The evidence about Johan Kotzé will be presented in the North Gauteng High Court in Pretoria on Wednesday.
Clinical psychologist Cobus Coetzee is expected to testify about his findings that Kotzé can be held criminally responsible for allegedly attacking his wife Ina Bonnette and killing her 19-year-old son Conrad.
The state has already handed in Coetzee's report as evidence, but will call him to the witness stand on Wednesday.
Kotzé, Andries Sithole, Pieta Mohlake and Sello Mphaka, are on trial for allegedly murdering Conrad and kidnapping, assaulting, repeatedly raping and attempting to murder Bonnette in Kotzé's home in Modimolle on January 3 2012.
Kotzé claimed he could not be held accountable for his actions.
Psychiatric diagnosis
His three co-accused testified he threatened them with a firearm and forced them to get on top of his wife. They said they did not rape Bonnette because they could not get erections.
Coetzee observed Kotzé at the Weskoppies psychiatric hospital in Pretoria, with two other psychiatrists, for two months.
The other two found no psychiatric diagnoses could be made.
The state has contended that there were insufficient grounds for clinical psychologist Tertia Spangenberg's findings that Kotzé could not be held accountable for his alleged criminal actions because of non-pathological criminal incapacity.
On Tuesday, the state concluded cross-examining Mphaka, who repeatedly denied looking on while Kotzé tortured and mutilated Bonnette before taking turns with the others to rape her. – Sapa