There are a number of defences in English criminal law (the US ones are not all the same), a bit like German Rechtfertigungsgründe and Schuldausschließungsgründe.
Some of them can be pleaded as defences to all offences (general defences). For instance, self-defence (Notwehr) is a general defence that applies to several offences against the person or against property – but actually not against all offences. Insanity is a general defence, but the consequences of being found insane are not much fun, so it’s only ever pleaded against murder.
Then there are special defences: diminished responsibility and provocation can only be pleaded if murder is charged. Those are also called partial defences, because if the defendant is successful in pleading them, the consequence will be a conviction for manslaughter, not an acquittal.
Some defences in the news recently: The ‘canoeist’s wife’ Anne Darwin pleaded that her husband forced her to lie about his death, relying on the defence of marital coercion. See Times Online article. Whether or not Anne Darwin would have had to be in her husband’s presence for the defence to apply, it was widely reported that she was ‘no shrinking violet’ and possibly ‘wore the trousers’ herself.
Today, the UK Ministry of Justice has published Murder, manslaughter and infanticide: proposals for reform of the law – the proposals include:
To abolish the existing partial defence of provocation and replace it with
new partial defences of:
o killing in response to a fear of serious violence; and
o (to apply only in exceptional circumstances) killing in response to words and
conduct which caused the defendant to have a justifiable sense of being
• To make clear that sexual infidelity on the part of the victim does not
constitute grounds for reducing murder to manslaughter.
• To remove the existing common law requirement for loss of self-control in
these circumstances to be “sudden”.
As the law stands, it has often been criticized that a man who lost his temper and killed his (stereotypically) smaller and weaker wife could plead provocation for this sudden loss of control, whereas a woman who was terrorized by her husband and gradually became depressed would be taken to have full malice aforethought. In fact, the courts are aware of the problem and do not now tend to convict wives of murder in this situation. See Guardian article. There is also a proposal to change the defence of diminished responsibility.