Devices worn by officers that collect audio and video evidence, usually during police encounters with the public.
Focus of the intervention
Body-worn cameras (BWCs) are small recording devices worn by officers, typically attached to their clothing or headwear. They can collect audio and video evidence, usually during police encounters with members of the public, such as verbal statements from suspects and victims, body language and behaviour, and footage of crime and anti-social behaviour as they occur. BWCs have been increasingly used in policing to improve accountability and transparency during police-citizen encounters, review the quality of interactions, as well as capture video evidence that may strengthen investigations and prosecutions. Guidance relating to BWC deployment and use differs across forces and jurisdictions.
What research is this summary based on?
This updated crime reduction toolkit summary is based on the findings of three systematic reviews. The reviews cover a range of outcomes relating to crime, criminal justice and police activity.
- The overall findings of this summary in relation to the effects of BWCs on crime are drawn from Review one, which consists of a meta-analysis combining 30 primary studies. 17 of these studies were conducted in the USA, two in the UK and one in Uruguay. The locations of the remaining 10 studies were not reported. The primary crime outcome reported by the review was assaults and resistance against police officers (15 studies). The review also examined the effect of BWCs on arrests (13 studies) and non-crime outcomes including officer use of force, complaints against officers, calls for service and stop and search.
- Review two is a systematic review covering 12 eligible studies, of which 11 were included in meta-analyses. The studies were carried out in the USA (six studies), the UK (five studies) and Australia (one study). The review examined the effects of BWCs on downstream criminal justice outcomes measured using police or court data. The primary outcome reported by the review was prosecution and charging decisions (based on eight studies). The review also measured the effects of BWCs on convictions, guilty pleas, all guilty outcomes (convictions and guilty pleas combined) and case processing times.
- Review three contributes additional information to the Mechanism, Implementation and Economic cost sections of the summary. This review covered 11 primary studies: six from the USA and five from the UK.
Effect – how effective is it?
- The research suggests that BWCs do not have a clear, statistically significant effect on crime. Primary studies have shown both increases and decreases in crime and criminal justice-related outcomes.
Review one
The primary crime outcome reported by Review one related to assaults on police officers, officer injuries and resistance. The meta-analysis did not find a statistically significant difference in assaults or resistance against officers with BWCs compared to those without.
Statistically significant results were found for two of the 12 reported outcome measures: complaints against police and minor non-traffic summary offences (for example, littering and drunk and disorderly behaviour). Complaints against police were 17% lower for those with BWCs compared to control groups without BWCs.
For minor non-traffic summary offences, officers with BWCs were significantly more likely to write citations (similar to penalty notices or warnings) than those without BWCs. This finding must be viewed with caution as it was only examined in two studies.
For all other outcomes including arrests and officer initiated calls for service, results were inconclusive with non-significant results.
In relation to study design, the review found that RCT studies were less likely than quasi-experimental studies to find that BWCs reduced arrests or increased calls for service.
Review two
Review two found no statistically significant effects, overall, for the criminal justice outcomes it examined.
However, a meta-analysis found a non-significant increase in prosecutions/charging decisions associated with BWCs, convictions, guilty pleas, guilty outcomes and in case processing time associated with BWCs. This indicates BWCs may help cases move towards a criminal justice outcome. However, since the overall results were not statistically significant, the evidence is too uncertain to conclude BWCs improve criminal justice outcomes overall.
Review two also examined whether findings differed by study design or context. It found that studies using weaker, quasi-experimental designs produced significantly larger effects than those using stronger, experimental designs. This matters if weaker studies represent a relatively large proportion of those in the review, as they may over-estimate the effects of BWCs on outcomes.
How robust is the review evidence?
- The quality of review methods used to assess the impact of body-worn cameras on crime was very strong.
Review one was sufficiently systematic that most forms of bias that could influence the study conclusions can be ruled out.
The evidence is taken from a systematic review covering 30 studies. The review had a well-designed search strategy, calculated effect sizes appropriately, assessed variability between studies and considered publication bias.
Some risks of bias were identified at the individual study level. These included baseline differences between groups, and potential risk of bias in outcome measurement. This could be caused by officers under- or over-reporting items such as use of force incidents, depending on whether they were assigned to the treatment or control group.
Review two used several good-quality methods. However, several sources of potential bias remained among the included primary studies. For example, Review two noted that most of the quasi-experimental studies used unmatched designs, often comparing incidents with and without BWCs with limited information on whether those incidents were truly comparable. The review also noted that half of the studies reported implementation problems, and that several studies risked treatment contamination because officers with and without BWCs sometimes attended the same incidents. A major caveat of the review was that the effects of crime type and study design could not be easily distinguished. All the quasi-experimental studies in the review were of domestic violence cases, so it was not possible to separate the effects of crime type from research design. Another limitation identified by the review was most studies could not tell whether BWC footage was captured or used in criminal justice processes.
Although Review three was systematic, many forms of bias that could influence the study conclusions remain.
Mechanism – how does it work?
- The quality of evidence in the reviews to assess how body-worn cameras work was low.
The reviews suggested several potential mechanisms by which BWCs may affect crime and other outcomes but did not empirically test them.
