Policing and collective efficacy: A rapid evidence assessment

Collective efficacy is a neighbourhood social process that has important benefits for crime prevention. Policing is thought to be one antecedent to collective efficacy, but the mechanisms by which police activity and officer behaviour are thought to foster collective efficacy are not well understood. This article presents findings from a rapid evidence assessment conducted to take stock of the empirical research on policing and collective efficacy. Thirty-nine studies were identified and examined. Overall, trust in police was the aspect of policing most consistently associated with collective efficacy. There was also some evidence that community policing activities, such as visibility and community engagement, predicted collective efficacy. Police legitimacy, on the other hand, was relatively unrelated to collective efficacy: a finding which suggests perceptions of police linked to the ‘action’ of individual officers may be more enabling of collective efficacy than perceptions of the policing institution as a whole. Implications and directions for future research are discussed.


Introduction
Decades of research have shown that neighbourhoods high in collective efficacy -a construct that relates to social ties among neighbours, combined with a willingness to intervene to solve local problems -experience fewer crime problems (Morenoff et al., 2001;Sampson et al., 1997). When residents know and trust one another, and are motivated to take collective action, crime decreases. These findings have been replicated across different neighbourhoods, cities and countries (Burchfield and Silver, 2013;Mazerolle et al., 2010;Morenoff et al., 2001;Sampson et al., 1997;Sampson and Wikström, 2008;Wikströ m et al., 2012;Zhang et al., 2007). Furthermore, when collective efficacy is strong, it has been shown to mediate the relationship between concentrated disadvantage and crime (Browning et al., 2004;Sampson and Raudenbush, 1999). The positive effects of collective efficacy also extend to other outcomes, including substance abuse by adolescents (Erickson et al., 2012), self-rated health (Browning and Cagney, 2002) and parental monitoring (Rankin and Quane, 2002). Yet, despite the crime-reducing (and other) benefits of collective efficacy, little research has examined the factors that generate and sustain it over time (Hipp and Wickes, 2017;Wickes et al., 2013).
Policing is thought to be one antecedent of collective efficacy, but the mechanisms through which it is expected to facilitate and encourage collective efficacy are not well understood. We know that police activity and officer behaviour can have positive consequences for a range of citizen outcomes, including people's propensities to cooperate with police, comply with the law and engage in other pro-prosocial behaviours (Sunshine and Tyler, 2003;Tyler, 2011;Tyler and Fagan, 2008). When people experience fair process at the hands of police, and view them as trustworthy, legitimate authorities, they are more likely to comply and cooperate with the structures and rules the institution represents Jackson et al., 2013;Tyler and Huo, 2002;Walters and Bolger, 2019). But despite a wealth of evidence linking police behaviour and activity to outcomes like cooperation and compliance, we know little about whether and how police activity might be linked to neighbourhood social processes such as collective efficacy. If police can develop strategies that facilitate collective efficacy, then this would go some way to helping neighbourhoods develop better defences against crime.
This article presents the findings from a rapid evidence assessment (REA) conducted to take stock of the empirical evidence base on policing and collective efficacy. We systematically searched the literature with the following question in mind: what does the evidence base tell us about the relationship between policing and collective efficacy? Given the limited research on this topic, our scope for this review was broad. We searched for and included in the REA any papers that assessed the relationship between a measure of policing and a measure of collective efficacy, even if this analysis was not the central point of the paper, or if the relationship assessed was in the opposite direction (i.e. if collective efficacy was used as an independent variable; for example, to predict police legitimacy).
The main objective of this article is to provide a narrative summary of the empirical research linking policing and collective efficacy and to provide guidance for future research and inquiry. We proceed by outlining the concept of collective efficacy and discussing some of the problems with its measurement, before describing the proposed theoretical mechanisms linking policing and collective efficacy. We then detail the search strategy for this REA, discuss the findings from the review and provide directions for future research.

