A 29-year-old man named Juan Carlos Castillo Ponce was renting a basement apartment in the New Jersey city of Elizabeth when he befriended his landlord’s daughters, who were aged three, six and 10. While he lived there between 2000 to 2008, he would take the girls out to dinner, and became a trusted friend of the family. He would also sexually assault them while their parents weren’t home, record them in their rooms through a pinhole video camera, and threaten that, if they told anyone about the assaults, no one would believe them and they would be taken away. When one of the girls finally did tell her parents, investigators recovered hundreds of DVD recordings they say Ponce made of the assaults, not just of the landlord’s daughters but of other girls as well.

Ponce was arrested and charged with aggravated sexual assault and endangering the welfare of a child, among other things. As the judge sentenced him to 27 years in prison, calling the crime a shock to his conscience, Ponce buried his head in his arms and, through a Spanish interpreter, asked for forgiveness. But to those privy to his psychological report, his contrition might not have rung true: Ponce had admitted that when he was around children – and around these four in particular – he knew that what he was doing was wrong but simply could not stop himself.

Those admissions don’t come as a surprise to experts who view not paedophilia but, rather, paedophilic behaviour as the truly dangerous thing. The distinction is critical: paedophiles are individuals with an attraction to children. Paedophilic behaviour is what happens when one acts on the urge; it is an attraction to children that one fails to control.

Now there’s a growing body of evidence suggesting that paedophilia might not be a learned desire but rather an in-born biological trait, like a cleft palate or a hook nose. And lack of emotional control, a separate trait, might be biologically-based as well.

If true, the insights raise a host of issues, from culpability (If they’re born that way, can they help themselves?) to prevention (Can you lock up someone simply for being a paedophile if he hasn’t molested a child?). For now, legal experts seem unmoved by the argument My biology made me do it. After all, we all have urges that we control. The prevention issue, however, is more nettlesome. We don’t detain people for simply having a capacity to engage in an unlawful act. They must commit a crime first. Yet can the urge to abuse children really be ignored? When it comes to paedophilic behaviour – one of the most heinous crimes – we might be entering a frustrating period in which we have a lot of good information and no good way of using it.

Until recently, paedophilia was thought to result from early childhood trauma, probably sexual abuse of some sort. But new research reveals that paedophiles tend to share unique brain structure and some unusual physical attributes. The scientist James Cantor and his colleagues at the Canadian Centre for Addiction and Mental Health (CAMH) in Toronto published studies in 2008 and again in 2015 showing that paedophiles had less white matter in their brains.

Reporting in the Archives of Sexual Behavior in 2015, the team found that paedophiles were more likely to be left-handed, a trait rooted in prenatal neural development; in fact, paedophiles are up to three times more likely than the general population to be left-handed, a rate similar to that observed in individuals with autism and mental retardation. In all, the Cantor team found that paedophiles manifest detached earlobes and misshapen ears, lesser physical height, lower mean IQs, poorer visuospatial and verbal memory test scores, lesser educational attainment, and an elevated propensity to have suffered head injuries before (but not after) age 13.

Cantor said he’s not looking for causality. He doesn’t believe there is a causal relationship between left-handedness, detached earlobes or shortness, and paedophilia. Rather, he studies these traits to determine when the paedophilia might have developed, in the same way that one might cut open a tree and examine the rings to determine its age. For most individuals, such traits are determined by genes or an infection or even maternal stress, in utero. These traits ‘are clues that whatever it is we’re looking for, we’re not just looking for something in the brain’, he adds. ‘We’re looking for something in both brain and body development. That’s the “A-ha” with this one.’ In fact, it constitutes extraordinary evidence that some people are born paedophiles, before environmental factors such as upbringing even come into play.

