Transparency data

The impact of overcrowding on assaults in closed adult public prisons

Published 19 June 2025

Applies to England and Wales

1. Rationale

Qualitative research has shown that overcrowding can worsen living conditions, disrupt daily routines, and put a strain on relationships between prisoners and staff – all of which are known to increase the risk of violence.

To better understand the relationship between overcrowding and prison violence, analysis was conducted to estimate its impact on a prisoner’s individual likelihood of being involved in an assault.

This document provides a summary of the analysis, conducted on the closed adult prison estate (male and female). The analysis looked at assaults and overcrowding in the closed estate during 2022 and the key finding is:

• Prisoners in overcrowded cells are 19% more likely to be involved in an assault over a one-year period than those in cells that are not overcrowded. This difference is statistically significant, with the true figure likely to lie between 15% and 24%.

2. Background: Trends in overcrowding and assaults

The percentage of prisoners living in overcrowded conditions in adult closed prisons (male and female) has fluctuated between 2012 and 2024 (Figure 1) and varies between prisons of different categories ^{[footnote 1]}. The highest and lowest levels of overcrowding were seen in 2015 with 28% and in 2021 with 22%, respectively. Although overcrowding levels are not currently at their highest point, they have been increasing again since 2021.

Assault rates have generally increased over the same period, except for a decrease shortly before and during the COVID-19 pandemic (2019-2021) ^{[footnote 2]}.

Figure 1: Trends in assault rates and percentage of the prison population living in overcrowded conditions – Adult closed estate only, 2012-2024

While overcrowding and assault rates do not rise and fall in perfect sync (which is expected, as many factors contribute to violence ^{[footnote 3]}), they often move in the same direction. This suggests a link between the two.

There has been limited research on this issue, so this analysis was carried out to help fill that gap.

3. Data

The analysis uses data from public sector prisons in 2022, taken from the Ministry of Justice’s (MoJ) prison case management system and its administrative Human Resources (HR) system.

Further information on prison crowding and assaults can be found as part of the annual HMPPS Digest and the quarterly Safety in Custody Statistics publication, respectively.

4. Methodology and Findings

Overcrowding is assessed based on whether the number of prisoners in a cell exceeds the Certified Normal Accommodation (CNA)^{[footnote 4]} for that cell. To understand the effect overcrowding has on causing assaults we worked with subject matter experts to first map out the different factors that could influence assaults, using diagrams called directed acyclic graphs (DAGs)^{[footnote 5]}. The DAGs identified that, by accounting for prison population size, staff levels, and sentence lengths we can better measure the true effect of overcrowding on assaults.

The DAGs allowed us to identify which variables should be used in our statistical analysis, to isolate the causal relationship between overcrowding and assaults. To quantify this impact, we used survival modelling^{[footnote 6]}, and compared the likelihood of involvement in an assault over a 12-month period between two groups of prisoners:

• Those in an overcrowded cell
• Those not in an overcrowded cell

This method allowed us to test whether the probability of an individual surviving the period without being involved in an assault differs amongst the two groups of prisoners, whilst controlling for factors other than overcrowding.

Applying this analysis to data from January to December 2022 within the closed estate we found that prisoners in overcrowded cells were 19% more likely to be involved in an assault over a 12-month period compared to those in non-overcrowded cells. At a 95% confidence level, the range of increased likelihood was between 15% and 24%.

As illustrated in Figure 2, the disparity in assault likelihood between overcrowded and non-overcrowded prisoners increases over time.

Figure 2: Probability that a prisoner is not involved in an assault over time, in overcrowded and non-overcrowded conditions

5. Limitations

While this analysis provides valuable new insights, there are limitations that should be considered when interpreting findings.

Data

The data which underpins this release is extracted from departmental operational systems, which are relied upon by MoJ for effective offender management. As with any large-scale recording system the data is subject to possible errors with data entry and processing.

A potential limitation arises from 2022 data being used for the analysis because the COVID-19 National Framework was still in place in some prisons until May that year. This means that crowded prisoners’ opportunity to assault may differ from other years during those months.

Our analysis concentrated exclusively on the closed estate. Assault rates in the open estate are significantly lower, as offenders must undergo a risk assessment before being transferred to open conditions. This process limits the number of offenders eligible for open facilities, resulting in minimal crowding. Given these factors, our research focused on the closed estate, where overcrowding and assault rates are markedly higher.

Methodology

It is possible that the production of the DAGs did not capture all the influencing factors, which means that the underlying effect of overcrowding on assaults may be biased. However, the DAGs were consistent with previous literature on drivers for assaults.

Contacts

Media contacts

Press enquiries should be directed to the Ministry of Justice press office.

020 3334 3536

Statistical contacts

Other enquiries about these statistics should be directed to the Prisons Directorate at the Ministry of Justice: Davies-Garton.Jobshare@justice.gov.uk

Ministry of Justice
Prisons Directorate:
7th Floor
102 Petty France
London
SW1H 9AJ

General enquiries about the statistical work of the Ministry of Justice can be e-mailed to: statistics.enquiries@justice.gov.uk

1. In the UK prison system, prisons are categorized based on security levels and risk, with the primary categories being A, B, C, and D. Category A prisons are the highest security, while Category D prisons are open prisons. Women’s prisons are also a separate category. ↩

2. See Safety in Custody Statistics, England and Wales: Deaths in Prison Custody to March 2025 Assaults and Self-harm to December 2024 - GOV.UK for details of assault rates over time. ↩

3. See Understanding prison violence: a rapid evidence assessment - GOV.UK. ↩

4. Certified Normal accommodation (CNA) is the official, uncrowded occupancy limit a prison cell can hold. Procedures for reviewing CNA are published in this policy document. ↩

5. A DAG is a simple and transparent way to identify and demonstrate knowledge, assumptions and theories about the causal relationship. Tennant, et al. Use of directed acyclic graphs (DAGs) to identify confounders in applied health research: review and recommendations. International Journal of Epidemiology. 2021;50(2):620-632. https://doi.org/10.1093/ije/dyaa213 ↩

6. Survival analysis was used as it allows us to understand the differences in time-to-assault incident for each condition. Survival analysis acknowledges that if an individual has not been involved in an assault, this may still happen after the end of the study period. ↩