BEFORE his release in 2001, having served seven years of a life sentence for the killing of James Bulger, Jon Venables was described in a psychiatric report prepared for his parole application as posing a "trivial" risk to the public. The chances of his successful rehabilitation were said to be "very high".

That assessment will now have to be revised. The 27-year-old's involvement in child pornography and his contact with other paedophiles ensures that he will be designated a continuing risk. This was recognised on Friday by the judge, Mr Justice Bean, who banned him for life from working with children.

The case revives the debate about whether people are born bad or made bad. Aged 10 when, with Robert Thompson, he killed the two-year-old in 1993, and 18 when he was released from prison in 2001, Venables has spent most of his life in custody or living with the fear of his identity being revealed.

Dr Alison White, consultant clinical psychologist at Llanarth Court Psychiatric hospital near Monmouth, Wales, who has dealt with hundreds of child sex offenders, said: "I have spent most of my career working with violent, dysfunctional men and in the nature versus nurture debate I am firmly on the nurture side.

"If you examine their backgrounds, it is easy to see where things went wrong. I recall reports about Venables that he had a very difficult childhood. If you are brought up with insecure attachments, no boundaries and a lack of guidance and support, it sows the seeds for later dysfunction. The role models that you have in childhood provide the building blocks for later development."

White said men who access child pornography on the internet usually fall into three groups. There were the curious who looked once, were repelled by what they saw and never looked again. There were those, like Venables, who used the images to fuel their fantasies and for masturbation. And there was a third group who looked at the images and then acted out their fantasies by gaining access to a child themselves.

"I had lots of men in the first group after the Pete Townshend case [the guitarist with The Who was cautioned by police after paying to access child pornography in 2003] who said they just wanted to see what was there," White said.

"Men in the second group, like Venables, often see it as preventive because they are not acting out their fantasies, though it obviously isn't because the children in the images are themselves being abused. In the third group are those for whom the world of the internet is not enough – they want more."

Latest British research suggested that between one in three and one in five men arrested for possessing child pornography progressed to abusing children, she said.

"The longer the fantasy is explored the greater the chances that the behaviour will be acted out in real life. The transition from hands-off to hands-on can be very rapid."

The internet offers unlimited access to children and cyberspace is notoriously difficult to police. The lack of self-regulation and the wide availability of child pornography almost normalised the behaviour, she added.

"Offenders have said to me, 'If it's so bad, why is there so much out there and why wasn't I arrested years ago?'" she said.

Her patients often claimed that the images were enough and they did not need to act out their fantasies.

"The question I have in my head is, for how long? They go to great lengths to access these images; it commands a lot of time and invariably it is not enough. If you then put them in risky situations – babysitting, say – you are asking for disaster."

In prison, Venables is likely to be referred to the Internet Sex Offender Treatment Programme (i-SOTP), which typically lasts between six and nine months and involves up to 90 two-and-a-half-hour group sessions with up to eight others.

The task of the therapy is to persuade offenders to accept responsibility for what they have done and empathise with the victim, instead of minimising it and blaming them – "she was asking for it", they might say. The final stage involves planning to avoid a repeat, by avoiding people, places and substances that might lead to similar incidents.

White praised the i-SOTP programme but said it "treated the symptoms rather than the cause". Venables' problems will likely need more intensive, one-to-one psychotherapy.

"This is a man with a very difficult past who is finding it difficult to make intimate relationships with other adults. Given his chaotic childhood it is almost inevitable that he will have a personality dysfunction that needs addressing.

"The people I'm asked to see shrink into one big pool of inadequate care, with parents who were involved in drink, drugs, domestic violence and with mental health problems. If you treat the symptoms without treating the cause, it is like building a house on shaky foundations. Putting him through the i-SOTP programme would be like applying a sticking plaster.

"People with personality dysfunction are notoriously difficult to treat and you are often looking at years of therapy. Typically they tend to disengage before you get anywhere. I am a firm believer that people can change, but a person's past remains the best guide to their future.

"I have not assessed him but, prognostically, it is disappointing [after the Bulger murder] he has now engaged in further deviant behaviour."