Conceptualising and measuring collective efficacy
The concept of collective efficacy was initially introduced in psychology as a way to explain group performance (Bandura, 1997). It refers to the collective sense of being able to accomplish a collective task. As Bandura (1982: 143) noted, 'perceived collective efficacy will influence what people choose to do as a group, how much effort they put into it, and their staying power when group efforts fail to produce results'. The concept was transplanted to the neighbourhood effects literature by Sampson and colleagues, who similarly conceptualised collective efficacy as a task specific property of groups, where neighbourhoods are the group of interest and the task is one of reducing crime and disorder through the provision of informal social control (Hipp and Wickes, 2017).
Informal social control relates to residents' willingness to act to address neighbourhood problems; for example, to break up fights or intervene if children are skipping school. There have been numerous discussions in the literature about how best to measure informal social control, not least because this type of behaviour can only occur if there is actually delinquent behaviour going on in a neighbourhood (Hipp, 2016). One approach used by researchers has been to measure the potential for informal social control by asking residents how likely they would be to engage in certain behaviours if the situation arose (e.g. 'how likely is it that you would intervene if . . . '; Warner, 2007). Individual responses from residents can then be combined at the neighbourhood level as a measure of the likelihood that residents in a given neighbourhood will engage in informal social control behaviour. Other studies have measured informal control behaviour by asking residents how often they have engaged in particular activities to address problems in their neighbourhood (Wells et al., 2006). Again, these responses can be combined into a neighbourhoodlevel measure of informal social control.
Most commonly, informal social control is measured by asking residents to report on what they think their neighbours might do in different scenarios (e.g. 'how likely is it that your neighbours would intervene if . . . '; Sampson et al., 1997;Wickes et al., 2013). Responses to these questions capture the extent to which residents expect others in their neighbourhood will engage in informal social control behaviour. When aggregated at the neighbourhood level, this measure can be conceptualised as the 'shared expectations' of informal social control (Morenoff et al., 2001;Sampson and Raudenbush, 1999).
Although some researchers conceptualise informal social control as being synonymous with collective efficacy (Hipp, 2016), other researchers suggest informal social control is only one part of the construct of collective efficacy. Social cohesion -which relates to the ties between neighbours and mutual trust -is thought to form the other component. Sampson and colleagues (1997: 918) define collective efficacy as 'social cohesion among neighbours combined with their willingness to intervene on behalf of the common good'. However, there is some debate in the literature about whether social cohesion and informal social control should be combined into a single construct ('collective efficacy') or treated separately. Although Sampson and colleagues (1997) originally proposed that social cohesion and informal social control combined to form the construct of collective efficacy, there is some evidence that social cohesion is distinct from informal social control. This evidence has shown, for example, that the two constructs are not always highly correlated (Horne, 2004), do not consistently load onto a single factor (Armstrong et al., 2015;Gau, 2014;Wickes et al., 2013) and that the causal relationship between them, and with downstream variables such as crime rates, might vary from context to context (Rhineberger-Dunn and Carlson, 2011).
In this review, we include studies that measure collective efficacy as either (a) informal social control or (b) a combination of informal social control and social cohesion ('collective efficacy'). We include studies that measure informal social control as shared expectations for behaviour (e.g. respondents' perceptions of the likelihood their neighbours would intervene), as well as those that use residents' own informal social control potential or behaviour, but only when these are aggregated at the neighbourhood level (e.g. we do not believe that an individual reporting they have, or would, intervene in a situation captures a neighbourhood's capacity for informal social control). We exclude studies that only measure social cohesion because: (a) informal social control is at the core of the concept of collective efficacy, and (b) social cohesion is a much broader (and non-task specific) concept that could have any number of antecedents and consequents other than those we are interested in here. 1

