Jackass star Johnny Knoxville was left stunned to discover he comes from a long line of in-breeders during a recent family tree climb.

The Tv and movie stuntman-turned-actor hired a top U.S. genealogist to trace his roots and he was shocked by his findings.

Knoxville says, "He calls me in... and he goes, 'Sit down, you know in these rural mountain regions you come from, no one ever goes into the community and no one ever leaves the community... so it's not uncommon that there's in-breeding in those communities.'

"I said, 'Is there in-breeding in my family?' and he says, 'A significant amount'."

But Knoxville, real name Philip John Clapp, admits he quickly shook off his shock.

The Dukes of Hazzard star explains, "I was so happy. He thought he was giving me a really horrible thing and I had tears rolling down my face."