Teina Pora has a mental age of nine or 10 years for the purpose of understanding, the Privy Council was told overnight as his appeal against a 1994 rape and murder conviction began.
None of the trial or appeal courts or any of the psychologists who examined him during that period were aware that "he suffers from a brain-based disability and has done since birth", Pora's lawyer, Jonathan Krebs, said in London at the start of a hearing set down for two days.
Pora's lawyers are seeking to persuade the Privy Council that had a diagnosis that he suffers from fetal alcohol spectrum disorder been known it would have made a difference to the outcome.
The consequences of the condition for Pora were a high degree of suggestibility, a tendency to be easily confused and a drive to please others.
Mr Krebs said this was relevant because the two juries that convicted Pora assessed his confessions as though they were made "by a cogent and mature person".
Pora was convicted in 2000 of the rape and murder of Susan Burdett at a retrial ordered after the semen from Ms Burdett was found to belong to Malcolm Rewa.
Rewa was a habitual lone offender convicted of sex attacks on 27 women including Ms Burdett, who was raped and murdered in March 1992 after returning to her Papatoetoe home from 10-pin bowling.
She was believed to have been battered. A softball bat she kept for her own protection was found beside her body.
No physical evidence was found to link Pora to the crimes. But Solicitor-General Michael Heron, QC, told the Herald prior to the hearing that "there are surrounding circumstances and some pretty special knowledge that Mr Pora exhibits that makes the convictions safe".
Mr Krebs is seeking to persuade the panel to consider evidence of Professor Gisli Gudjonsson, an expert in false confessions, who has concluded that Pora's confessions are unreliable and may have been motivated by a desire to gain reward money.
The panel watched a section of a police interview video with Pora where he is unable to identify Ms Burdett's house, and when it is pointed out to him comments that the hedge had grown when it had been trimmed to about half the size of the time of her murder.
The Crown is represented by Mr Heron, QC, Dr Mathew Downs and Zoe Hamill. Pora is represented by Mr Krebs, Ingrid Squire and Dr Malcolm Birdling.
Dame Sian Elias is on the panel of the Judicial Committee of the Privy Council with Lord Kerr, Lord Hughes, Lord Reed and Lord Toulson.
The key questions:
• Would fresh expert opinion that Pora made a false confession have made a difference to the verdict.
• Would new evidence that Pora suffers from foetal alcohol spectrum disorder have made a difference to the verdict.
• Would the jury have reached a different verdict had it known that Malcolm Rewa (convicted of raping Susan Burdett) suffered from erectile dysfunction.