Policing and its role in building collective efficacy
Most of the research on collective efficacy has focused on its outcomes, rather than its antecedents. Given the crimereducing effects of collective efficacy, there are obvious benefits in understanding the factors that might generate and sustain collective efficacy over time. Policing is assumed to be one antecedent of collective efficacy (Sargeant, 2017), but the role of police in building collective efficacy has not been adequately explored empirically. Three potential mechanisms have been proposed in the literature: (a) that trust and confidence in police fosters collective efficacy; (b) that police legitimacy encourages collective efficacy; and (c) that certain policing strategies help build collective efficacy.
First, some scholars have suggested that trust or confidence in the police may encourage collective efficacy. When residents view the police as a capable and effective resource, believe they exercise their authority in a fair and just manner, and are consequently willing to call upon or otherwise invoke the police, they may be more inclined to take collective action to address neighbourhood problems and may feel more empowered 'to intervene when confronted with local acts of deviance' (Silver and Miller, 2004: 558). Conversely, when residents do not see the police as a viable resource, they may feel too vulnerable to intervene in neighbourhood issues, because they may see their own actions as both less effective and more risky (Drakulich and Crutchfield, 2013;Kubrin and Weitzer, 2003). In other words, when deciding whether or not to intervene, residents weigh up the potential costs and benefits of participating in collective action, and this process is strongly influenced by perceptions of formal social control efforts (i.e. the ability and readiness of the police to step in if necessary; Drakulich and Crutchfield, 2013). When police are not seen to be capable of controlling crime in their neighbourhood, or that they do not care about the people who live there, 'in a simple utility maximization sense this lowers the potential rewards of action relative to the costs or effort needed to take it' (Drakulich and Crutchfield, 2013: 385).
Ethnographic research has highlighted the extent to which residents depend on police to exercise informal social control (Carr, 2003). In what Carr referred to as the 'new parochialism', his study of a predominantly white working-class neighbourhood in the United States showed that residents depend on and act through agents of formal social control in their informal attempts to address crime. And in general, for residents to feel confident personally intervening in neighbourhood problems they need to trust that police are a reliable resource who will arrive quickly and effectively address the problem (Kochel and Weisburd, 2019).
An alternative argument proposed by some scholars is that when police are seen to be effective, residents will be less likely to exercise informal social control because they believe the police are capable of dealing with local issues on their own (Silver and Miller, 2004). Correspondingly, police ineffectiveness may actually encourage informal social control actions because residents react to perceived police deficiencies to instil order in their communities (Kochel, 2018a;Kubrin and Weitzer, 2003).
Second, researchers have suggested that police may facilitate collective efficacy through their legitimacy (LaFree, 1998). Legitimacy refers to the extent to which people believe police behave in an appropriate manner, and feel a normatively grounded obligation to obey police (Jackson et al., 2013). Through their role as moral guardians, the police construct and enforce shared norms and values, and provide guidance about appropriate and inappropriate behaviour (Kochel, 2012;LaFree, 1998;Triplett et al., 2003). When residents do not see the police as legitimate, the validity and force of these mutually shared norms and values diminishes, and residents are less willing to cooperate with officers, less willing to grant them discretion and even less likely to obey the law (Jackson et al., 2013;Sunshine and Tyler, 2003;Tyler and Fagan, 2008;Van Damme et al., 2013). Similarly, when police lack legitimacy, neighbourhood social processes may break down, such that residents 'struggle to develop a working trust and so cannot be confident that other neighbours will act in the best interest of the neighbourhood' (Kochel, 2012: 389). Conversely, when police are seen as legitimate authorities, shared norms and values are reinforced, and people are more willing to take pro-social action to uphold these values. Of course, police legitimacy is closely related to the first proposed mechanism -trust -and it is likely these concepts would interact with each other to foster collective efficacy.
The third proposed mechanism is that certain types of policing strategies -those that are place-based and involve police-community engagement -will increase collective efficacy within communities. Community policing (otherwise known as 'neighbourhood' or 'local' policing) is a law enforcement approach that emphasises community involvement in crime prevention and seeks to increase contact between the police and local residents (Gill et al., 2014). Community policing scholars argue that, if crime is the result of social disorganisation, policing strategies should seek to build and sustain vital social processes within neighbourhoods (Rosenbaum, 1987;Skogan, 1990). Community policing is expected to increase collective efficacy by providing more opportunities for residents to interact with one another, by increasing access to police resources and by stimulating 'self-help' within communities (Renauer, 2007;Sargeant et al., 2013;Scott, 2002). As a result of community policing -and the specific strategies it entails, such as neighbourhood watch meetings and other community events -residents are expected to build new social ties and expand their neighbourhood networks, develop mutual trust with other residents and become emboldened to work collectively with neighbours to solve local issues. In other words, police 'lay the foundation for enhanced social interaction' and stimulate residents to regulate conduct within their neighbourhoods (Kochel and Gau, 2019: 4).
Another aspect of community policing thought to contribute to collective efficacy is police presence. Similar to the proposed mechanism of perceived police effectiveness, it is anticipated that increased visibility will reassure residents of their safety and reduce fear of crime, allowing them to more confidently engage in their own informal social control behaviours (Kochel and Weisburd, 2019). However, although one of the expected outcomes of community policing is a reduction in residents' levels of fear of crime, there have been mixed findings about whether increased police presence does have this effect (Gill et al., 2014). In an experimental study, Van de Veer and colleagues (2012) found that fear of crime reduced when police were present in an 'unsafe' environment, but increased when they were present in a 'safe' environment, suggesting that neighbourhood context is an important consideration in understanding the potential effects of police presence on collective efficacy.
Although in this section we have presented possible mechanisms for how policing might foster collective efficacy, collective efficacy has also been theorised in the literature as a predictor of trust and legitimacy (Jackson and Bradford, 2009;Jackson and Sunshine, 2007;Jackson et al., 2013;Kochel, 2018b;Nix et al., 2015). Scholars have argued that neighbourhood context plays an important role in shaping attitudes toward police. In what has been termed the 'neo-Durkheimian' model, confidence and trust in the police is thought to be shaped more by 'day-to-day' concerns about social order and control, than instrumental concerns about safety and crime (Jackson and Bradford, 2009a;Jackson and Sunshine, 2007). In other words, people look to the police to defend and uphold the moral structure of society, and they lose faith and confidence in the police when community values, beliefs and norms are seen to be deteriorating. Of course, it is likely that, in reality, any association between police activity and collective efficacy is bi-directional.

