Firearms and violence in Europe-A systematic review.

BACKGROUND: Higher availability of firearms has been connected to higher rates of interpersonal violence in previous studies. Yet, those studies have focused mainly on the United States, or used aggregated international data to study firearm violence. Whether those aggregated findings are applicable to understanding the phenomenon in continental Europe specifically remains unclear. The aim of this systematic review is to bring together all studies that exclusively use European data.
METHODS: Nine databases were searched, resulting in more than 1900 individual studies. These studies were assessed on relevance and eligibility for this study, based on their title, abstract and full text. Information on study characteristics, operationalizations of main concepts and study results were extracted from the six eligible studies.
RESULTS: Four studies assessed the impact of firearm restrictive regulations on the rate of firearm homicides. Two other studies correlated rates of firearm availability and -violence. Results vary: some studies show a clear decline once availability of firearms is restricted, while others indicate a limited effect on only a very specific subgroup, such as female victims, or national guards with weapons at home. Moreover, studies used various operationalizations for firearm availability, thereby decreasing the comparability of findings.
CONCLUSION: Empirical research exclusively using European data is still lacking. To increase comparability of future studies, methodological inconsistencies and regional gaps need to be overcome. Assessing how firearm availability can be measured with reliable and valid proxies across countries will be a crucial first step to improve future research on the link between firearms and firearm violence.

Introduction

It is estimated that around 7000 people (0.9 per 100.000 population) die of gunshot wounds each year in continental Europe, including suicides, unintentional accidents involving firearms, and criminal homicides [1]. Although many types of weapons can cause death or bodily harm, firearms are of specific interest when studying violence, due to their high lethality, widespread use on a global scale and value for criminals [2]. How firearms are linked specifically to violent death has been studied extensively, but existing studies heavily focus on the US context, where more than 12 people per 100.000 population die of gunshot wounds each year [3, 4]. Most research to date focuses on the firearm availability hypothesis, which assumes that an increase in firearm availability leads to an increase in violent crime [5-7]. Yet, even with these existing studies, as causal links between the prevalence of firearms and violence remain unclear, heated discussions on the connection between the two phenomena continue both in academia and beyond [8].

The question arises whether findings from US-based studies are applicable to other global regions, such as Europe, given variations in existing gun cultures and firearm legislations. In contrast to the US, where the right to own firearms is implemented in the Constitution under the Second Amendment, European countries have strict regulations that mostly ban civilians from bearing guns, with only few exceptions. Member states of the European Union follow the same framework of regulations regarding civilian access to firearms, which leaves room for proportionate national variations between the member states [9, 10]. Such national variations between member states seem irreconcilable, given the difference in gun culture not just on a global scale, but also amongst European countries, as exemplified by the challenging approval of the 2017 Firearm Directive in the European Council in which Poland and the Czech Republic voted against the amendment for being too strict, whereas Luxembourg wished for harsher restrictions [10]. In addition, Europe does not only differ significantly from the US in terms of existing gun cultures and firearms legislation [11], but also in terms of other cultural and socio-economic factors, such as the overall crime rate or levels of inequality [12]. Such factors have been found to act as moderators in violent crime rates [13, 14]. As these factors vary across countries, the validity of findings from US studies for the European context could be questioned. Therefore, a review of empirical studies testing a potential link exclusively based on European data is required.

With this systematic review, we aim to inspect existing studies that empirically examine the link between firearm availability and firearm-enabled interpersonal violence in Europe, since such an assessment does not yet exist to the best of our knowledge. To increase comparability of studies and their findings, we focus particularly on criminal forms of interpersonal violence, thus excluding firearm-enabled suicides or accidental fatal and non-fatal injuries. With the findings of our study, we seek to inform researchers, practitioners and policymakers in the domains of public health and criminal justice about the current state of knowledge regarding the association between firearm availability and violence. Furthermore, we aim to identify the lacunae of knowledge and the methodological challenges which can be addressed by future research.

Methods

This study made use of PRISMA guidelines for conducting systematic review [15].

Eligibility criteria

For this systematic review, we included studies that empirically examined a potential link between firearm availability and firearm enabled crimes, including homicide, non-fatal assaults or robberies. Studies that focused on the relationship between firearm enabled crimes and other factors, such as mental illness, but included a measurement for firearm availability were eligible as well. We excluded non-criminal forms of violence, such as suicide by gunshot or accidental shootings as we expected that those forms of violence could have underlying explanatory variables that are different to criminal forms of violence and should therefore be studied separately. Furthermore, we excluded studies that did not examine the link between measurements of those two main concepts in a statistical manner to eliminate potential subjectivity from our synthesis of findings. Studies that did not explicitly differentiate firearms from other types of weapons (e.g. knives) were also excluded to enhance comparability of the eligible studies, but when a clear differentiation was provided, we included the studies.

Concerning our geographical focus, we only included studies that are based on continental Europe (excluding Turkey and Russia, which are countries situated on two continents). US-based studies and studies that combined data from non-European and European origin, which inhibited us to assess data sources independently, were not further examined. When we were able to isolate the results of European data from non-European data, we included the studies.

All studies published after 1991 were selected, as 1991 marks the year the first European directive in firearm acquisition and possession was introduced in the European Union [16]. This cut-off was chosen as it can be expected that most countries on the European continent would have been following similar definitions of and regulations for firearm restrictions since then. Our expectations were that this directive might have led to empirical cross-national studies relevant for this research. Studies were excluded when they were published before 1991 or when they used data only from years before 1991. Included studies had to be written in English, German or Dutch, due to the researchers’ capabilities of understanding those languages.

