One of three deaf people accused of murdering a profoundly deaf man who fell 12 metres from an apartment balcony had an IQ in the lowest 0.03 per cent of the population, a court has heard.

Forensicare psychologist Professor Michael Daffern told a Supreme Court jury on Friday that Jake Fairest, 27, had a profound intellectual disability and would not understand what was going on if he went on trial for murder.

Jake Fairest, accused of murdering another deaf man. Credit:Jason South

Giving evidence during a special hearing to formally confirm Mr Fairest was unfit to stand trial, Dr Daffern said Mr Fairest's intellectual ability fell within the mentally retarded range and was well below average.

Dr Daffern said Mr Fairest, who has severe cognitive and developmental disabilities following aggressive chemotherapy treatment for a brain tumour when he was seven, did not believe there was any evidence against him and wanted to plead not guilty.