Rapid evidence assessment
Despite much theorising about the proposed mechanisms linking policing and collective efficacy, there has been no systematic review of the evidence base. This article presents the findings from an REA of the empirical evidence linking policing and collective efficacy. It is important to acknowledge at the outset that an REA is no substitute for a systematic review or meta-analysis. Although an REA shares many of the same features as a systematic review (e.g. transparent search criteria, strict inclusion/exclusion criteria), it is not as extensive or exhaustive as a systematic review and there is more room for bias. For example, an REA can be carried out within a much shorter timeframe and there is less focus on the overall quality of the source material. Further, unlike a meta-analysis, it is not possible to draw conclusions about overall effect sizes from an REA. Nonetheless, the aim of this article is to take stock of the evidence base and to provide a narrative summary of the findings from which a systematic review and metaanalysis could follow. The main research question guiding this REA is: what does the evidence base tell us about the relationship between policing and collective efficacy?

Method
The search strategy used in this study involved keyword searches of six relevant electronic databases, including dissertation databases. 2 Literature searches were first conducted on 1 August 2019 and updated on 4 September 2020. Search terms were modelled around the two key areas of interest: policing and collective efficacy (see Table 1 for search terms).
Records identified through the database searches were then sifted to identify relevant studies. In selecting the studies, the following inclusion criteria were used: a. The study must report the findings of a quantitative empirical research project exploring the relationship between a measure of policing and a measure of collective efficacy; 3 b. Be available in English; c. Have been published from the year 2000 onwards.
EPPI-Reviewer 4 software 4 was used for all information management in this REA. Studies were first screened on title and abstract to remove any that were ineligible, based on the above inclusion criteria. Full-text screening was then conducted, and the inclusion criteria were again applied to exclude ineligible studies. 5 During the full-text screening, each study's reference list was also screened to identify any relevant studies that did not appear in the database search. These studies were then screened, and the inclusion criteria applied. The screening process and final studies included are shown in Figure 1.

Results
A total of 39 studies were included in this REA (see Supplementary Appendix for descriptive information about each, along with a summary of the main findings related to the current research question). The studies were predominantly from the United States (n ¼ 24, 62%), with others coming from the United Kingdom (n ¼ 5, 13%), Trinidad and Tobago (n ¼ 4, 10%), South Korea (n ¼ 2, 5%), Taiwan (n ¼ 2, 5%), and one each from Australia and Turkey.

Research designs
Research designs were predominantly cross-sectional (n ¼ 31, 79%), with only three studies (8%) using a longitudinal survey design. The remaining five studies (13%) were field experiments testing the effect of a policing intervention on residents' perceptions of collective efficacy. All five field experiments included a control group, and four of five randomly allocated to conditions. Three of the field experiments used a panel survey design (e.g. the same residents were surveyed before and after the intervention and change in residents' responses in the intervention site was compared with a control site), one used a longitudinal (but not panel) survey design, and one included a post-test survey only (e.g. residents in the intervention site were surveyed after the intervention and their responses were compared to residents in a control site).
Survey respondents were predominantly residents of a particular neighbourhood (n ¼ 36), with two studies focusing on neighbourhood leaders (n ¼ 2) and one on college students (n ¼ 1). Overall, survey sample sizes tended to be large. Across all studies, the mean sample size was 3,665 respondents (SD ¼ 6,397; range 77-38,346).

Measurement of key constructs
Collective efficacy. Just over half of studies included a measure of informal social control (n ¼ 22, 56%), whereas 44% (n ¼ 17) included a measure of collective efficacy (a composite of social cohesion and informal social control). 6 Informal social control was predominantly defined as people's perceptions of the likelihood their neighbours would intervene to solve neighbourhood problems (n ¼ 36, 92%). Three studies measured informal social control as respondents' own action or intention to intervene (aggregated at the neighbourhood level). In all composite measures of collective efficacy, social cohesion centred around the concept of trust and shared values with neighbours. Collective efficacy was included as a dependent variable in 21 studies (54%), and an independent variable in 18 studies (46%).
Policing. The policing measures included in studies were predominantly different aspects of people's perceptions of the police in their neighbourhood. There was considerable variation in the way the policing constructs were conceptualised and measured, as well as variation in what the constructs were called (e.g. trust, confidence, satisfaction, performance effectiveness, police competence, police service quality). To make sense of the different measures, and to ensure we were comparing like with like, we assessed the individual items included in the policing measures and grouped them according to their content (see Table 2 and Supplementary Appendix). As shown in Table 2, police perceptions typically centred around four main areas: (a) trust (i.e. whether people see the police as a capable and effective resource, and believe they exercise their authority in a fair and just manner); (b) legitimacy (i.e. whether people feel an obligation to obey police and feel a normative alignment of values); (c) community policing (i.e. whether the police engage with the community and provide a visible policing presence); and (d) a global