It wasn’t until the British neuroscientist Simon LeVay found that the brains of gay men were anatomically different than those of heterosexual men that people began to believe that perhaps atypical sexual behaviour, such as homosexuality, was not a choice but rather the result of biological differences. LeVay discovered back in 1991 that the third interstitial nucleus of the anterior hypothalamus in the brain, a region that helps to regulate sexual behaviour, tended to be smaller in gay men than in straight men. LeVay’s work was the first evidence that basic sexual orientation really was a brain phenomenon.

But it was difficult to study other groups in the same intensive way. Most of LeVay’s research was done using gay men who had died of HIV, making their brains available to autopsy and analyse.

‘We didn’t have an analogous thing going on with paedophiles,’ Cantor said. That changed about 15 years ago when scientists gained widespread access to highly sensitive Magnetic Resonance Imaging machines (MRI), making it easier to detect very small differences between brains in those who were still alive.

A pioneer of the technique was the neuroscientist Kent Kiehl, who examined some of the most dangerous criminals currently in jail in the Sand Ridge Secure Treatment Center in Mauston, Wisconsin. Kiehl had access to these prisoners as part of his research, and he had a portable MRI machine that his team wheeled around the prison in a semi-trailer. Kiehl’s study population, of course, had a wide array of issues, from substance abuse to uncontrollable anger; sexual deviancy was just one of them. Thus, distinguishing between brain differences that might result in sexual versus other unrelated issues remains a work in progress.

Yet other researchers have been focusing on family genetics and epidemiology. One study, published in the April 2015 issue of the International Journal of Epidemiology, compared relatives of 21,556 male sexual offenders with relatives of matched non-offender controls, and found substantial evidence of moderate to strong family risk for sexual offending among men. Having a father or a brother convicted of a sexual offence increased the odds of being convicted oneself by four to five times, compared with just 3.5 times the odds for children of male violent offenders, or double the odds for suicidal behaviour in children of individuals who committed suicide.

The problem is when paedophiles commit paedophilia. The issue is not so much the motor, it’s the brakes

Anna Carol Salter, a clinical psychologist in Madison, Wisconsin who has worked extensively with sex offenders, said she’s not surprised to learn that paedophilia is genetically influenced and hardwired in the brain. In her work with the Wisconsin Department of Corrections, she’s found that the majority of paedophiles, when faced with the threat of a polygraph, acknowledge they were not sexually abused as children – a rationale they sometimes give to explain their behaviour.

People are wired the way we are wired – and that goes for the heterosexual who is attracted to the 16-year-old babysitter or their best friend’s wife. Paedophiles just happen to have a sexual attraction to children. ‘Everyone has sexual impulses that they don’t think it’s a good idea to act on,’ Salter said. The problem is when they act on them – when paedophiles commit paedophilia. The issue is not so much the motor, she said. ‘It’s with the brakes. It is not who people are attracted to. It’s what they do with that attraction.’

In one highly publicised case, a 40-year-old school teacher in Virginia developed sudden and uncontrollable paedophilia, making sexual advances to his pre-pubescent stepdaughter and accumulating child pornography. He was removed from the house, found guilty of child molestation and, in lieu of prison, a judge ordered him to undergo treatment at a 12-step programme in a rehabilitation centre for sex offenders. But even in treatment, he kept soliciting sexual favours from staff and other clients. He was kicked out of the programme after just a few days and was headed for jail. The night before he was to go, he walked into a hospital emergency room complaining of a headache. Upon examination, he was found to have a brain tumour sitting on his orbitofrontal cortex, the part of the brain that regulates judgment and ability to control impulses.

As soon as the tumour was removed, the man’s sex-obsession disappeared, prompting his doctors to claim that the tumour had actually caused his paedophilia. The school teacher was allowed to re-enter the sex-offender rehab programme and, this time, he graduated. He even moved back in with his wife. But 10 months later, he again began collecting child pornography and complaining of headaches. Upon re-examination, doctors found that the tumour had grown back. It was again removed and, once more, the hypersexual behaviour largely fell away.

