DPP v Heffernan: Burden of proof for defence of diminished responsibility is the balance of probability

Here, the Supreme Court held that “establish” in s 6(2) of the Criminal Law (Insanity) Act 2006 requires a murder accused to prove on the balance of probability that, at the time the offence was committed, they suffered from a mental disorder which justified a finding of not guilty of murder by reason of diminished responsibility.


Criminal Law (Insanity) Act 2006

6. – (1) Where a person is tried for murder and the jury or, as the case may be, the Special Criminal Court finds that the person –

(a) did the act alleged,

(b) was at the time suffering from a mental disorder, and

(c) the mental disorder was not such as to justify finding him or her not guilty by reason of insanity, but was such as to diminish substantially his or her responsibility for the act,

the jury or court, as the case may be, shall find the person not guilty of the offence but guilty of manslaughter on the ground of diminished responsibility.

(2) Subject to section 5(4), where a person is tried for the offence specified in subsection (1), it shall be for the defence to establish that the person is, by virtue of this section, not liable to be convicted of that offence.



The Central Criminal Court convicted Heffernan of the murder of Eoin Ryan at Ennis, County Clare in June 2011. At trial, Heffernan admitted to causing Ryan’s death but raised a defence of diminished responsibility under s 6(2) of the 2006 Act. The trial judge instructed the jury that the burden of proof for that defence is the balance of probabilities. Heffernan appealed his conviction to the Court of Appeal on grounds that the trial judge had misdirected the jury: he claimed that the defence of diminished responsibility is proven where the accused raises a reasonable doubt. The Court of Appeal held that the word “establish” in s 6(2) means that the accused bore the burden of proofit is not enough that the accused raise a reasonable doubt.

Heffernan applied to the Supreme Court for a further appeal on the correct interpretation of the burden of proof. Granting leave, the Court determined that he had raised an issue that may arise in other cases and that was of general public importance.


Supreme Court

Dismissing the appeal, O’Malley J wrote the judgment for the Court; Charleton J wrote a concurring judgment (here).

On appeal, Heffernan argued that s 6 must be interpreted in light of the general rule that the prosecution must prove guilt and that the accused need not prove anything. Where an accused raised a reasonable doubt concerning an s 6 defence the prosecution was obliged to prove otherwise. The alternative interpretation would breach the presumption of innocence, as a jury may be obliged to convict even where they had a reasonable doubt about guilt.

The Court rejected that argument. Firstly, the prosecution must prove all the elements of a murder conviction; thereafter, s 6 “creates a new, mitigatory defence which reduces the consequences of a proven offence” [64]. It does not affect the presumption of innocence [65]. And there are good policy reasons to justify the higher standard of proof: the subjective nature of the defence, the fact that the accused cannot be compelled into a medical examination and because the prosecution would need to introduce expert medical opinion to prove the accused did not suffer from diminished responsibility [66].

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