Data sources

Peer-reviewed academic articles, books, book chapters, and (unpublished) doctoral dissertations were included. We searched seven databases that cover academic studies in the relevant disciplines of public health, sociology and criminology: Criminal Justice Abstracts, Embase, MedLine, PsyInfo, PubMed, Sociological Abstracts and Web of Science. In addition, we searched ProQuest and EThOS for relevant (unpublished) doctoral dissertations. If results from a dissertation were also published in the form of an academic article, the latter was chosen. Additional records were found by searching the bibliographies of relevant studies. To overcome the danger of publication bias, we emailed relevant researchers in the field and conducted a web search using Google and Google Scholar search engines on 21 April 2020 to find grey literature, such as research reports. The results of those searches were filtered based on the eligibility criteria.

Search process

The search queries used to find relevant studies include terms concerning firearms, their prevalence, and firearm violence. Previous published systematic reviews on firearm availability in the non-European context served as an inspiration for the chosen search terms (2,6). In particular, three specific search queries were used in each of the nine databases:

◦ firearm AND availability AND violen*

◦ (firearm OR gun) AND (availability OR access) AND (violen* OR homicide)

◦ (firearm OR gun) AND (availability OR access) AND (violen* OR crim*)

Study selection

Databases were searched between February and April 2020, using the three search queries. As shown in Fig 1, a total of 8,179 studies in the nine databases were found and reduced to 238 studies by elimination of duplicates and screening of titles and abstracts on relevance. The full texts of the remaining 238 studies were assessed on relevance using the eligibility criteria. Authors of studies missing full text were emailed with the question to provide full access to their study, resulting in ten additional studies that were included in the assessment. For 23 studies, the author did not respond, or no contact details were available or found online. In total, 232 studies were excluded during that process because they missed full-text (n = 23), discussed irrelevant topics (n = 14), did not focus exclusively on Europe (n = 107), missed empirical data (n = 45), or data measuring either firearm-enabled crimes (n = 13) or firearm availability (n = 30). As a result, six studies were included in the synthesis of results for this systematic review (Fig 1).

Fig 1

Flow chart of systematic search.

The quality of each eligible study was assessed based on six factors which have been used in a previous, similar systematic review about firearm laws and firearm homicides in the US [17]. Next to the study design, the authors of that review evaluated studies based on five additional measurements:

“(1) Were appropriate data source(s) and outcomemeasure(s) used for the study question?

(2) Was the time frame studied adequate (eg, sufficient surveillance before and after a law)?

(3) Were appropriate statistical tests used?

(4) Were the results robust to variations in the variables and analyses?

(5) Were the disaggregated data and results of control variables consistent with the literature?” [17 p108].

A study’s quality was qualified as good when they scored high on all those elements, and as fair when they scored high on three to four of the factors. Studies were considered poor if they scored on only two or less factors. We have followed the same quality assessment.

Results

Study characteristics

In total, only six studies remained eligible for this study. The background information on each of those studies can be found in Table 1.

Table 1

Characteristics of included studies, including study locations, time frame studied, study design, sample, quality and type of study.

Study	Location	Time Frame	Self-reported Study Design & Sample	Quality Assessment	Type
Duquet & van Alstein (2015) ‘Firearms and violent death in Europe’ [1]	Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, FYR Macedonia, Malta, Moldova, Montenegro, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, United Kingdom	2007	Cross-sectional correlational design; 33 European Nations	Fair	Report
Gjertsen et al. (2014) ‘ Mixed impact of firearms restrictions on fatal firearm injuries in males: a national observational study‘ [18]	Norway	1969–2009	Cross-sectional time series design; 434 cases of male accidental and homicidal deaths caused by firearms	Fair	Journal Article
Hurka & Knill (2018) ‘Does regulation matter? A cross‐national analysis of the impact of gun policies on homicide and suicide rates’ [19]	Austria, Belgium, England and Wales, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland	1980–2010	Cross-sectional time series design; 16 West European Nations	Good	Journal Article
Kapusta et al. (2007) ‘ Firearm legislation reform in the European Union: impact on firearm availability, firearm suicide and homicide rates in Austria’ [20]	Austria	1985–2005	Longitudinal time series design; Firearm homicide rate 1985–2005	Fair	Journal Article
Killias & Haas (2002) ‘ The role of weapons in violent acts: Some results of a Swiss national cohort study ‘ [21]	Switzerland	1997	Cross-sectional correlational design, 21.315 surveys by Swiss male soldiers	Fair	Journal Article
König et al. (2018) ‘ Austrian firearm legislation and its effects on suicide and homicide mortality: A natural quasi-experiment amidst the global economic crisis’ [22]	Austria	1985–2016	Cross-sectional time series design; firearm homicide rate 1985–2016	Fair	Journal Article

Five studies were published in peer-reviewed journals [18-22] and one report was published by researchers associated with the Flemish Peace Institute in Belgium [1]. Two studies were published between 2000 and 2010 [20, 21]. Four studies were published between 2010 and 2020 [1, 18, 19, 22]. Four of the six studies limit their research to examining the link between access to firearms and firearm-enabled violence to one country [18, 20–22]. Those studies originate from Austria (n = 2) [20, 22], Norway (n = 1) [18] and Switzerland (n = 1) [21]. The other studies include data from 16 [19] up to 33 countries [1]. Four studies use non-aggregated data [18, 20–22], whereas the two other studies rely on aggregated data from the national level, namely national homicide rates [1, 19]. All studies have scored as either fair or good in the study quality assessment.

