A unique insight into why young children sexually abuse other children is to be revealed in a ground-breaking study.

The research, which has yet to be formally published, was on boys aged 10 or under who have molested siblings, classmates, or friends. It found that they are invariably born into families in which abuse, violence and neglect has become routine over several generations.

The peer-reviewed study found that the boys were unable to form healthy relationships as a result of neglectful and hostile parenting. Even before starting school, they were anxious, angry and detached; bed-wetting, nightmares, self-harm and eating problems were common.

All of the boys in the study, which is to be published in Child Abuse Review next year, started abusing after being sexually abused themselves. By the time they received specialist help they had all perpetrated serious abuse against several children. This was not childhood experimentation: their victims were as young as six months; penetration and violence were common.

The research, conducted in the London-based National Clinical Assessment and Treatment Service (NCats), found that the authorities, as well as teachers, social workers and doctors, often missed numerous opportunities to intervene.

Colin Hawkes, the study's author and NCats service manager, said that professionals often ignore, dismiss or punish early warning signs such as a child exposing himself or talking explicitly about sex because they find it difficult to believe that children are physically or emotionally capable of such things. The study also found that in a third of the 27 cases in its sample group the birth mother was suspected of sexually abusing her child. But this social taboo was never tackled by the authorities.

So why does a six year old sexually abuse a three year old? The study asserts that in many cases they copy what adults around them are doing. They may also be seeking control in response to the cruelty and loneliness of their own lives, while spoiling the life of a "luckier or happier" child. Researchers were most shocked to find that many of the boys, like adult offenders, had learnt to groom and target vulnerable children.

The findings add to growing evidence about the devastating impact of early childhood abuse and neglect on brain development, which can lead to serious violence against one's self and others in later life. About a third of all convicted sex offenders carried out their first assault before age 18.

The study comes after two brothers, from Edlington, near Doncaster, were sentenced to a minimum of five years last April for beating, torturing and sexually abusing two boys. The attackers were then aged 10 and 11. The crime was the most notorious committed by British children since the murder of James Bulger in Liverpool in 1993 by 10-year-olds Jon Venables and Robert Thompson. NCats helped assess the perpetrators in both cases.

Mr Hawkes said: "This small minority cannot think straight. They have never experienced calm, coherent parenting. By the time we see them they have been spinning through a spiral of thoughts and feelings and sexually harmful behaviours for several years. Early intervention is key as the longer you leave it, the more likely these harmful patterns become fixed [in the brain]... especially after puberty. If the carers cannot, then the state must respond calmly to the first signs of worrying sexual behaviours."

The NSPCC-funded service treats around 25 of the country's most dangerous boys, and girls, each year.

Susan Haacke, a specialist social worker at NCats, said, "These children are not born bad or evil, they have been born into horrific circumstances. They are rarely naughty, more often sad, ashamed and secretive. Each one is a victim as well as a perpetrator. We use a cognitive-behaviour threrapy model to help them understand their thoughts and feelings, so that they can gain some sense of control."

Camila Batmanghelidjh, director of the Kids Company charity, said: "We have created a perverse system... a child must commit a crime before they get help."

Case study...

Jane and Chris met their adopted son Jack, 10, four years ago. His past included neglect, domestic violence and possible physical and sexual abuse.

Within months of coming to live with them, the sensitive, anxious little boy was using sexually explicit language to other children. They were horrified. It has taken four years to secure local authority money for specialist therapy and Jack is now improving, but they must always remain vigilant.

"Jack's a charming little boy who was all over us straight away. In hindsight, this was not natural, but reflected his need to please adults in the hope of staying safe.

"Within months, the sexual language started: a six year old should not be using the C-word. There have been several incidents where he has tried to sexually harm another child – new situations which make him feel anxious, as if he is merging into the background, are the riskiest as he needs to feel some control.

"We are always vigilant, but I can't watch him all the time, so we say no to play dates and holidays. It doesn't ruin everything: he has brought us so much joy and it isn't his fault, but there is a lot of added stress."