Area Keywords
Police ("Police" OR "Policing" OR "Law enforcement") Collective efficacy ("Collective efficacy" OR "Social cohesion" OR "Informal social control") measure of confidence/trust (i.e. one item capturing overall trust or confidence). In addition to these measures, five field experiments tested the effect of a policing intervention on collective efficacy. Of the policing interventions tested, three were of some form of community policing intervention (Kochel and Weisburd, 2019;Lombardo and Donner, 2018;Tuffin et al., 2006), one was of an intervention specifically designed to increase collective action and collective efficacy , and one was of a broken windows style intervention (Weisburd et al., 2011).

Analytical approaches
Analytical approaches, on the whole, tended to be sophisticated. Of the 34 (non-experimental) survey-based studies, 22 (65%) used multilevel modelling -where individual residents were nested within neighbourhoods -or structural equation modelling. Another 11 studies (32%) used multivariate linear, ordinal or logistic regression. Of the 33 studies using regression-based approaches, all studies included multiple control variables in their models, including: respondent demographics (e.g. age, gender, ethnicity, income); neighbourhood variables (e.g. structural disadvantage, crime rate, population density); psychological variables (e.g. fear of crime, perceived safety); and experiences of crime and the police (e.g. victimisation, contact with police).

Relationship between policing and collective efficacy
This next section provides a narrative summary of the findings from the REA, specifically focusing on the relationship between policing and collective efficacy. The findings are organised based on the policing measure included in the study. The five field experiments are discussed separately at the end of this section.
Trust. Trust in police was the most common measure of policing included in studies. On the whole, studies that included a measure of trust tended to find positive associations with collective efficacy. For example, using three waves of resident surveys from Trinidad and Tobago, Kochel (2018a) found a direct positive relationship between trust (a combined measure including aspects of procedural justice, community engagement and effectiveness) and collective efficacy. A number of cross-sectional studies also found relationships in the expected directions, with higher levels of trust predicting higher levels of informal social control and collective efficacy (Drakulich and Crutchfield, 2013;Kochel, 2012Kochel, , 2013Pabayo et al., 2020;Pattavina et al., 2006;Sargeant, 2017;Silver and Miller, 2004;Warner and Burchfield, 2011;Yun, 2009). Trust was included as an independent variable in these studies, suggesting that trust in police leads to higher levels of collective efficacy. Studies that included trust as a dependent variable also tended to find positive associations between collective efficacy/informal social control and trust (Bradford et al., 2009;Jackson et al., 2013;Kirk and Papachristos, 2011;Kwak and McNeeley, 2019;Lai, 2016;Lai and Zhao, 2018;Maxson et al., 2003;Rodrigues, 2010), suggesting that the relationship between these variables may be bi-directional (although the cross-sectional nature of the data makes it impossible to be sure).
Most of these studies measured trust in police effectiveness and/or trust in procedural justice, suggesting that perceptions of collective efficacy are related not only to whether residents feel the police are a capable and effective resource, but also to whether they believe police exercise their authority in a fair and just manner. Two studies that included trust as an independent variable, and three that used trust as a dependent variable, found no association between trust and collective efficacy/informal social control (Baek et al., 2020;Cantora et al., 2019;Correia, 2010;Renauer, 2007;Warner, 2007), but by far the majority of studies found significant and positive relationships between these variables.
Legitimacy. Six cross-sectional and two longitudinal studies included a measure of legitimacy. In contrast to the findings for trust, these studies tended to show no association between legitimacy and collective efficacy. For example, using legitimacy as an independent variable, both Kochel (2012, 2018a) and Sargeant (2017) found no significant relationship between perceptions of police legitimacy (obligation to obey and/or moral alignment) and collective efficacy. The authors suggested these results could indicate that perceptions of the policing institution as a whole (legitimacy) may have less relevance to neighbourhood social processes than perceptions of the actions of individual officers. This assertion is consistent with the literature on trust presented above: perceptions of how the police act and behave significantly predicted, and were predicted by, collective efficacy.
