Fritzl has been in custody since the case came to light in April
The Austrian father who allegedly imprisoned and abused his daughter for 24 years has said he was "born to rape", according to a leaked report.
The report by psychiatrist Adelheid Kastner says Joseph Fritzl's mother humiliated him in childhood, creating his need to "possess a human being".
Mr Fritzl said he had an "evil streak" and that he "could have behaved a lot worse than locking up my daughter".
Mr Fritzl fathered seven children with his daughter in a cellar he built.
Dr Kastner says in her 130-page report that Mr Fritzl is fit for trial and would be "highly" likely to reoffend if he had the opportunity. A trial could begin in the next few months.
Mr Fritzl has been in custody since the case came to light in April.
Dr Kastner's report has been submitted to the prosecutor's office but was leaked to the Austrian media.
In the report, Mr Fritzl, 73, tells the psychiatrist: "I was born to rape and I held myself back for a relatively long time."
Fritzl, here as a 16-year-old in 1951, spoke of humiliations as a child
Dr Kastner says Mr Fritzl spoke of humiliating and unprovoked attacks by his mother in childhood.
"His childhood made him susceptible to an emotional handicap," she writes, creating the need for him "to possess an entire human being".
The report says Mr Fritzl believed incarcerating his daughter, Elisabeth, meant he would have someone "just for me".
He believed having children with her would mean she would have to stay with him as she would "no longer hold any attraction for other men".
Mr Fritzl speaks of himself as a "volcano" who has a "flood of destructive lava that was barely controllable".
Prosecutors say Mr Fritzl has confessed to keeping Elisabeth captive in a cell at his home in Amstetten, west of Vienna.
DNA tests have shown he is the father of six of Elisabeth's children - a seventh child is believed to have died shortly after birth.
Elisabeth, 42, her six children and her mother Rosemarie have been undergoing psychiatric treatment and are thought to be at a secret location learning to resume a normal existence.
The case first came to light in April after 19-year-old Kerstin, one of the children fathered by Mr Fritzl, became seriously ill and was taken to hospital.