Outcomes

An overview of the operationalizations used for firearm availability and violence, as well as the outcomes of each of the studies can be found in Table 2. The outcomes are presented in three subsections: results from national studies on the impact of firearm legislation of -violence, cross-national studies regarding such an impact and correlational studies.

Table 2

Study design, level of analysis, operationalizations for gun violence and availability and outcome of included studies.

Study	Study Design	Level of Analysis	Operationalization (Gun) Violence	Operationalization Gun Availability	Outcome
Duquet & van Alstein (2015) [1]	Cross-sectional, correlational analysis	Cross-National, aggregated data	Gun related homicide according to WHO codes	Civilian firearm ownership rate	The higher the availability of firearms, the more women are killed by firearms; other relationships statistically insignificant
Gjertsen et al. (2014) [18]	Cross-sectional time series analysis	National, non-aggregated data	Accidental + homicide firearm related deaths according to WHO codes	Firearm restricting regulations	Removing firearms from private homes of National Guards associated with reduced firearm homicides
Hurka & Knill (2018) [19]	Cross-sectional time series analysis	Cross-National, aggregated data	Gun related homicide according to WHO codes	Firearm restricting regulations	Stricter firearm policies associated with less firearm- and non-firearm homicides
Kapusta et al. (2007) [20]	Cross-sectional time series analysis	National, non-aggregated data	Gun related homicides	Number of firearm licences	Stricter firearm policies is associated with a decrease of firearm licenses and gun homicides in Austria
Killias & Haas (2002) [21]	Cross-sectional multivariate regression analysis	National, non-aggregated data	Self-reported purposeful infliction of injury to another person	Number of handguns owned; Number of rifles owned; Frequency of carrying a weapon	Both owning and carrying a firearm increased the risk of injuring another person intentionally.
König et al. (2018) [22]	Cross-sectional times series analysis	National, non-aggregated data	Gun related homicides	Number of firearm licences	Stricter firearm policies is associated with a decrease of firearm licenses and gunhomicides in Austria

National studies on the impact on firearm legislation of violence

Of the six included studies, four discuss the impact of legislations that restrict or limit the access to firearms on firearm-related violence on a national level [18, 20–22]. Two studies [20, 22] assess the impact of a singular law: a 1997 law adapted in Austria, that introduced stricter controlling mechanisms, such as background checks and mental health checks, the process of obtaining a firearm, as well as safe storage regulations. Both studies use official statistics as data sources and operationalize the measurements of availability of firearms and firearm-enabled violence using the same proxies: number of licenses for firearms and homicide by gunshot respectively. Consequently, the outcome of the studies regarding the impact of the law after 1997 on national firearm homicide rates are similar: both studies report a decline of firearm homicides post-regulation ranging from 9,6% [22] for the period of 1998 to 2008 to 9,9% [20] between 1998 and 2005. The change in rates is statistically significant in both of the studies. Additionally, König et al. examine the percentage of firearm enabled homicides in relation to the total number of homicides and found a decrease from 9% in 1998 to 2,6% in 2008 [22]. After 2008, the authors see the decline reversed into an increase of firearm homicides. They ascribe this rise to the economic crisis, which in turn led to an influx of migrants and an increase of issues of firearm licenses. Kapusta and colleagues also report a decline in firearm licenses after 1998 [20]. As none of the confounding variables used in both studies–unemployment rate, alcohol consumption and proportion of young men–show any significant effects, authors of these two studies cautiously provide support for the theory that higher availability of firearms is connected to increased violence.

Gjertsen et al. examine the impact of four different singular laws in Norway that were implemented between 1986 until 2003, aimed at regulating access to firearms by implementing new tests for hunters, permissions by police as a requirement for obtaining a shotgun, safe storage regulations and the removal of military firearms from private homes [18]. Because of a low incidence of female victims in relation to firearm deaths, the authors examined only male firearm deaths (N = 276) between 1969 and 2009. The only significant findings in relation to firearm homicides are found after the implementation of the latest firearm restricting regulation in 2003, that removed military firearms from private homes. After the implementation of the regulation, firearm homicides decreased by 64%. Rates of non-firearm homicides show no significant changes in the same period. The authors warn that their findings should not be overinterpreted, as the law under investigation targeted a very specific subgroup of Norwegian National Guards storing their weapons at home, and other factors not included in the analysis, such as changes in perceptions and behaviour regarding gun safety could have had an additional effect on fatal firearm violence in Norway.

