Kaea Karauria's teen killer on cusp of intellectual disability - expert

• Teen murder-accused on cusp of intellectual impairment
• Development likely affected by mother's drinking and drug use during pregnancy
• Accused to give evidence in his defence

A teen accused of murder was exposed to large quantities of alcohol and drugs while in his mother's womb, and this could have affected his brain's development, a court has heard.

The teen's mum regularly consumed 18 packs of bourbon and cola, as well as methamphetamine and cannabis.

The teen is on trial in the High Court at Palmerston North, accused of murdering 15-year-old Kaea Karauria in the early hours of 11 May last year in Napier. The accused, whose name is suppressed, was also 15 at the time. He has prominent Mongrel Mob tattoos on his face.

The Crown says he was part of a group that attacked Kaea over a minor incident that happened weeks before at a party.

The defence says the accused acknowledges stabbing Kaea, but is guilty of manslaughter not murder. The court has heard that the accused has significant mental impairments.

After more than a week of Crown evidence, the defence opened its case on Wednesday.

Accused on cusp of intellectual disability - expert witness

Registered clinical psychologist Erin Eggleston said the accused had an IQ score of 70, which would put him in the lowest 2 percent of the population and on the cusp of having an intellectual disability.

"Most people fall within the range of an IQ of 85 to 115 - 68 percent of the population," Eggleston said.

"If we went out into Palmerston North today and we found... 16-year-olds randomly and then we ranked them in order of their intelligence, with one being the lowest and 100 the most intelligent, [the accused] would be number one or number two."

The IQ score of 70 was made up of smaller assessments. The accused was around the 70 mark for processing speed and verbal comprehension, but was significantly higher for visual and spatial exercises, which was an unusual "relative strength".

"He's hitting the bottom of that normal range," Eggleston said.

The accused functioned similarly to a seven-year-old, but that age-equivalent rating, while useful for the likes of reading comprehension, failed to take into account wider life experiences in assessing a person. So, Eggleston said he also drew on information from youth workers and the accused's mother.

From her he learnt that her consumption of alcohol was "very high" when pregnant. She would consume 18 packs of Cody's pre-mix bourbon and cola about three times a week, as well as use methamphetamine three or four days a week, and cannabis for one or two days.

"What we've come to learn is that alcohol during pregnancy is the most damaging drug in terms of impact on the developing foetus," he said.

The accused met the guidelines for pre-natal alcohol disorder. Eggleston said this wasn't his area of speciality, so he couldn't make a definitive diagnosis. He said, typically, people's brains wouldn't develop until they were 25. Those under that age would have less self-control and act more impulsively.

Among Eggleston's overall findings were that the accused had reduced capacity in fast-changing situations and for reflective decision making and to foresee consequences. He would be less likely to regulate his emotions and exercise impulse control.

Accused to give evidence

In a brief opening to the jury, defence lawyer Tim Braithwaite said the accused had chosen to give evidence, but it was still up to the Crown to prove its prosecution.

"He wants the jury to hear what actually happened - what he did and, more importantly, what he was thinking," Braithwaite said.

The accused would tell the jury about the day he stabbed Kaea - where he went, how the incident started and what substances he had taken.

"His memory is impacted by alcohol and drugs, but he will remember the key points. He accepts he assaulted Kaea with a knife and that he caused Kaea's death."

But the accused would say he only assaulted Kaea once and, after Kaea ran off, never saw him again. The Crown says Kaea was attacked twice.

"[The accused] will tell you that he did not mean to cause Kaea's death and that he did not think about death being a possible outcome. Of course, he will do that in his own particular way."

The court would hear that the accused would use threats and violence as a way of dealing with stress, and that he was easily taken advantage of by others, Braithwaite said.

The trial continues.