On 20 January 1843, Daniel MNaghten, a lathe worker from Glasgow came to London to kill the prime minister. Suffering under the delusion that he was being persecuted by spies in the control of Sir Robert Peel, he was desperate to put an end to his misery, but in a case of mistaken identity, MNaghten shot the prime minister’s private secretary, Edward Drummond, who later died of his injuries.

When MNaghten was questioned at Bow Street police court the following day, he claimed that the Tories had made him do it (no comment!). His statement was later reported in the press: ‘the Tories in his native city had driven him to this, and had followed him to France, Scotland and other parts, and he could get no sleep from the system they pursued towards him. They had driven him into a consumption’ (Freeman’s Journal, 6 Mar 1843). MNaghten was detained at Bethlem and later moved to Broadmoor, when the hospital opened in 1864.

At his trial at the Old Bailey, Daniel MNaghten was found not guilty, by reason of insanity. Following public outrage, a House of Lords committee was convened to assess the verdict. The resulting ‘MNaghten Rules’ were a measure of ascertaining whether a defendant was sane or insane, following the premise that a defendant may not be able to distinguish right from wrong and therefore, could not be held culpable for the consequences of his/her actions. If offenders were judged to be ‘insane’, then they were labelled ‘criminal lunatics’ and detained ‘at Her Majesty’s pleasure’. The modern defence of ‘diminished responsibilty’ evolved from this mid-Victorian precedent.

Although the MNaghten Rules changed the law regarding pleas of insanity, there have always been cases where defendants have claimed madness or the ‘influence of evil spirits’ as their motivation for crime and I have uncovered some good examples, both in Detective Caminada’s casebook and my own family tree to share with you at a later date. But, in the meantime Chris Martin, head of David Cameron’s Private Office, (not the guy from Coldplay!) would be wise to watch his back…