Cross-national study on the impact of firearm legislation on violence

Hurka and Knill assess a potential impact of firearm regulations regarding availability and firearm enabled violence [19]. They examine firearm homicide in 16 Western European countries over a timespan of thirty years. In contrast with the previously discussed studies, Hurka and Knill created an index to assess the level of restrictiveness in firearm regulations across the 16 countries included in the sample, thereby using an aggregated measure of firearm control policies, rather than assessments of individual regulations [19]. Through a cross-sectional time-series analysis, the authors conclude that the higher the level of restrictiveness regarding firearm control, the fewer firearm and non-firearm enabled homicides take place. They further specify that a difference of one value on their firearm control index (ranging from one to three) makes a difference of 0.2 homicides per 100.000 citizens a year. The control variables unemployment and urban population showed an increasing (0.01) and decreasing (-0.0) effect on firearm homicides respectively. Finally, they not only conclude that more restrictive firearm policies are associated with fewer homicides committed with firearms, but also that it is not likely that potential homicide offenders would switch to a different kind of weapon, given that stricter gun policies also appeared to have a decreasing effect on the overall homicide rate.

(Cross-) national correlational study

The remaining studies that fit our eligibility criteria do not assess the impact of firearm regulations but use a proxy for firearm availability to assess its effect on firearm-enabled violence using correlational analyses. Duquet and van Alstein include data from 2007 for a broad sample of 33 European nations [1]. Data on firearm homicides are derived from the WHO’s Detailed Mortality Database, whereas estimates for civilian firearm ownership from the Small Arms Survey serve as a proxy for firearm availability. The correlational analysis between civilian firearm ownership and firearm homicides reveals a moderate positive, yet statistically insignificant, relationship–both for firearm homicides and the overall homicide rate. Only when distinguishing between gender of firearm homicide victims, a significant moderate relation exists, suggesting that the greater the availability of firearms, the more women are killed by firearms within the countries under study. The authors suggest that this shows a specific effect of firearm availability on fatal domestic violence [1]. Other explanatory variables that could affect firearm homicides were not included in the analysis.

Killias and Haas, on the other hand, used confidential survey responses from a Swiss sample of male army recruits to assess whether owning a handgun or rifle, and carrying a weapon on a regular basis had an impact on committing a violent act against another person [21]. The descriptive results indicate that participants in the survey who own a handgun have injured (10,7%) and shot at other people (4,4%) more often than non-owners of handguns (2,1%; 0%). Logistic regression analysis further supports the hypothesis that an increased number of handguns owned significantly increases the likelihood of inflicting injury on someone else intentionally (OR = 1,024). Analyses also reveal that every additional handgun owned raises the risk for violent incidents by 60 percent. Again, similar to previous studies presented in this review, the authors suggest that restricting access to firearms might lead to an overall decrease of violence committed with this type of weapon [21].

Conclusion and discussion

This systematic review sought to assess all studies that statistically examine a potential link between the prevalence and availability of firearms with criminal forms of violence committed with firearms. A search of nine relevant databases revealed six studies that matched all eligibility criteria for this review. Four of these studies examined the impact of firearm-restricting regulations on the rate of firearm homicides, and two others used proxies for firearm availability to conduct correlational analyses with rates of firearm and non-firearm homicides.

Findings differ: whereas a firearm restricting regulation in Austria was associated with a decrease of almost 10 percent in firearm homicides in the following 10 years [20, 22], Norwegian scholars concluded that similar laws had little to no significant effect on a specific target group of male National Guards who stored their firearms at home [18]. Hurka and Knill’s comparative analysis of Western European nations show that more restrictive regulations concurred with less firearm homicides, as well as a lower homicide rate overall [19]. In a correlational study, Duquet and van Alstein found no significant correlations between those two variables, except for female victims with fatal gunshot wounds [1]. Similar results were found by Killias et al. who sought to correlate homicide data with ownership rates of firearms from European and other nations [23]. Yet, the study by Killias and Haas reveal that owning a handgun significantly increases the risk of committing violent offences. Even more so, their data show that owning several handguns further elevates the risk by 60 percent per handgun owned [21].

One explanation for these varying results lies in the lack of comparability between the studies. International frameworks for firearm legislations, such as the firearm directives set out by the European Union [24], do exist. However, differences in implementations of those regulations on a national level, as well as cultural- and socio-economic backgrounds with regards to the use of firearms and crime in general lead to incongruent legislations across nations in Europe [25]. Another factor inhibiting the comparability of these studies is the range of operationalizations for availability of firearms. Even though various firearm directives by the EU encouraged all Member States to file and register civilian firearm ownership, European nations do not have identical systems to register legal firearm possession by civilians [26]. Moreover, in cases where reliable national registers exist, the prevalence of illegal firearms is not accounted for. Therefore, cross-national studies have diverted to using different proxies for firearm ownership and availability, such as survey data from the International Crime Victims Survey [23, 27, 28], firearm suicide rates [29, 30], accidental firearm death rates [30, 31], or the often-cited Cook’s Index, which uses the average of the percentages of US suicides and homicides committed with firearms to estimate levels of ownership [32, 33]. In the two cross-national correlational studies included in this systematic review, three different proxies for firearm availability were used: accidental firearms deaths, suicides by gunshot and rate of civilian firearm ownership as reported by the Small Arms Survey, which in turn is based on multiple sources such as national registries, population surveys and expert estimates [34]. Although these three proxies have been used by previous published studies, it should nonetheless be noted that the validity of many proxies for firearm prevalence has been questioned due to limitations of each proxy [for an overview, see 35]. As such measurement biases might have impacted the results, they need to be considered, especially when these studies are used by policymakers and practitioners to address the public health issue of firearm-related violence.

