Blame The Brain?

Simon Rippon wonders why we think it's normal to say diseases can mitigate legal responsibility for crimes, but the structure of your brain itself cannot:

[C]onsider what the brain is: it is, essentially, a biological machine; 100 billion nerve cells living in a chemical soup and firing electrical impulses at each other. And in years to come, as neuroscience improves and expands our knowledge of the brain, we may reach the stage where your lawyer will be able to explain any particular criminal misjudgement as a result of this-or-that chemical overdose or deficit, this-or-that badly routed synapse, the growth of this-or-that cell, or – perhaps – this-or-that quantum random occurrence.

How will we respond to these future lawyers? What’s the difference between the idea that this-or-that bit of your brain (albeit perhaps a microscopic bit) made you irrational, and the idea that a large tumour made you irrational? A tumour is not some alien invader: it is a proliferation of your own cells. Is it, then, size that matters here? Surely not!