Ranui abduction: What made him do it?

Police and Security at the Ranui train station. Picture / Jason Oxenham

People who commit sexual offences against children do so for a variety of different reasons and not only because they are sexually attracted to minors, one expert says.

Police are still looking for a man who abducted an 11-year-old boy on Thursday afternoon, taking him to an unknown location and sexually assaulting him before dropping him back at a West Auckland train station four hours later.

Forensic psychologist Sarah Christofferson said a common factor in child sexual abuse cases is a sexual deviance where the offender is attracted to children.

However, this sexual deviance is not always present and is not enough on its own to guarantee offending.

"You also need to get past the fact that even a lot of people who have that deviance they don't act on it," Christofferson told the Herald.

Fear of judgement, fear of harming themselves and knowing they are harming someone else can stop people offending even when they have motivation to.

"The key is there will form of disinhibition as well."

This could be a trigger like substance abuse or the collapse of social support system which was stopping them from offending like a relationship break up.

It was uncommon for an offender to sexually assault a child they did not know, Christofferson said.

However without profiling the person individually it was hard to know what exactly caused Thursday's predator to offend.

Christofferson is a clinical psychologist who has completed a PhD in the area of sexual offenders.

She has worked as a forensic psychologist assessing and treating sexual offenders for nearly a decade and is an academic at the University of Canterbury.

Despite the awful nature of Thursday's offence, Christofferson said parents should not feel unduly scared for their children's wellbeing.

"I would just want to hold onto the ... rareness of it and it doesn't mean there's any greater risk we were unaware of to our own children than there was before."

Because of this, it was necessary to avoid reactionary changes in policy and rehabilitation should be considered as well, she said.

Without knowing what had caused this man to abduct and sexually assault a child it was impossible to say what their chances of rehabilitation were, but overall Christofferson was optimistic offenders could be released into society safely.

"Rehabilitation is definitely possible, I don't agree with the sentiment of there's no hope. I think there's a number of factors which might contribute to how successful [rehabilitation [might be]," she said.

"They come to this abhorrent action for any number of different pathways and motivations and with an individualised assessment you can really isolate what is going on for them and rehabilitation efforts have been found to reduce reoffending significantly."

While sexual attraction to children was deemed a paraphilia, it was not the same as mental illness and offenders were no more or less likely to be mentally ill than the rest of the population, although in some cases that could be a factor.

She said committing an offence like this was not a symptom of an illness but rather a behaviour.

While police were right to check known sex offenders in the area where the boy was taken, it was highly likely the man was not already known to police.

"Out of everyone who received a conviction of sexual offence against children in 2015 in New Zealand, 16 per cent it was people who were repeat offenders," Christofferson said.

The boy, who was snatched from Ranui train station on his way home from school, was recovering with his family at home.

Police have asked anyone with information about the case to contact Detective Senior Sergeant Jason McIntosh on 021 191 2659.

Information can also be given anonymously to Crimestoppers on 0800 555 111.