Overall, this systematic review highlights a lack of available studies based exclusively on European data. That is, however, not to say that the link between firearm prevalence and firearm violence has not been addressed in academic literature at all. There are number of widely cited cross-national correlational studies that include European data [23, 28, 29, 33, 36–38]. Yet, those studies also include data from other non-European nations, often the US, Canada, Japan or Australia, in their statistical correlational analysis, which makes it impossible to reveal findings based on European data only. Data from other included non-European countries might influence the overall results, considering cross-national large differences in firearm legislations and gun cultures [35]. Moreover, these studies show similar methodological weaknesses in terms of operationalizations and a lack of control for other factors influencing homicide rates [2, 7]. Based on this assessment, a systematic review including these cross-national studies with non-European data would encounter similar limitations regarding comparability of studies.

Nonetheless, a few results from relevant global studies should be discussed, in particular in relation to the findings of the six included studies. Similar to results presented in Duquet and van Alstein’s study [1], Killias and colleagues found a strong positive correlation between firearm availability–operationalized as gun ownership according to the international victimization survey–and female gun homicide victimization, even when outliers are removed (Estonia, Malta, USA) [23]. In addition, both studies found no such significant correlation between firearm availability and male firearm homicide victimization [1, 23]. Other global studies have not differentiated between the gender of the victims of gun violence. Moreover, similar to the European findings discussed above, global studies show no conclusive, but varying results, ranging from no significant [23], to positive [28] as well as negative correlational [39] links between firearm availability and violent death by firearms. Although we have not conducted a systematic review of all global studies investigating the link between firearm availability and firearm violence, these varying results in the European context and beyond underline the importance of conducting in-depth, rigorous research that also include other factors relating to socio-economic and cultural factors that determine the context in which firearm violence takes place.

Next to international studies using non-European data, the findings of this systematic review can be complemented with other studies addressing the link between firearms and violence with a descriptive non-statistical approach. In doing so, several studies have linked an increase of (mostly illegal) firearms on the national level to an increase of firearm homicides or other types of crimes [40, 41]. In Sweden, for example, the percentage of firearm homicides in relation to other homicides has increased significantly, and a number of Swedish researchers have associated this increase with the rise of motorcycle- and other types of gangs who not only commit the crime, but also bring more weapons into the country [41-43]. In Switzerland, on the other hand, many firearm homicides take place in the private home in the context of domestic disputes. Killias and Markwalder relate this trend to a Swiss law that allowed (former) soldiers to keep their weapons after service at home [44]. Unfortunately, a small sample size did not allow the researchers to assess whether changes to the size of the army and therefore number of guns stored in private homes affected the homicide rate.

Even though such descriptive studies offer valuable insights into the context in which firearm violence takes place, they cannot sufficiently address the lack of insights into the association between firearm prevalence and–victimization. The main finding of our systematic review regarding firearm availability and–interpersonal violence in Europe is that empirical studies are still rare and existing studies lack comparability due to both a national focus and to variations in measurements of firearm availability. Future empirical research should not only address this research gap but should also critically engage with the identified methodological difficulties, for example by evaluating various proxies and their reliability for measuring firearm availability based on European data. Moreover, future research should overcome some of the limitations of this systematic review. For example, we could only include studies that were published in English, German or Dutch. We cannot rule out that there are studies in other languages that would have been relevant for this review. Another limitation concerns the indirect publication bias present in this systematic review, as we only included published studies. To address this issue, we decided to include grey literature as well. Future research examining the link between firearm availability and violence should further pay specific attention to Eastern-European countries, which are now grossly underreported, as well as to types of violence other than homicide, such as non-fatal assaults, or robberies. More specifically, such research would benefit from addressing the methodological shortcomings of existing research by empirically testing the assumption that availability of firearms is associated with forms of firearm violence rather than relying on descriptive statistics. Furthermore, they can take socio-economic and cultural factors into account. In addition, an empirical examination of the validity of proxies used to measure firearm availability, following the example of Cook’s index [32], could form the basis for above proposed empirical research and also increase comparability across studies. Only if those challenges and gaps are addressed will future studies become more comparable and valuable for public health and criminal justice researchers, policymakers and practitioners alike.

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4 Nov 2020

PONE-D-20-22981

Firearms and violence in Europe - A systematic review

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Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The purpose of this paper was to both summarize the European studies on how firearm availability connects to rates of interpersonal violence and to inform the potential effects of firearm restrictive legislations and future trends. It was an informative and thorough search of the literature that exposed the need for further work in this field. The writing is clear, concise, and explains the literature in appropriate detail. The first goal is met in that only six studies were appropriate for inclusion in this study. The second goal is not as strongly supported due to the small sample size of this study. Therefore, I recommend major revisions.

The major issues are as follows:

1. There is a need to either restate the second goal of the paper (lines 68-71), or to add more information that supports it. Because the papers have few common findings, it does not seem reasonable to use this work to inform potential effects and future trends. The reasons behind this incongruence are explained in the discussion. If this is still a goal of this manuscript, then far more information needs to be added and synthesized to make specific statements. Conversely, if this goal can be restated, this is no longer a major issue.

2. Another major issue is the number of studies reviewed. A sample size of six makes it difficult to accept any conclusions, even if the conclusions are that there are not enough studies. This can be remedied by either relaxing the inclusion criteria and including more studies or discussing some of the results from excluded studies to give more support to specific conclusions.

