It is difficult to be certain about the numbers of people in contact with the criminal justice system who have a mental health need.

In 1998, the ONS report on prisoners in England and Wales found that in the 12 months before entering prison, about 20% of male prisoners and 40% of female prisoners (both remand and sentenced) had received help or treatment for a mental or emotional problem. When it came to receiving help in prison, the figure was about 15% for male remand and sentenced prisoners – and for women, 23% for remand prisoners and 30% for sentenced prisoners.

Two related studies were carried out using the same data:

• One focused specifically on women prisoners.
• Another focused on young offenders aged 16 to 20. This report highlighted a parallel distinction between morbidity among men and women in this age group. In the 12 months before entering prison, 11% of sentenced and 13% of remanded male young offenders had received help or treatment for a mental or emotional problem, compared with 27% of female young offenders. Almost 1 in 10 of the female sentenced young offenders reported having been admitted to a mental hospital.

However, in relation to police custody and courts there have only been a limited number of studies often based on a single court or a single custody centre. A report for the Office for Criminal Justice Reform (OCJR) looking at arrests in London and Devon and Cornwall estimated that 269,000 offenders are routinely identified with mental disorder at arrest (12% of all arrested offenders) with 40% of this group (108,000) going on to be charged, accounting for 14.7% of the charged offender population. It further estimated that 33% of all those serving community sentences (56,000) have identified mental disorder.

In respect of people subject to Part III of the Mental Health Act, in January 2010 the Ministry of Justice published the Statistics of Mentally Disordered Offenders 2008.