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A clinical psychologist says it is possible a man accused of wounding his 23-day-old son with intent to cause grievous bodily harm last year had "retarded cognitive function" at the time of the incident.
The Queenstown Lakes resident, who has interim name suppression, denies the charge which stemmed from an incident on March 13 last year.
At issue is his intention at the time.
The man had been stressed about providing for his young family and was sleep-deprived, the court was told.
He and his infant son had been asleep about half an hour when the baby started crying.
In the Queenstown District Court on Wednesday the man, who was 32 at the time, said he was feeling "angry and frustrated" and "half asleep" when he picked the infant up by the left elbow, moved the child in front of him and squeezed him around the mid-section for 10 to 15 seconds.
His son sustained a fractured left upper arm, which required surgery during which two pins were inserted, and 13 fractured ribs, on both sides of his body near his spine.
Registered clinical psychologist Geoffrey Shirley, of Queenstown, provided an expert witness statement to the court yesterday.
Mr Shirley said immediately before the incident, the man was on parental leave.
"During this time he found himself without the distraction of work, ruminating about a number of frustrations [and] disappointments, all centering on financial issues."
His concerns included "significant" debt he had incurred providing for his wife and two children and financially assisting his retired parents, and the state of the Queenstown housing market, Mr Shirley said.
"He did not express concern to his wife or anyone else, but was suffering progressive psychological deterioration."
At that stage, he was "collapsing into clinical depression" and despite exhibiting a "number of signs" did not seek help.
"When I saw him two months later, he was clinically depressed."
At the time of the incident, given his emotional state, Mr Shirley said it was "reasonable to assume" the defendant had "retarded cognitive function".
"[It’s the] diminished ability to form clear, rational thought which may include the consequences of any such action."
Crown prosecutor Riki Donnelly submitted the defendant’s evidence of "forming emotions of anger and frustration" when he was woken demonstrated he was capable of "some form of cognitive function".
However, Mr Shirley said those feelings did not necessarily have any relation to his thought process and he believed the defendant was "somewhat confused" about the difference.
"Which, probably, is crucial."
Mr Shirley believed the defendant was likely suffering from anxiety which, physiologically, was "identical to anger".
The anxiety could have impaired his thinking and been converted to anger "simply at an emotional level" when he was woken from a deep sleep.
"Most of us know what it’s like to be woken.
"There is a period where the brain simply doesn’t function the way it does when it’s fully conscious."
He believed it was possible, in the circumstances, to have impaired the man’s cognitive function "for a number of seconds".
The trial continues.