In terms of minor revisions:

1. For the Introduction (lines 47-71), include more information regarding firearm use in Europe. For non-European readers, it would be helpful to understand more about the legislation that is in place and the culture surrounding firearms in Europe. Some of this is discussed in lines 258-262, but may be better suited as background information. Similarly, include more information about past analyses of legislation. Particularly, The Science of Gun Policy (Smart 2020) conducts these kinds of studies in the United States and there are likely others. Highlighting some of these past works and the methods used to synthesize multiple papers or pieces of legislation will give support to your claims.

2. For Table 1, add the title of the manuscript to the table. Even though the bibliography contains this information, readers are more likely to look at the studies if the title is readily available.

3. Clarify the differing levels of analysis and potentially define the study design types (Table 1, Table 2, lines 169-173). Especially when discussing levels of analysis, some readers may understand ``individual'' to mean specific people and it is unclear whether this is the case in your analysis.

4. Within the conclusion and discussion, present some of the ways in which future studies could meet inclusion criteria for a future review. There are many mentions of the need for consistency across studies, but tying back in the reasons why certain studies were or were not included in your analysis would give more guidance to researchers in the field.

Thank you for your work on this manuscript and your contribution to the field!

Reviewer #2: Please see attached. I'm copying and pasting the first bit to get over 200 characters. This paper fills an important gap in the literature, by conducting a systematic review, following the PRISMA guidelines, of the corpus of papers that use data to study the Firearms Availability Hypothesis in Europe. This hypothesis states that greater availability of firearms is associated with more incidents of firearm-enabled criminal violence. The authors exhaustively searched nine databases and some 8,000 papers to find all papers satisfying their desiderata. The result is six papers that address this question, using data, during the appropriate time frame, using control variables, and producing robust results. The authors do an excellent job explaining their methodology, describing these six papers, distilling conclusions, and making recommendations for future work. I expect that future papers will build on and cite this paper.

I have two substantive questions/comments for the authors, and then a small number of line by line comments.

(1) The authors mention that they ruled out 25 papers where they were unable to get a full-text copy. Have they succeeded in getting any of these in the months since? It seems like Feb - April was a hard time everywhere, and so the authors they emailed might have dropped the ball on writing back. Were those authors ever emailed again?

(2) In the Outcomes column of Table 2, many of the papers are summarized using overly causal language. For example, when describing reference [14], the table writes ``Removing firearms from private homes of National Guards reduces firearm homicides'' but [14] uses less causal language, saying the laws ``could have contributed.'' Similarly, in describing [15] the table says ``Stricter firearm policies lead to less firearm and non-firearm homicides'' but the methods of [15] (linear and Poisson regression) do not support causal conclusions. Similarly, ``lead to'' is used in the description of [16] and [18], while for the description of [17], ``Both owning and carrying a firearm increased the risk of injuring another person intentionally'' again suggests a causal link. In all these cases, the authors of those studies are careful to avoid overly causal language. Furthermore, all six of the study designs (except possibly the quasi-experimental design of [17]) do not seem capable of proving causation. I encourage the authors to modify their language to strip out unsupported causal claims. For example, `lead to" can be replaced (in both Table 2 and the subsequent discussion of the six papers) with ``is associated with."

As the authors themselves write in the first paragraph of the introduction, ``causal links between the prevalence of firearms and violence remain unclear." It's best not to accidentally insert causal claims where they were not supported by the papers in question. For example, [14] is an observational study. Most of the six papers use time-series methods and observe that gun violence dropped when new laws were put into effect. But this approach ignores the possibility that culture is a confounding variable that causes both the change in laws and the drop in gun violence.

My last comment before moving on to the line-by-line remarks is that I want to encourage the authors to take up a similar literature review for the interplay between firearm availability and suicide rates, as well as accidental shootings. Since you've already done the first step, of combing through all the papers, this would be easier for you than for other researchers and it's arguably just as important as the systematic review you've just completed.

There are also attached line-by-line remarks.

**********

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Reviewer #1: Yes: Shelby M Scott

Reviewer #2: No

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Submitted filename: referee report on gun paper for PLOS One.pdf

Click here for additional data file.

22 Jan 2021

Reviewer 1

Comment 1: There is a need to either restate the second goal of the paper (lines 68-71), or

to add more information that supports it. Because the papers have few

common findings, it does not seem reasonable to use this work to inform

potential effects and future trends. The reasons behind this incongruence are

explained in the discussion. If this is still a goal of this manuscript, then far

more information needs to be added and synthesized to make specific

statements. Conversely, if this goal can be restated, this is no longer a major

issue.

Response: First of all, thank you for your clear comments throughout your review. It is

indeed clear that we did not meet our second goal, given the low quantity of

eligible studies and their methodological characteristics, which did not allow

for us to make any clear conclusions regarding the effects of firearm legislation.

We have restated our goal to match the information gained from this systematic

review more clearly.

Changes: We have changed our second goal to: ‘inform researchers, practitioners and

policymakers in the domains of public health and criminal justice about the

current state of knowledge regarding the association between firearm

availability and violence, remaining lacunae of knowledge to be filled by and

methodological challenges to be addressed by future research’.

The text has been adapted in the introduction (lines 79-83) and the conclusion.

Comment 2: Another major issue is the number of studies reviewed. A sample size of six

makes it difficult to accept any conclusions, even if the conclusions are that

there are not enough studies. This can be remedied by either relaxing the

inclusion criteria and including more studies or discussing some of the results

from excluded studies to give more support to specific conclusions.