Review one suggested that the use of BWCs increased self-awareness brought about by the knowledge that police interactions with members of the public are being recorded and watched. This is thought to have a deterrent effect on excessive use of force by police, as officers know that they are being recorded and therefore exercise restraint.
From a member of the public’s perspective, the fact that they are being recorded causes them to moderate their behaviour during encounters with police and not exhibit socially unacceptable behaviour.
Review two suggested that BWCs may work by improving the quantity and quality of criminal evidence available to prosecutors and courts. For example, cameras may capture injuries, statements, demeanour, behaviour and details of searches or arrests that might not be fully recorded in written reports alone. The review highlighted that this type of evidence could be especially useful in cases where proof is often difficult to establish, such as domestic violence, drug and alcohol offences, or offences committed by or against police officers. It also noted, however, that an increase in the quantity and quality of evidence could potentially benefit either side in a criminal case (defence or prosecution), and that increased quantity of evidence can potentially slow down criminal justice processes.
Review three suggested that, in addition to the theories of self-awareness and deterrence, BWCs may also have an impact by assisting evidence gathering. They could support memory of key events as officers’ memory retention may be affected by the experience of a traumatic event. This improved evidentiary capture could lead to increased early guilty pleas.
However, none of the three reviews directly tested these proposed mechanisms. Discussion of mechanisms therefore remained largely theoretical.
Moderators – in which contexts does it work best?
- The quality of evidence in the reviews to assess the contexts where body-worn cameras might work best was very strong.
Review two compared the effect of BWCs across several important contexts, including crime types, crime severity and geographic locations. The clearest finding was that results differed significantly by crime type. Across nearly all outcomes (except prosecution and charging decisions), effect sizes were considerably larger in studies focusing on domestic violence, as opposed to studies focusing on aggregate crime or other crime types. However, since all the domestic violence studies also used quasi-experimental designs, it is not possible to tell whether the size of the effects for domestic violence were due to the study design used.
No clear moderator effects were found for crime severity or geographic location. The effect of BWCs did not differ in any significant way by seriousness of offence, nor between studies from the United States, the United Kingdom, and Australia.
Implementation – what can be said about implementing this initiative?
- The quality of evidence in the reviews to assess how to implement body-worn cameras was low.
Review three provided a limited account of how the intervention was implemented to ensure BWCs were used in line with requirements. One study described how resistance to using BWCs was overcome by daily reinforcement, and compliance was best maintained when a senior officer was present.
Review three found that police perceptions of BWCs were generally positive.
Review two identified several practical issues in the underlying studies that may affect implementation, evaluation or the credibility of results:
- BWC discomfort, poor video quality and technological malfunctions were reported in several studies.
- Low or selective activation and usage rates meant BWCs were not always switched on or used consistently.
- Treatment contamination was a recurring problem, with officers with and without BWCs sometimes attending the same incident.
- Many quasi-experimental studies used unmatched designs, making it difficult to know whether BWC and non-BWC incidents were comparable.
- The review highlighted a major practical gap: most studies could not show whether BWC evidence had been captured, used or thought to have been useful in charging, plea or court decisions.
Overall, the review identified barriers that could weaken the apparent effect of BWCs, but it did not test implementation strategies or provide actionable guidance on the best way to roll cameras out.
Economic considerations – how much might it cost?
- The quality of evidence in the reviews to assess how much body-worn cameras might cost was low.
None of the systematic reviews conducted formal economic analysis. However, the primary studies included in Review three did mention costs. One study reported a £50,000 saving in court, prosecution and police costs over the course of two trials, resulting in an estimated £125,000 saving per year. However, the author notes that considerable caution should be taken when considering these cost estimates.
General considerations
- Most studies in the reviews were from the United States, with fewer from the United Kingdom and other countries. Caution is therefore needed when applying the findings to other contexts, for example, countries with different legal systems, regulatory requirements or caseload pressures.
- Small sample sizes and wide variation in effects between studies meant Review two could not attribute effects to BWCs.
- Findings in relation to domestic violence should be treated with caution, as Review two was not able to tease apart the effect of crime type from study design.
- A major limitation identified by Review two was that the studies tended not to report whether BWC footage had been captured by officers and used during prosecution or court decision-making. This makes it hard to judge whether null findings reflect an ineffective intervention or limited use of the evidence.
Summary
- Overall, the evidence suggests that body-worn cameras do not have a consistent effect on crime.
Evidence from Review one indicates that BWCs do not have a significant impact on crime-related outcomes including assaults and resistance against police officers. While Review two suggests that BWCs may help cases move towards a criminal justice outcome, the available evidence is too uncertain to indicate a positive effect on criminal justice outcomes in general.
In terms of non-crime outcomes, evidence from Review one suggests that use of BWCs can reduce complaints against officers.
Reviews
Review one
Reference
Lum C and others. (2020).’ Body-worn cameras' effects on police officers and citizen behavior: A systematic review’. Campbell Systematic Reviews, volume 16, article e1112.
Review two
Reference
Petersen K and Lu Y-F. (2023). The downstream effects of body-worn cameras: A systematic review and meta-analysis. Justice Quarterly, volume 40, issue 6, pages 765–790.
Review three
Reference
Cubitt T and others. (2017). Body-worn video: A systematic review of literature. Australian & New Zealand Journal of Criminology, volume 50, issue 3, pages 379–396.