According to the study, led by Flinders University, the defence of mental impairment can have lingering negative effects on a person’s social standing and even civil rights.

A defendant’s plea of reduced agency as a defence against criminal responsibility can lead to the person being seen as less morally deserving of certain rights in the community, Australian and US experts have said.

Dr Melissa de Vel-Palumbo, from the Flinders University College of Business, Government and Law, co-wrote the study — and explained how the research came to this conclusion.

“We argue that although this strategy may help defendants evade blame, it may carry longer-term consequences because lay people in society often perceive a person’s agency can be linked to some of the moral rights they grant them,” she said.  

“After serving their sentence, people who have used this defence can be given fewer rights, as reduced agency can be perceived as increased dangerousness. This was observed through a range of different types of mental impairment, offences and sentences, and have broad implications for the legal and justice system — and defendants.”

For example, a person using a defence of mental impairment may be subjected to exclusionary policies and treatment, making it harder for them to reintegrate into the community. 

Using online scenarios, the study examined randomised responses from 1,600 people to assess their perceptions of a hypothetical defence of mental impairment to reduce their level of responsibility and punishment relative to a guilty plea. 

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“The research also speaks to broader philosophical questions of who is granted moral standing in the community, and why,” Dr de Vel-Palumbo added.

“Typically, we see vulnerable people as needing protection — but not in this case.”

However, co-author Dr Rose Ferguson, from the Institute of Health and Wellbeing at Federation University, said the study also found possible ways to mitigate the negative effect of the mental impairment defence on moral rights. 

“When participants are given information that a defendant using this defence had subsequently engaged in treatment and was able to manage their illness independently, they were perceived as higher in agency and more worthy of rights,” she said.   

“This could suggest that providing the community with information about successful treatment outcomes may increase acceptance of defendants who had been found not criminally responsible for their actions due to mental impairment.”