A jury has heard that a boy accused of murdering another teenager has autism, ADHD and extremely low cognitive abilities.

The trial is underway of a 15-year-old accused of murdering 14-year-old Christopher McBurnie, who was stabbed to death in Ramsey on May 29 last year.

The defendant, who cannot be named for legal reasons, admits inflicting the fatal wound but claims he acted in self-defence. He has pleaded not guilty to murder.

On the tenth day of the trial, jurors heard expert evidence about the defendant's mental health and intellectual abilities.

The defence called consultant clinical psychologist Dr Rhiannon Lewis to give evidence.

She agreed with diagnoses of autism and attention deficit hyperactivity disorder (ADHD) made by the Child and Adolescent Mental Health Service (CAMHS) in March last year.

Dr Lewis also assessed the defendant over two days in April this year, carrying out tests which indicated his cognitive abilities and intelligence were extremely low, placing him in the ‘bottom 2%’ of the general population.

She explained that this meant the defendant struggled to manage the challenges associated with his neurodiversity and compensate for his difficulties in social settings. She said this had resulted in him withdrawing from school.

Dr Lewis said autism made it difficult for the defendant to interpret social situations and understand unwritten rules and nuances, including sarcasm. He also tended to take things literally.

His low intelligence meant he struggled to compensate for these difficulties, while his ADHD contributed to impulsive behaviour.

Dr Lewis carried out five cognitive tests assessing the defendant's vocabulary, ability to absorb and process visual information, problem-solving, working memory and speed of processing complex information.

He was placed in the ‘extremely low range’ in all five tests. His performance was assessed as equivalent to that of children aged between six and 12, with his overall cognitive ability broadly comparable to that of a nine-year-old.

Dr Lewis also said the defendant had been socially isolated for 18 months and suffered from severe social anxiety.

However, she stressed that autism and ADHD were not, in themselves, risk factors for violence or criminality.

She said the combination of autism, ADHD, low intelligence and social anxiety affected the defendant's ability to cope with the situation on May 29 last year.

‘He found himself in a situation where he could not cope with the social demands,’ she said.

The court also heard from prosecution expert Dr John Marshall, a forensic psychologist who assessed the defendant over two days in July this year.

He agreed with Dr Lewis that the defendant had autism, ADHD, social anxiety and low intelligence.

However, Dr Marshall assessed the defendant's IQ at 70, just above the threshold for intellectual impairment, while Dr Lewis placed it at 69, just below that threshold.

He emphasised that the ‘large majority’ of people with these conditions do not commit violent acts or crimes.

Dr Marshall also questioned the usefulness of attributing a mental age to someone, explaining that a 14-year-old - the defendant’s age at the time of the offence - with such difficulties could not be directly compared with a typical child aged six, nine or 12.

He confirmed that, despite his conditions, the defendant would have ‘a general understanding of the consequences of his actions’.

In the afternoon, further expert evidence was heard from psychiatrists for both the prosecution and defence.

Child psychiatrist Dr Joanna Sales, instructed on behalf of the defence, told the jury that the defendant's 'world had completely shut down'.

She said he hadn't been going to school for the best part of two years, he was not participating in family life and he wasn't going out with friends.

'He was barely functioning,' she said. 'His functioning was not going to improve seeing a group of possible assailants at his gate.'

A joint report with prosecution expert, child psychiatrist Dr Richard Church, highlighted the defendant's mental state alongside his underlying psychiatric vulnerabilities, limited life skills and limited problem solving skills.

Earlier on Wednesday, the prosecution closed its case with the reading of agreed facts.

These included details of two previous convictions involving Christopher.

One was for assault causing actual bodily harm after he punched a fellow pupil at school 40 times.

The second was for common assault after he and another boy attacked a pupil, punching him and tripping him as he ran away.

Both assaults had been recorded by fellow pupils, and the videos were played in court.

A third video was also shown, depicting Christopher and another boy attacking a fellow pupil in school changing rooms.

Statements were read from bus passengers who saw Christopher and the other boy travelling to Ramsey that day and heard them discussing having a fight.

On Friday, a defence statement from the defendant was read to the court.

He said: ‘I was frightened because there were so many people outside. I grabbed the knife and put the knife in my pocket.

‘I had the knife to protect myself because I thought I was going to get jumped.

‘I just wanted to scare him, I did not want to use the knife.’

The trial continues.