There was something wrong with the schoolteacher with the headache -- doctors could see that from the start.

Though charming and intelligent, the 40-year-old man couldn't stop leering at female nurses. He had been in trouble with the law for sexual advances toward his stepdaughter, and now he was talking about raping his landlady.

University of Virginia Medical Center neurologists Dr. Russell Swerdlow and Dr. Jeffrey Burns had never seen a case like this.

The man had an egg-sized brain tumor pressing on the right frontal lobe. When surgeons removed it, the lewd behavior and pedophilia faded away. Exactly why, the surgeons cannot quite explain.

'It's possible the tumor released some pre-existing urges,' Burns said. 'But that's a tough debate, we just don't know.'

The outcome raises questions not only about how tumors alter brain function, but also how they can influence behavior and judgment.

Daniel T. Tranel, a University of Iowa neurology researcher, said he has seen people with brain tumors lie, damage property, and in extremely rare cases, commit murder.

'The individual simply loses the ability to control impulses or anticipate the consequences of choices,' Tranel said.

Vast implications

Dr. Stuart C. Yudofsky, a psychiatrist at the Baylor College of Medicine who specializes in behavioral changes associated with brain disorders, also has seen the way brain tumors can bend a person's behavior.

'This tells us something about being human, doesn't it?' Yudofsky said. If one's actions are governed by how well the brain is working, 'does it mean we have less free will than we think?'

It's a question with vast implications in the criminal justice system.

The U.S. Supreme Court has ruled that executing mentally retarded murderers is unconstitutionally cruel because of their diminished ability to reason and control their urges.

Chris Adams, a death penalty specialist for the National Association of Criminal Defense Lawyers, thinks the next logical step would be to include people who have brain tumors.

'Some people simply don't have the frontal lobe capacity to stop what they're doing,' he said.

Human behavior is governed by complex interactions within the brain. But scientists think most 'executive functions' -- decisions with major consequences -- are controlled within the frontal lobes, the most highly evolved section of the brain.

Tumors in that area can squeeze enough blood from the region to effectively put it to sleep, dulling someone's judgment in a way that's similar to drinking too much alcohol.

Only in very rare cases would the tumor turn the person to violence or deviant behavior on its own, Tranel said.

Dr. Patrick J. Kelly, chairman of the Department of Neurosurgery at New York University Medical Center, said he's never seen a tumor turn someone into a pedophile.

'I've seen them make people hyperactive, forgetful, apathetic,' Kelly said. 'And it usually takes a fairly extensive tumor to do that ... the size of an orange maybe.'

'Pleasure principle' override

The Virginia schoolteacher with the tumor didn't respond to written interview requests by The Associated Press made through his doctors. But according to his case report, which Swerdlow and Burns wrote in the Archives of Neurology, the man didn't remember having abnormal sexual urges for most of his life.

In 2000, the man began collecting sex magazines and visiting pornographic Web sites, focusing much of his attention on images of children and adolescents.

Eventually he couldn't stop himself, telling doctors 'the pleasure principle overrode' everything else. When he started making subtle advances on his young stepdaughter, his wife called police. He was arrested for child molestation.

The man was convicted and failed a 12-step rehabilitation program for sexual addiction because he couldn't stop asking for sex favors, according to the case report.

The day before he was to be sentenced to prison, the man walked into the emergency room with a headache. He was distraught, Swerdlow said, and was contemplating suicide.

He also was 'totally unable to control his impulses,' Burns said. 'He'd proposition nurses.'

An MRI revealed the tumor, and it was cut out days later. The man's behavior began to improve. Swerdlow said the judge allowed him to complete a Sexaholics Anonymous program. The man eventually moved back home with his wife and stepdaughter.

About a year later, Swerdlow said, the tumor partially grew back and the man started to collect pornography again. He had another operation last year, and his urges again subsided.

'That's one of the interesting things about frontal lobe damage,' Swerdlow said. 'This guy, he knew what he was doing was wrong, but he thought there wasn't anything wrong with him, and he didn't stop.'