Sometimes it is clear that an individual cannot be held responsible for their actions, however, inexplicable that action is. This might even be the case with murder.
A horrific murder took place in St Ives in 1907. A baby was found decapitated in a tray of water in the town, and suspicion fell on a local ‘idiot’ boy, Thomas Polmear.
Polmeor, described by The Times as a ‘demented lad’, was taken to the police station, where he confessed to the crime. His confession made explicit his learning difficulties:
“Me put baby in tray of water. Me washed baby’s legs. Me cut baby’s head off. Baby kicked cradle and cried. Baby teasy old thing. Won’t do it anymore, policeman.” 
Polmear clearly failed to realise how decapitating the child would be seen by others. In his mind, he was simply playing with it, and responding to the baby’s ‘teasing’. He thought that simply promising not to ‘do it anymore’ would be enough to enable him to leave the police station.
The defendant appears to have been Thomas Paynter Polmear, who would have been 13 at the time of the crime, which fits with the description of him as a ‘lad’.
He was born in the first quarter of 1894, baptised at St Ives on 19 May 1894 and in the 1901 census he is listed as living in Back Road, St Ives, the son of Peter, a mariner. Although the 1901 census makes no reference to him being ‘demented’, the 1911 census is more explicit.
This shows that Thomas was sent to an asylum after admitting killing the unnamed baby. The newspapers do not appear to have covered the case apart from the one reference above; if the offence had reached the Cornwall Assizes, he would have been found not guilty of murder by reason of insanity, under the terms of the 1883 Trial of Lunatics Act.
If he HAD been found guilty, he could have been given the death sentence despite his age – it was not until the following year, 1908, that the Children’s Act was passed, setting out that only those over the age of 16 could be executed.
In 1911, he is listed as a patient of the Cornwall County Lunatic Asylum on Whiteheath Road in Bodmin, and in the infirmity field of the census it is recorded that he was ‘imbecile from birth’.
Thomas died in 1938, aged 41. His death was registered in Bodmin, so it is likely that he spent the rest of his life in the lunatic asylum.
1: The Times, 6 April 1907