The neurologist Russell Swerdlow, then part of the University of Virginia team that made the discovery, was heralded as having found the part of the brain that deals with paedophilia. But instead, he said, his team simply confirmed the idea that, when there is damage to the orbitofrontal cortex, people can lose their ability to inhibit their sexual urges, whatever those might be. He likened such out-of-control paedophiles to the iconic brain-damaged patient Phineas Gage, a blasting foreman on railway construction projects in 1848, who was using explosives to build a tunnel when a large iron rod went sailing through the air and impaled him in the head just above his right eye. The rod went through a part of the brain that affected his ability to control his anger. ‘He maintained his intellect and was fine, and you would never know something was wrong until he would get irritated, at which point he would just be a brute,’ Swerdlow, now at the University of Kansas Medical Center, said. ‘The guy with the tumour knew what he was doing, but there was no physiologic feedback to stop him.’

The biological roots of paedophilia raise the issue of culpability. Should people be absolved of their sexual urges simply because they were born with them? Absolutely not, said Jessica Oppenheim, a former Deputy Attorney General in New Jersey. Heterosexual men are not allowed to have sex with women without their consent.

‘It’s really the same,’ said Oppenheim, who now heads the criminal justice advocacy programme for the ARC of New Jersey, an advocacy group for the developmentally disabled. ‘Just because it’s genetic doesn’t absolve me. I still have to learn to control myself.’

They need to get help, like any other addict, said Michael Haney, Executive Director of the American Professional Society on the Abuse of Children. Just as there are alcoholics who give in to drinking and those who don’t, there are paedophiles who give in to their desires, committing paedophilia, and those who don’t. ‘It’s kind of like an alcoholic who gets in the car after drinking and kills someone. They’re still held accountable for manslaughter or murder because they had choices. Yeah, they have a compulsion, maybe an addiction, but they’re still held accountable, legally.’

If paedophiles can now be viewed as people with a strong biological proclivity, the legal question is whether that means one is automatically programmed to engage in this behaviour, or whether it simply means they are unusually likely to, said Robert Weisberg, a Co-Founder of the Stanford Criminal Justice Center in California. If child molestation is not an automatic result of having this trait, then it’s no different than a lot of character traits that one might develop as a result of environment or genes, he said. From a legal standpoint, the only defence one could use in a court of law would be to say that this inborn trait automatically resulted in an involuntary reflex, over which one has no control, like an epileptic person having a seizure that resulted in striking someone.

And, Weisberg noted, ‘that would be an almost impossible standard to meet’. A reflex is instantaneous. An act of paedophilia lacks the immediacy required for it to be viewed as an involuntary reflex. In light of the new biological component of paedophilia, the diagnosis might be viewed as a mitigating factor, but hardly exculpatory.

An insanity defence wouldn’t work either, Weisberg said. While someone could be born with mental illness, the legal definition of ‘insanity’ involves one’s ability to know right from wrong. A very mentally ill person, for instance, might feel compelled by the voice of God to kill but, if he nevertheless knows right from wrong, he will not be deemed legally insane. Those who treat paedophiles say they often know that what they did was wrong.

Nor would it help much to argue ‘diminished capacity’, a defence used in California by Dan White, a member of the city’s board of supervisors who was convicted of shooting to death George Moscone, the Mayor of San Francisco, and his fellow supervisor Harvey Milk in 1978. In what came to be known as the ‘Twinkie Defence’, White claimed that his brain had malfunctioned due to low blood sugar. The strategy worked to get White’s charges dropped from second-degree murder to voluntary manslaughter, but caused such an uproar that California’s murder laws were changed to prohibit the defence in the future. Relying on such quasi-insanity defences, Weisberg noted, is a good way to lose your case.

