WASHINGTON » A co-author of a disputed Heritage Foundation report on a new immigration bill has resigned amid controversy over claims he made about immigrants having low IQs.

A spokesman for the conservative think tank confirmed Jason Richwine’s resignation Friday without offering details.

Richwine was one of two authors of a report released Monday that said immigration legislation pending in the Senate would cost $6.3 trillion over 50 years as immigrants consumed federal benefits without making up for it in taxes. The report quickly came under attack as critics from the left and right said it didn’t account for economic benefits from immigration.

Attention turned to Richwine when his 2009 Ph.D. dissertation from Harvard University came to light in which he asserted that: “The average IQ of immigrants in the United States is substantially lower than that of the white native population, and the difference is likely to persist over several generations.” He argued that the result is low socioeconomic assimilation and other problems and argued that allowing in immigrants with high IQs would help solve the problems.

Richwine’s comments got attention from immigrant activist groups, on left-leaning blogs and elsewhere, and helped muddy Heritage’s attempt to refocus the immigration debate on what it says are unsustainable costs from the new legislation.

“Jason Richwine let us know he’s decided to resign from his position. He’s no longer employed by Heritage,” Mike Gonzalez, Heritage vice president of communications, said in a statement Friday. “It is our long-standing policy not to discuss internal personnel matters.”