Other studies by Karakus (2017) and Kochel (2018b), this time using legitimacy as a dependent variable, found no direct association between collective efficacy/informal social control and legitimacy. However, in Kochel's (2018b) study, there was a significant interaction with perceived victimisation risk, which suggested that when residents feel at risk and neighbourhood capacity is weak, residents may depend more on police and grant them more legitimacy. Three studies had somewhat different results, finding positive associations between legitimacy and collective efficacy (Jackson et al., 2013;Lammers, 2019;Schuck, 2019), but, overall, the empirical literature shows that legitimacy has less promise as a mechanism to foster collective efficacy.
Community policing. Five studies included perceptions of community policing, which were most commonly perceptions of how visible police are and how well they engage with the community. Findings here were generally positive. For example, Kochel and Gau (2019) used a panel community survey and tested the impact of perceptions of police visibility and police-community engagement on perceptions of informal social control. They found that both satisfaction with police visibility and police-community engagement (at Wave 1) were significant predictors of social cohesion (at Wave 2) and, through social cohesion, these policing measures indirectly predicted informal social control (Wave 3). Further, using multiple waves of survey data, Schuck (2019) found that police presence was significantly related to higher neighbourhood levels of informal social control (although this finding became non-significant once willingness to work with the police was added to the model). Jackson and Sunshine (2007) also found a positive association between collective efficacy and perceptions of police engagement in the local community.
Two studies had somewhat contrasting results, but both used surveys of neighbourhood leaders as opposed to residents (Renauer, 2007;Scott, 2002). Both studies measured three aspects of police-community engagement: police encouragement (the extent to which police are involved in community meetings); resident involvement in problem-solving (the extent to which police and residents work together); and police accessibility. In the first study, only police accessibility was related to collective efficacy (Scott, 2002). In the second, the frequency of attendance at community meetings was related to informal social control, but in a negative direction (Renauer, 2007).
Global measure of confidence or trust. The last policing measure included in studies was a global perception of trust or confidence in the police, which was typically measured with just one question (e.g. 'How good a job do you think the police in your area are doing?'). There were mixed findings. Some studies found a significant association between collective efficacy and people's overall impressions of the police (Drakulich and Crutchfield, 2013;Jackson and Bradford, 2009a;Kochel and Gau, 2019;Nix et al., 2015), whereas others found no association (Cantora et al., 2019;Park et al., 2020;Renauer, 2007). One further study found that 'satisfaction with police' was positively associated with taking an informal social control action (e.g. talking to neighbours about a problem), compared with a formal action (e.g. contacting the police; Wells et al., 2006). Given the broad nature of the measures used, the findings from these studies are less informative about what exactly it is about police activity and behaviour that might encourage collective efficacy.
Field experiments. Three field experiments tested the impact of a community policing intervention on collective efficacy. Two of these interventions found significant positive effects. First, Kochel and Weisburd (2019) tested the impact of two types of community policing approachescollaborative problem-solving and police presence (directed patrol) -on collective efficacy. The authors found that, over time, an increase in police presence promoted modest improvements in collective efficacy (Kochel and Weisburd 2019). They found the impact of increased police presence first benefited informal social control in the short term, then social cohesion in the long term. Second, Lombardo and Donner (2018) tested the effect of the Chicago Alternative Policing Strategy (CAPS) on informal social control. The authors found the presence of community policing led to increased levels of informal social control, but this effect was mediated by satisfaction with police, suggesting an indirect effect of the CAPS programme on informal social control. The third community policing intervention found no significant effect on collective efficacy (Tuffin et al., 2006).
Two other field experiments were included in this REA. The first tested an intervention specifically designed to increase collective action and collective efficacy at crime hot spots (Assets Coming Together; Weisburd et al., 2020) via three primary mechanisms: establishing proximal relationships with and between residents; increasing trust between police and residents; and developing shared expectations that empower residents to take action. The authors found the intervention had little impact on collective efficacy, although it did increase citizen reports of participation in collective actions (e.g. collaboration in problem solving). The second field experiment tested a broken windows style intervention, also in crime hot spots, and the authors found no significant impact of the intervention on levels of collective efficacy (Weisburd et al., 2011).