The new knowledge won’t help paedophiles prevail in court, but can it be used to prevent them from abusing children in the first place? Some insight comes from Wisconson’s Kiehl, who has studied impulsivity and recidivism across all kinds of criminal behaviour, and sees a link between self-control and the amount of grey matter in the brain. His research, published in the Proceedings of the National Academy of Sciences in 2013, shows that, when people have less density in a part of the brain called the anterior cingulate cortex (ACC), they are less able to control their impulses and more likely to reoffend.

To do the study, Kiehl and his team scanned the brains of 96 inmates with an MRI machine as they completed an exercise aimed at testing their impulse control. Inmates were told to press a button with their right index finger as quickly and accurately as possible whenever they saw the letter ‘X’ appear onscreen, but not to press the button if they saw the letter ‘K’. Since ‘X’ came up about 85 per cent of the time, the inmates became so used to pressing the button, it took them an effort to suppress the same impulse when ‘K’ popped up. Scientists found that inmates with less ACC activity made more errors on the task, suggesting a correlation with poor impulse control. And since past research has shown impulsivity is a strong risk factor for recidivism, inmates who showed poor impulse control on the test were viewed as more likely to be rearrested after they were released, researchers said.

Kiehl has yet to repeat the study to look at paedophiles specifically. But even if there was an MRI indication that there was a problem at the impulse control centre of the brain, even if we accepted that was admissible in court, said by John Esmerado, the prosecutor who tried Juan Carlos Castillo Ponce, ‘I don’t think we can causally say they will use that impulse to act sexually against a child. The best use of this information right now would be to ensure that certain caretakers such as teachers and coaches with this indication are never alone with children, ever.’

‘We can’t prosecute people or hold them in state supervision if they haven’t done anything wrong,’ Oppenheim added. She likens the scenario to Minority Report (2002), a futuristic movie about an elite crime-prevention unit of psychics who can foresee murders and arrest the would‑be killer before a crime is committed. It could be fairer and more helpful, she suggests, to identify the paedophiles and figure out how to treat them, without incarcerating them before their impulse control breaks down and a child gets hurt.

Science is not yet advanced enough to pick out potential offenders – those who will act on their paedophilia – in advance. Yet there are ways to try to stop convicted paedophiles from reoffending. The most well‑known is the sex-offender registration and notification statutes, such as New Jersey’s Megan’s Law, in which the community is told when a convicted sex offender lives nearby.

Some US states have civil commitment statutes, in which people who have already committed offences undergo rigorous examination to determine whether they are still considered violent sexual predators. If so, they can be committed to a sex‑offender treatment facility indefinitely, even though they’ve already served their jail sentences.

‘Sex offences are a very special class. There’s just no tolerance for mistakes. What happens if someone you released sexually assaults someone’s daughter?’

Minnesota, for instance, currently confines 733 sex offenders at two treatment facilities in St Peter and Moose Lake: more, per capita, than any other state; these individuals are retained after they’ve completed their prison terms for ongoing treatment. But the law permitting this is now being challenged based on claims that the offenders aren’t getting treatment at all.

Others advocate parole supervision for life. Such individuals would be subject to heightened scrutiny forever. Their computers and apartments could be searched without a warrant, and they would be prohibited from living with children unless an exception has been made.

Alternatively, there are medications that can alter the brain’s grey matter – for instance, the medicines already used to treat the symptoms of multiple sclerosis. ‘The brain is very plastic,’ said Kiehl. ‘If I’ve taught you anything during our conversation, I’ve just changed your hippocampus.’

But concepts such as civil commitment, lifelong parole and forcibly scanning and then medicating predators put us on a slippery slope towards violating human rights. So we continue to gather data but, until we can establish that paedophilia begets paedophilic behaviour, there might be little we can do with that information. ‘We currently can’t predict human behaviour. We know what factors can give us an indication that someone might be more likely than someone else to carry out a certain behaviour, but we can’t predict they’re going to do it. It’s almost a crap shoot to make those kinds of judgments,’ said Haney. ‘Clearly, if we had that ability, we could stop a lot of things.’