Response: We agree that the low N of eligible studies presents a disadvantage, as already

stated in the response to your first comment. In the preparation of the search

protocol, we have tried different inclusion criteria and agreed on the necessary

practical, as well as logical inclusion criteria, such as location and timeframe

that allow us to answer the research question. To overcome some of the

limitations of the low N, we placed our findings in a broader context by

comparing them with findings from international studies that include, amongst

others, European data in the discussion. To emphasize the relevance of those

studies, we now added a more concrete summary of those international studies

in the discussion, as well as a short comparison of the findings from the global

studies with the included European studies. A challenge in such a comparison

– yet at the same time important conclusion – is that comparisons are difficult

due to the varying results found in each of the studies, mainly due to socio-

economic and cultural differences between the countries under study. We have

now explicitly stated this in the discussion. Moreover, we hope that the low

number of empirical studies will show the need for methodologically robust

research on this topic.

Changes: We have added a paragraph (lines 315-329) in the discussion with some

additional observations.

Comment 3: For the Introduction (lines 47-71), include more information regarding firearm

use in Europe. For non-European readers, it would be helpful to understand

more about the legislation that is in place and the culture surrounding firearms

in Europe. Some of this is discussed in lines 258-262, but may be better suited

as background information. Similarly, include more information about past

analyses of legislation. Particularly, The Science of Gun Policy (Smart 2020)

conducts these kinds of studies in the United States and there are likely others.

Highlighting some of these past works and the methods used to synthesize

multiple papers or pieces of legislation will give support to your claims.

Response: Thank you for this valuable comment.

Changes: We have added a summary (lines 59-74) of relevant research on gun

culture and -legislation in Europe to the introduction.

Comment 4: For Table 1, add the title of the manuscript to the table. Even though the

bibliography contains this information, readers are more likely to look at the

studies if the title is readily available.

Response: We agree.

Changes: Titles of studies are added in table 1.

Comment 5: Clarify the differing levels of analysis and potentially define the study design

types (Table 1, Table 2, lines 169-173). Especially when discussing levels of

analysis, some readers may understand ``individual'' to mean specific people

and it is unclear whether this is the case in your analysis.

Response: This is an important comment, thank you. To clarify the kind of data used in the

different studies, and to avoid misunderstanding, we have adapted the

information presented in table 2 to reflect whether a study was conducted

nationally vs. cross-nationally, using aggregated vs. non-aggregated data.

Moreover, we have added a column in table 2, describing the research design

of each study.

Changes: In table 2, the level of analysis has been adapted. A clarification has also been

added in lines 170-178. A column ‘study’ design was added in table 2.

Comment 6: Within the conclusion and discussion, present some of the ways in which

future studies could meet inclusion criteria for a future review. There are many

mentions of the need for consistency across studies, but tying back in the

reasons why certain studies were or were not included in your analysis would

give more guidance to researchers in the field.

Response: In addition to the suggestions for future research mentioned in the

original manuscript, we have added two additional suggestions, which – we

believe – would increase the quality of future research: first, although various

proxies of firearm availabilities are used, there is no examination of the validity

of these proxies. Therefore, we suggest that future studies should focus on

available proxies, such as data from the Small Arms Survey, the

International Crime Victim Survey or European firearm suicide data. Such a

research has been carried out in the past in the US context. Our second

suggestion is that more studies need to carry out empirical tests to assess the

link between firearm availability and -violence. Although various studies on

fatal and non-fatal violence in Europe mention firearm availability as a factor,

most assumed the association between availability and violence, rather than

testing such a link.

Changes: We have added above described suggestions for future research in lines 357-

365 in the revised manuscript.

Reviewer 2

Comment 1: The authors mention that they ruled out 25 papers where they were unable to

get a full-text copy. Have they succeeded in getting any of these in the months

since? It seems like Feb - April was a hard time everywhere, and so the

authors they emailed might have dropped the ball on writing back. Were those

authors ever emailed again?

Response: This is a very good point. The pandemic has changed all of our private and

professional rhythms and it would be more than understandable if our

contacted authors were not able to respond to our initial email. Therefore, we

have send out another email again, on November 10th, as well as follow-up

reminder on November 24th. As a result, we have received two more studies,

which we have assessed according to our eligibility criteria. Unfortunately, we

had to exclude both studies on the merit that they did not (exclusively) include

European data. We have adapted the text, as well as the PRISMA flow chart

accordingly.

Changes: Numbers of studies received and excluded were adapted in the text (lines 142-

147) and the PRISMA flow chart.

Comment 2: In the Outcomes column of Table 2, many of the papers are summarized using

overly causal language. For example, when describing reference [14], the table

writes “Removing firearms from private homes of National Guards reduces

firearm homicides” but [14] uses less causal language, saying the laws “could

have contributed.” Similarly, in describing [15] the table says “Stricter firearm

policies lead to less firearm and non-firearm homicides” but the methods of

[15] (linear and Poisson regression) do not support causal conclusions.

Similarly, “lead to” is used in the description of [16] and [18], while for the

description of [17], “Both owning and carrying a firearm increased the risk of

injuring another person intentionally” again suggests a causal link. In all these

cases, the authors of those studies are careful to avoid overly causal language.

