topping the ranking included the Center for Global Development and the Woodrow Wilson International Center for Scholars, both based in Washington.

Scoring worst among the 43 U.S.-based groups was OSF for a third straight year.

The organization, which was established by investor and philanthropist George Soros, failed to disclose the names of some of its donors, Transparify said.

OSF spokeswoman Laura Silber challenged the findings, saying that the group had incorrectly been labeled a think tank and should not have been included in the ranking.

"We are a private grant-making and operating foundation that advances human rights, justice and democratic practice across the world," she said.

"Our expenditures and budget are available online."

The OSF funded Transparify's rankings research.

"We are aware of the irony," Dustin Gilbreath, a Transparify spokesman, told the Thomson Reuters Foundation. "Transparify applies the same criteria to all organizations, including our own donor."

Human Rights Watch and USIP fared slightly better, listing all or most of their donors but little or no financial information online, according to the report.

Human Rights Watch said more detailed information was published in hard copies of its annual report and is "public for the most part."

"We don't publish online because many people (particularly older donors) don't want us to," spokeswoman Emma Daly said in an email. "But anyone can contact us and get the annual report."

USIP, which works on such issues as gender and conflict analysis, did not immediately respond to a request for comment.

(Reporting by Sebastien Malo, Editing by Ellen Wulfhorst and Katie Nguyen. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights and climate change. Visit