Discussion
The main purpose of this article was to take stock of the evidence base on the relationship between policing and collective efficacy. It aimed to provide a narrative summary of the literature and go some way to furthering our knowledge about whether, and how, policing can foster important social processes within neighbourhoods. A total of 39 studies were included in the review; however, only five of these studies used an experimental design. The vast majority of studies were cross-sectional, limiting the conclusions that can be drawn from this review about causal processes. Furthermore, there was huge variation in how policing was conceptualised and measured across studies. Nevertheless, some promising findings emerged.
The most compelling evidence from the REA related to trust in police, particularly across the dimensions of effectiveness and procedural justice. A strong majority of studies that included these measures found positive associations. These findings indicate either that (a) people's trust in police informs their beliefs about collective efficacy, or (b) people's views about (or experiences of) collective efficacy shape how they view police. The crosssectional nature of many of the studies makes it impossible to determine the direction of the relationship, and indeed both may be true at the same time. Two longitudinal studies found evidence for the former assertion: people's views about police tactics and procedural justice significantly predicted collective efficacy measured at a later point in time (Kochel, 2018a;Kochel and Gau, 2019). These findings seem to suggest that when people feel the police are an effective and supportive resource, they may be more inclined to take collective action. We found no support for the alternative argument proposed by some scholars that police effectiveness will have a negative relationship with informal social control because belief that the police are capable of dealing with issues will lead to inaction by residents (Kubrin and Weitzer, 2003;Silver and Miller, 2004).
In contrast to the findings for trust, we found little compelling evidence that police legitimacy fosters collective efficacy. Although legitimacy has been shown to have positive effects on a range of individual outcomes, such as people's willingness to cooperate and comply with police (Sunshine and Tyler, 2003;Tyler, 2011;Tyler and Fagan, 2008), because collective efficacy is, in criminological research at least, a construct of neighbourhoods, how people see individual, local, officers may be more important than how they view the police institution (Kochel, 2018a;Sargeant, 2017). And this is what the research seems to suggest: trust (whether people view police to be effective and procedurally fair) had a much more consistent association with collective efficacy than legitimacy. It may also be the case that trust in police is more strongly linked to 'action'; in other words, whether an individual believes officers would turn up and try to help if they called them and, crucially, their willingness to act on this belief. Indeed, this comes very close to the increasingly widely accepted definition of trust: a willingness to be vulnerable to another premised on beliefs about their competency and good intentions (i.e. action based on expectations that the trustee will behave in a dependable, predictable manner; Hamm et al., 2017;Jackson and Gau, 2016). Expectations about (potential) police action may be more enabling of engagement in informal social control than a sense of duty toward police generated by legitimacy. Further research is needed to confirm this hypothesis.
The evidence relating to community policing was mostly positive, and the strongest methodologically. For example, two studies using longitudinal panel surveys (Kochel and Gau, 2019;Schuck, 2019) found positive associations between aspects of community policing (e.g. community engagement and police presence) and collective efficacy. Furthermore, two of the three field experiments testing the impact of a community policing intervention found significant and positive effects on collective efficacy (Kochel and Weisburd, 2019;Lombardo and Donner, 2018). There is a relative scarcity of policing field experiments that use collective efficacy as an outcome measure, compared with other outcomes, such as crime, perceived disorder, fear of crime, citizen satisfaction and legitimacy of police (Braga et al., 2019;Gill et al., 2014;Hinkle et al., 2020;Mazerolle, 2013). More of these types of studies are needed to understand the links between police activity and behaviour, and social processes within neighbourhoods, such as collective efficacy.
An important issue arising from this review was the variation across studies in the conceptualisation and measurement of the policing constructs. The extent of this variation made the task of summarising and comparing these studies difficult. Trust in police was variously conceptualised as perceptions of fairness, as perceptions of effectiveness and, in some cases, as a global measure of trustworthiness (i.e. how much do you trust the police). No studies measured trust as expectations about police action (i.e. a willingness to be vulnerable), despite this being an increasingly widely accepted definition (Hamm et al., 2017;Jackson and Gau, 2016). Police legitimacy was also conceptualised in a number of ways in the studies reviewed: as perceived obligation to obey (Sargeant, 2017), as a combination of moral alignment and obligation to obey (Jackson et al., 2013), as confidence in police and police accountability (Schuck, 2019), and as trust and satisfaction with various police organisations (Karakus, 2017). That there was no consistent definition of legitimacy is a feature of ongoing debates in the literature about the appropriate operationalisation of this construct (see Cao and Graham, 2019;Jackson and Bradford, 2019;Trinkner, 2019 for a recent discourse centred on competing views over how to conceptualise and measure legitimacy).
There was more consensus in the studies reviewed about the measurement of informal social control and collective efficacy. Most studies measured informal social control as residents' perceptions of the likelihood their neighbours would intervene to solve neighbourhood problems. Only three studies measured informal social control differently, as residents' own informal social control potential or behaviour. 7 There have been many discussions in the literature about the best way to measure informal social control (Gau, 2014;Hipp, 2016). Although the most common approach, it has been argued that getting residents to answer questions about what their neighbours would do is difficult if they have never seen their neighbours in a type of situation where they would be required to exercise informal social control before (e.g. in neighbourhoods with low levels of crime and disorder; Hipp, 2016;Hipp and Wo, 2015). This is also true when asking respondents to determine their own potential for informal social control. Future research should continue to refine the measurement of informal social control, and to consider how best to capture a neighbourhood's capacity to address local issues.
Lastly, it is important to recognise the different policing jurisdictions and cultural contexts across the studies included in the review. Although the majority of research was conducted in the USA, two-fifths of studies took place in countries that have objectively different policing styles and different historical relationships between the public and police from the USA (e.g., UK, Turkey and Trinidad and Tobago). Further, collective efficacy may function differently in collectivist versus individualist societies, both of which were included in this REA (e.g., Taiwan and South Korea versus the USA and UK). However, in this review we were interested in the relationship between policing and collective efficacy, which was found to be remarkably similar across contexts.

