The Austrian man who locked his daughter in a cellar and fathered her seven children sees himself as a "born rapist" and hatched the plot to imprison her while he was in jail for rape.
Leaked accounts of Josef Fritzl's interviews with a forensic psychiatrist have been reported by The Times in Britain, and Austrian newspapers are expected to publish further details on Wednesday.
The Times said Fritzl, 73, hatched his plan to imprison his daughter Elisabeth in the late 1960s, while he was serving an 18-month jail term for the brutal rape of a woman at knifepoint in her home.
"I have realised that I had a mean streak. For someone who was born to be a rapist, I have managed to contain myself for a relatively long period," Fritzl is quoted as telling the psychiatrist.
He is awaiting trial for sexually abusing and incarcerating Elisabeth, 42, in a specially built dungeon under his house in the town of Amstetten.
He fathered seven children by her, one of whom died shortly after birth and Fritzl burnt his body in an oven.
Three of the surviving children were allowed to live upstairs with him and wife, Rosemarie, 69, while their three siblings were condemned to a secret, shadowy existence with their mother in the cellar.
They were finally freed by police in April after the eldest daughter Kerstin, 19, fell ill and was rushed to hospital after Elisabeth begged Fritzl to let her see a doctor.
After the family was liberated, it emerged that Fritzl had been jailed in 1967 for the knifepoint rape of a 24-year-old woman.
According to the report, compiled by forensic psychiatrist Dr Adelheid Kastner and cited by The Times, Fritzl hatched the dungeon plan as an "ideal solution" to his deranged fantasies.
He decided to lock up his daughter in the cellar so that he could "live out" his "evil side" while leading a seemingly normal life in the flat upstairs, the Times report said.
Fritzl - who participated in in-depth interviews with Kastner - told the psychiatrist that his relationship with women was shaped by his experience with his abusive mother.
He said his mother had beaten him and isolated him from other children until he started school. He also claimed his mother only had him to prove to her partner that she was not sterile.
"He was not only incredibly able to lead a double life but also managed to maintain a triple life without any problems," Kastner wrote, indicating that Fritzl played down the gravity of his crimes in his mind.
"Mr Fritzl resembles a volcano; under the surface that appears almost banal there is an evil streak. He is torn apart by his desires that he cannot master."
The report declared Fritzl clinically sane and fit for trial, but also diagnosed a severe combined personality disorder and a sexual disorder, The Times said.
"It is to be expected that Mr Fritzl would perpetrate deeds with severe consequences also in the future," Kastner concluded.
Fritzl's trial is expected to start early next year.