Oscar Pistorius listens to his lawyer, Barry Roux, ahead of his trial for the murder of Reeva Steenkamp. File picture: Siphiwe Sibeko

Pretoria - A psychologist gets onto the stand, waves her magic diagnostic wand, and the accused has an explanation for why he committed the crime for which he is standing trial.

But there’s a fine art to it. Wave that wand a little too hard, and the evidence you’re trying to submit swings to an explanation of behaviour popularly known as the “insanity defence”.

Paint your client as a little too unstable or incapacitated, and you land yourself with more problems.

This is what seemed to have unfolded on Monday when psychiatrist Dr Merryl Vorster got onto the stand and explained that Oscar Pistorius had a general anxiety disorder.

Professor James Grant, from the Wits School of Law, said testimony from a psychiatric professional could explain that Pistorius did not intend to kill but made a mistake based on his high state of anxiety.

But the defence got onto a slippery slope when Vorster said Pistorius suffered from a mental disorder that is dangerous and may have diminished his capacity for the crime.

“The mistake that they have made, they have been vague about the target (of her testimony),” said Grant.

Grant said that by trying to clear Pistorius of all blame because of his psychiatric issue, they had opened themselves up to the issue of criminal capacity.

Professor Stephen Tuson, of the Wits School of Law, said criminal capacity could be broken into two parts: the ability of an accused to appreciate the difference between right and wrong, or the ability of an accused to control themselves.

“If you have insight and you can control yourself, then you have what we call criminal capacity,” said Tuson.

Lack of capacity at the time of the crime is often referred to as the insanity defence.

But the law does recognise a halfway point, known as diminished capacity. This is what prosecutor Gerrie Nel raised as an issue – that if Pistorius wanted to argue he had diminished capacity, he should subject himself to psychiatric observation at a state facility.

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The Star