3 December 2012 Last updated at 14:44 The number of children arrested by the police has fallen by a third since 2008, figures compiled by a charity suggest. Police arrested 209,000 under-18s in 2011, down from 315,923 in 2008. The Howard League for Penal Reform said 10 forces in England and Wales had halved the number of children taken into custody. The Association of Chief Police Officers said the detention of children was authorised for a number of reasons. The figures, compiled through a freedom of information request to all 43 police forces in England and Wales, include more than 2,100 children aged 10 - which is the age of criminal responsibility - and 11. The greatest fall in the number of children arrested in England and Wales was in Dorset, where police held just over 1,000 17-year-olds and under - about 70% fewer than in 2008. Other forces that had more than halved the arrest rate were Humberside, Hertfordshire, Suffolk, West Midlands, West Mercia, Gloucestershire, North Wales, Warwickshire and Lancashire. The Metropolitan Police arrested almost 40,000 juveniles in 2011, down 10,000 on 2008. Four other forces arrested more than 10,000 under-18s: Hampshire, West Midlands, West Yorkshire and Greater Manchester. In 2008, there were 10 forces which had arrested more than 10,000. Low-hanging fruit The director of campaigns at the Howard League, Andrew Neilson, said a move away from a mandatory target system for police was likely to be the main reason for the third successive fall. He said: "In 2008, the first year we have data for, the target culture was coming to an end. At that time, police services were being ranked on the number of arrests they were processing, and to a degree children were low-hanging fruit that helped the police reach their arrest targets. While some police services still use these targets informally, we are seeing a move back towards a discretion-led, common sense approach, and that's reflected in the figures. I suspect there are a lot of things that the police are doing to reduce arrests, ranging from restorative justice disposals to triage arrangements at police stations, where a youth offending team worker will interview a young person to see if there are any welfare issues. An Association of Chief Police Officers spokesman said: "As with adults, detention of children in custody is authorised for a number of reasons, including to further a criminal investigation, to uncover the identity of any suspects or because the disappearance of that person would hinder any prosecution. The rules for the detention of suspects are set down in law and on every occasion must be authorised by a custody officer. Detentions of both children and adults in police custody are reviewed regularly to ensure that they are being held in accordance with the law and not for any longer than required for police investigations. The latest Crime Survey for England and Wales, released by the Office of National Statistics, found that crimes recorded by the police fell by 6% for the year ending in June, compared with the previous year.