Biological Models of Depression and Criminal Culpubility

Mood disorders such as depression cannot be used to claim an ‘insanity defence’  or to otherwise reduce criminal culpability in most Western countries, including Australia.

However, a diagnosis of a depressive illness may play a major role in mitigating sentences post-conviction on the grounds of diminished responsibility and a high likelihood of rehabilitation.

In Australia, mood or affective disorders are separated from other forms of mental disorder on the basis that they do not affect ‘rational decision-making’. This is due to the influence of the 1843 M’Naghten Rules for determining a ‘disease of the mind’, which in their most basic description require proof that at the time of offence:

the accused was suffering from a mental impairment; and that the mental impairment affected the accused so he or she either did not understand the nature and quality of the conduct, or did not know that it was wrong.

It should be noted that in some circumstances major forms of depression can lead to irrational and psychotic thinking, and therefore the offender can avail themselves of the insanity defence. For example, Andrea Yates from Houston, Texas was found not guilty by reason of insanity for the murder of her children on the basis that severe post-partum depression created an ‘irresistible impulse’ to commit the act. However, these circumstances are limited.

There are major issues with the current use of the M’Naughten Rules in common law countries and some good reasons why it should no longer remain the standard for insanity used in criminal trails.

The first issue, is that the rules have almost no reference to the kinds of considerations and diagnosis used in modern psychology. Despite claiming to define an a defence of ‘insanity’ -the rules themselves have very little relevance to the practice of diagnosing a mental disorder.

Secondly, the rules assume a level of control of behaviour that appears unlikely given new insights in neuropsychology. Indeed, in the words of neuroscientist David Eagleman if what is indicated by  the latest neuro-imaging research is accurate, we are all functionally suffering from a condition of legal automatism.

Finally, – and this is particularly true for depression – the M’Naughten rules ignore the biological nature of mental illness. In particular they ignore  the large scale changes in brain function caused by mental illness, and their effects on personality, behaviour and cognition.

Below is a brief overview of the kinds of biological changes in the brain that occur when someone is suffering from major depression.  Clearly, this indicates that our existing standard of criminal insanity is out of touch with our modern understanding of the brain.


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