In 2007 the United Kingdom amended Mental Health Act legislation. This was driven by a public outcry over homicides perpetrated by mentally ill offenders. There was a very large and unsuccessful campaign against these changes led by the Royal College of Psychiatrists. One of the central objections to the changes was the belief that the amended Act would introduce new criteria for detention that would result in a rise in the number of detentions.

To measure whether there was any change in outcomes associated with the amended criteria for detention.

Data is routinely collected in the county of Norfolk on over 99% of all assessments for possible detention. Data covering the period 2001 – 2011 covering 11505 (7885 pre amendments and 3620 post amendments) individual assessments for possible detention was analysed as part of the Norfolk Mental Health Act Assessments Outcomes Study. Outcomes measures were the number of admissions and the number of detentions that followed from Mental Health Act assessments.

There was a highly significant 27% rise (p 0.000) in the proportion of people detained and a 16% fall (p 0.000) in the proportion of people admitted voluntarily following Mental Health Act assessments under the new amended Act.

The changes to the law appear to have led to approximately 33 people being detained every year in Norfolk who would not previously have been detained. If extrapolated to the whole of England this would amount to over 1500 additional annual detentions.