As a young boy, Paul Christiano loved the world of girls - the way they danced, how their spindly bodies tumbled in gymnastics. In adolescence, as other boys ogled classmates, he was troubled to find himself fantasizing about 7- to 11-year-olds.
His desires remained stuck in time as he neared adulthood. Despite a stable home life in suburban Chicago, he was tortured by urges he knew could land him in prison.
"For having these feelings, I was destined to become a monster," he said. "I was terrified."
In 1999, Christiano was caught buying child pornography. Now 36, he said he has never molested a child, but after five years of state-ordered therapy, the attraction remains.
"These people felt they could snuff out the desire, or shame me into denying it existed," he said. "But it's as intrinsic as the next person's heterosexuality."
In the laboratory, researchers are coming to the same conclusion.
Like many forms of sexual deviance, pedophilia once was thought to stem from psychological influences early in life. Now, many experts view it as a sexual orientation as immutable as heterosexuality or homosexuality. It is a deep-rooted predisposition - limited almost entirely to men - that becomes clear during puberty and does not change.
The best estimates are that between 1 percent and 5 percent of men are pedophiles, meaning that they have a dominant attraction to prepubescent children.
Not all pedophiles molest children. Nor are all child molesters pedophiles. Studies show that about half of all molesters are not sexually attracted to their victims. They often have personality disorders or violent streaks, and their victims are typically family members.
By contrast, pedophiles tend to think of children as romantic partners and look beyond immediate relatives. They include chronic abusers familiar from the headlines - Catholic priests, coaches and generations of Boy Scout leaders.
Other pedophiles are "good people who are struggling," said Dr. Fred Berlin, a psychiatrist who heads the Johns Hopkins Sexual Behaviors Consultation Unit. "They're tortured souls fighting like heck not to do this. We do virtually nothing in terms of reaching out to these folks. We drive it underground."
Some of the new understanding of pedophilia comes from studies done on convicted sex criminals at the Center for Mental Health and Addiction in Toronto, where researchers use a procedure known as phallometry to identify men whose peak attraction is to children.
A man sits alone in a room viewing a series of images and listening to descriptions of various sexual acts with adults and children, male and female, while wearing a device that monitors blood flow to his penis.
Like men attracted to adults, nearly all pedophiles respond most strongly to one gender or the other - females far more often than males.
In searching for causes of pedophilia, researchers have largely dismissed the popular belief that abuse in childhood plays an important role. Studies show that few victims grow up to be abusers, and only about a third of offenders say they were molested.
Scientists at the Toronto center have uncovered a series of associations that suggest pedophilia has biological roots.
Among the most compelling findings is that 30 percent of pedophiles are left-handed or ambidextrous, triple the general rate. Because hand dominance is established through some combination of genetics and the environment of the womb, scientists see that association as a powerful indicator that something is different about pedophiles at birth.
"The only explanation is a physiological one," said James Cantor, a leader of the research.
Researchers have also determined that pedophiles are nearly an inch shorter on average than non-pedophiles and lag behind the average IQ by 10 points - discoveries that are consistent with developmental problems, whether before birth or in childhood.
In a 2008 study, Cantor's team conducted MRI brain scans on 65 pedophiles. Compared with men with criminal histories but no sex offenses, they had less white matter, the connective circuitry of the brain.
The evidence also points to what Cantor explained as "cross wiring": Seeing a child sets off the same neural response that men typically experience around an attractive woman.
More evidence of brain involvement comes from scattered examples of men with brain tumors or neurological diseases affecting inhibition.
In one case, a 40-year-old teacher in Virginia with no history of sexual deviance suddenly became interested in child pornography and was arrested for molesting his prepubescent stepdaughter.
The night before his sentencing, he showed up at an emergency room with a bad headache. An MRI revealed a tumor compressing his brain's right frontal lobe.
When the tumor was removed, his obsession faded, according to Dr. Russell Swerdlow, a neurologist on the case. A year later he again became sexually fixated on children. The tumor was growing back.
Swerdlow and others said the case suggests that the man's attraction to children may have always been present - the tumor simply took away the man's ability to control it.
Strong impulse control may help explain why some pedophiles never break the law.
Most clinicians have given up on changing the sexual orientation of pedophiles in favor of teaching the how to resist their unacceptable desires.
Experts believe that pedophiles who also have a significant attraction to adults stand the best chance of staying out of trouble, because of their capacity for some sexual fulfillment that is legal. For others, injections of hormones to reduce sex drive are often recommended.
Most pedophiles, however, don't receive any attention until they've been arrested.
In an attempt to change that, sex researchers in Germany launched an unusual media campaign in 2005.
"You are not guilty because of your sexual desire, but you are responsible for your sexual behavior," said billboards urging them to contact the Institute of Sexology and Sexual Medicine in Berlin. "There is help! Don't become an offender!"
More than 1,700 men have responded to the print, television and online ads for Project Dunkelfeld - literally "dark field." As of August, 80 had completed a one-year program aimed at teaching them to control their impulses. Some received hormone shots. Compared to men still on the waiting list, those who received treatment were deemed less likely to molest children, according to an analysis of risk factors.
The German researchers promise patients confidentiality. About half of those assessed admitted to having already molested a child.
Though extolled by many researchers, the same program could not be conducted in the United States or many other countries, where clinicians and others are required by law to notify authorities if they suspect a child has been or could be harmed.
There have been some grass-roots efforts to bring pedophilia out of the shadows. Anton Schweighofer, a Vancouver, Canada, psychologist, said he recently referred one of his patients to Virtuous Pedophiles, an online support group for men who have never acted on their desires and want to keep it that way.
"I just don't want to get myself in trouble," said the man, a factory worker who spoke on the condition that he not be identified. "I really don't want to harm anybody."
For many pedophiles, a fundamental part of life will always be a shameful secret.
In his late teens, Christiano taught gymnastics and supervised hundreds of young girls. He fasted at work to distract himself from his erotic feelings.
"My hand never slipped," he said. "There were students I loved and adored. In a perfect world, I could sweep them off their feet and live happily ever after."
In this world, however, he has tried to commit suicide three times, he said.
In 1999, he stepped into a federal sting operation when he ordered pornography. He avoided prison but was permanently added to the Illinois sex offender registry.
Once lauded in the Chicago press for his promise as a dance choreographer, Christiano now lives off unemployment, help from his parents and low-paying jobs. He has lost apartments and jobs because of his felony.
"PEDO PIECE OF GARBAGE," read one of many emails he received after an activist group posted a notice about his case online.
His mother, Jennifer Christiano, said that as far back as she could remember, he had always been different from other boys - an odd and creative soul who loved to perform and seemed to worship his female classmates.
"I can't tell you how hard it is," she said. "He's my only child. He'll never truly be happy. He'll never have someone he can truly love and who can love him back."