Limitations and future research
Of course, there are a number of limitations to this article that should be acknowledged. First, although both academic and dissertation databases were searched, there was no rigorous search of the grey literature. The studies that were included from the grey literature (Maxson et al., 2003) were mostly found through searching the reference lists of other studies. As a result, there may be some relevant research that is missing from this review. Second, a handful of studies included in the REA used the same data set as another study in the review (Kochel, 2012(Kochel, , 2013(Kochel, , 2018bKochel and Gau, 2019;Kochel and Weisburd, 2019); the findings from these studies can therefore not be considered unique.
Another limitation that should be acknowledged is that most of the studies included in the REA measured perceptions of police instead of actual police activity. How people feel about the police in their neighbourhood is an important consideration, but to understand the specific policing strategies that bolster collective efficacy, a more objective measure of policing is needed. Studies that measure actual aspects or types of police activity (e.g. the number or visibility of patrols in an area, level of police-community engagement) and link these to outcomes are few and far between. As stated previously, randomised field experiments that manipulate the quantity and type of policing should be a priority for future research. In particular, research is needed that measures the length of time it takes for changes in local policing strategies to affect collective efficacy.
Lastly, there are limitations to the review method used in this study. Although rapid evidence assessments share many of the features of a systematic review (e.g. transparent search criteria, strict inclusion/exclusion criteria), there is inherently more room for bias in a REA than a systematic review. Caution should therefore be taken when interpreting the findings. As a next step, we plan on carrying out a systematic review to follow on from this article. The systematic review will involve a more comprehensive search of the literature and a more thorough appraisal of the studies. If the data allows, a meta-analysis will also be conducted to provide an empirical conclusion about the relationship between policing and collective efficacy.

Conclusion
This REA sought to take stock of the evidence base on the relationship between policing and collective efficacy, and to provide a narrative summary of the literature. Findings from this review seem to suggest that perceptions of police linked to the 'action' of individual officers are more compelling enablers of collective efficacy than perceptions of legitimacy. When people feel the police are a trustworthy and effective resource -and when they are visible and engage with the community -perceptions of collective efficacy are higher. The implications of this seem to suggest that any benefits on collective efficacy of changes to policy or practice are likely to come from the actions and behaviour of individual officers, as opposed to the policing institution as a whole. Future research should prioritise field experiments and longitudinal methodologies to capture the causal mechanisms of police activity on collective efficacy. A systematic review of the literature will be an immediate next step.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
This work was supported by the first author's Economic and Social Research Council (ESRC) New Investigator Grant [ES/ S010629/1].

Supplemental material
Supplemental material for this article is available online.

1.
We also exclude studies that measure informal social control as whether residents would call the police to report crime (Goudriaan et al., 2006), although some scholars argue that calling the police can also be a form of informal social control (Carr 2003). 2. ASSIA, Criminal Justice Database, ProQuest Dissertations and Theses, Scopus, Sociological Abstracts, Web of Science. 3. Studies were included if they measured informal social control or collective efficacy (a combined measure of informal social control and social cohesion). We included studies that treated collective efficacy as an independent or a dependent variable. Qualitative studies were originally included in the selection criteria but only two qualitative studies were identified, and both had methodological problems. 4. This is a web-based software program developed by the Social Science Research Unit at the Institute of Education, University of London. https://eppi.ioe.ac.uk/CMS/Default.aspx? alias¼eppi.ioe.ac.uk/cms/er4&. 5. Studies where a full-text version was not available were also excluded. There were only two instances where a full text was not available, both of which were doctoral theses. In addition, two doctoral theses were excluded because the results of the dissertation had been written up and published as a journal article (which was included in the review). 6. Fourteen studies included a separate measure of social cohesion, but these findings are not reported here. 7. There were a handful of studies which were excluded from this REA because they measured respondents' informal control potential or behaviour without aggregating it at the neighbourhood level.