Furthermore, all six of the study designs (except possibly the quasi-

experimental design of [17]) do not seem capable of proving causation. I

encourage the authors to modify their language to strip out unsupported causal

claims. For example, ‘lead to” can be replaced (in both Table 2 and the

subsequent discussion of the six papers) with “is associated with.” As the

authors themselves write in the first paragraph of the introduction, “causal

links between the prevalence of firearms and violence remain unclear.” It’s

best not to accidentally insert causal claims where they were not supported by

the papers in question. For example, [14] is an observational study. Most of

the six papers use time-series methods and observe that gun violence dropped

when new laws were put into effect. But this approach ignores the possibility

that culture is a confounding variable that causes both the change in laws and

the drop in gun violence.

Response: This is a very valuable comment, thank you. Although we have tried to adapt

the language used by the authors of the included studies, we see that some of

the statements made in our review require careful revision to avoid making

causal claims that are not supported by the data. We have critically and

carefully reviewed the language used throughout and adapted where necessary.

Changes: Language has been adapted throughout the analysis, to avoid overly causal

claims that are not supported. For example in table 2.

Comment 3: My last comment before moving on to the line-by-line remarks is that I want

to encourage the authors to take up a similar literature review for the interplay

between firearm availability and suicide rates, as well as accidental shootings.

Since you’ve already done the first step, of combing through all the papers,

this would be easier for you than for other researchers and it’s arguably just as

important as the systematic review you’ve just completed.

Response: We agree that such a review would be a great addition to the body of scholarly

work. For our study, however, we have decided to exclude accidents and

suicides, due to our expectation that those forms of violence have different

underlying causes. However, we will keep the extracted literature and hope to

continue and explore the association of firearms and suicides and accidents in

the future .

Changes: No changes made.

Comment 4: Around line 116, the authors write that their approach was inspired by [2] and

[6]. It would be good to say a word here about why this paper is different from

[2] and [6], and still needed despite [2] and [6] already doing a systematic

review of literature related to firearm availability. I’m guessing it’s because

[2] includes data from outside Europe and does not include control variables,

and [6] includes data from outside Europe, but this would be good to say.

Response: A good point. We have added the reason to our text.

Changes: See line 129-130.

Comment 5:

• Line 162 has a dash that it doesn’t seem to need.

• Same in the Outcomes column for the Kapusta paper and for the K ¨onig paper.

• Same on lines 304 and 305.

• Line 241: “This systematic review sought to assessing all studies”

• Line 262: should “incongruent legislations” be “legislation” instead? I’m not sure.

• Line 282, should “in academic literature” be “in the academic literature”?

• Line 295, I think either the dash after ‘motorcycle’ should be dropped, or you have to add another dash after ‘gangs’ on line 296.

Response: Thank you for your detailed observations. The dashes were included to indicate

(for example in line 304) that we want to make a statement about firearm

homicides, not homicides in general. We left ‘legislations’ instead of

‘legislation’, given that many European countries – namely EU member states

– follow the same framework of regulation, yet national differences exist.

Changes: Adapted in the text.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

5 Mar 2021

PONE-D-20-22981R1

Firearms and violence in Europe - A systematic review

PLOS ONE

Dear Dr. Krüsselmann,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

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The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

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PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The manuscript entitled, ``Firearms and Violence in Europe'' has been improved greatly by the authors and I appreciate the attention given to initial calls for revision. The purpose of this article is to summarize the existing studies of firearm violence in Europe, but has been revised to highlight the need for further studies before specific conclusions can be drawn. With the revisions that have been made, I recommend acceptance with minor revisions. The minor revisions are listed below.

Minor Revisions

1. Line 49: Put this statistic in per capita form and potentially compare it to the continental United States or another similar region. As a researcher from the U.S., 7,000 seems like a small number, but is even smaller (relatively) when considering the population of continental Europe. Comparing it to another country will put it into perspective for readers.

2. Line 60: Change ``vultures'' to ``cultures.'' This is an understandable typo, but I wanted to be sure it was highlighted!

3. Lines 157 - 160: italicize or bold good, fair, and poor when initially defining them. This will make it easier to find the definitions for readers as they work through the review.

4. General: Extend the captions for figures and tables. Add a brief statement about what should be taken away from each table and figure so that they would be able to stand alone, without the text of the main manuscript

Thank you for your willingness to address the concerns from the first reviews and for completing this necessary work that will add to the field of firearms violence research.

Reviewer #2: This paper is ready to be accepted. I did, however, spot a few more typos:

Line 61: please capitalize Constitution

Line 62: "baring" should be "bearing"

Lines 154-157: Did you mean to have (1)-(5) display as one long sentence, or did you mean to have one item per line? I think it would be easier to read if it was one item per line, but it's up to you.

Line 316: perhaps this colon should be a period since the next word is capitalized

Line 323: there is a hyphen before a comma that does not seem to belong

**********

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Reviewer #1: No

Reviewer #2: No

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5 Mar 2021

We have adapted all the recommended changes in the revised manuscript.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

9 Mar 2021

Firearms and violence in Europe - A systematic review

PONE-D-20-22981R2

Dear Dr. Krüsselmann,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Sergio A. Useche, Ph.D.

Academic Editor

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Additional Editor Comments (optional):

Dear Authors: thanks for the adequacy of the amendments made. I will now proceed to accept the paper in its present form.

18 Mar 2021

PONE-D-20-22981R2

Firearms and violence in Europe – A systematic review

Dear Dr. Krüsselmann:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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