The judge in the Oscar Pistorius trial will decide on Wednesday whether he should be referred to a mental health hospital to see whether he suffers from an anxiety disorder that could help explain why he shot Reeva Steenkamp.
Pistorius, who denies murder, has always maintained he shot Steenkamp accidentally, believing her to be an intruder, in the early hours of Valentine's Day, 2013.
The prosecution has maintained he shot her deliberately after an argument.
Now, Judge Thokozile Masipa will rule whether he should be referred to a mental health hospital for 30 days to assess whether he is suffering from Generalised Anxiety Disorder (GAD), which makes people unduly fearful.
The referral application came from prosecutor Gerrie Nel, who said: "If there is a reasonable possibility that some mental illness or disorder played a part, the court must refer."
Dr Merryll Vorster, the defence's psychiatrist, said she had diagnosed him with GAD, adding he had been suffering from it since he was a baby, when he had both legs amputated, which someone so young would have perceived to be a "traumatic assault".
She added: "There is no doubt that children who have bilateral amputations at the age of 11 months find that very stressful … It's a factor that contributed to the development of GAD."
Dr Vorster said the condition could leave him "hypervigilant" and said it would have been worsened by the fact he grew up with an absent father and the death of his mother when he was 14.
Pistorius' mother used to sleep with a firearm beneath her pillow and had an irrational fear of intruders, the court heard on Tuesday.
The trial also heard how, growing up, Pistorius slept with the door locked and was anxious about the safety of his siblings when he travelled.
Pistorius' lawyer Barry Roux opposed the application to refer his client, saying it was "premature" as the defence still had another witness to call.
Earlier, the court heard Pistorius was "heartbroken" in the days after Steenkamp's death.
Social worker and probation officer Yvette van Schalkwyk told the court: "He was sorry for his loss, he was sorry for her parents."