Using social network analysis to examine alcohol use among adults: A systematic review

Background Alcohol use and abuse constitute a major public health problem and identifying their determinants is a priority. Social network analysis can indicate how characteristics of social networks are related to individual health behaviors. A growing number of studies have used social network analysis to examine how social network characteristics influence adult alcohol consumption, but this literature has never been systematically reviewed and summarized. The current paper systematically reviews empirical studies that used social network analysis to assess the influence of social network characteristics on drinking behaviors in adults. Methods A literature search of PubMed/MEDLINE, EMBASE, PsycINFO and Web of Science databases and a review of the reference lists of retrieved articles was conducted in March 2019. Two reviewers independently screened 5,510 non-duplicate records, and further screened the full text of 150 articles to determine their eligibility for inclusion. Seventeen articles were judged eligible and included. Results Most studies were conducted among young adults (mean age<30), in university settings or follow up visits with adolescent networks moving into adulthood. The objectives and methods of the included studies were heterogeneous. All included studies reported a statistically significant association between a social network characteristic and an alcohol consumption-related outcome. Social network members drinking behaviors were associated with participants’ drinking behaviors in multiple ways. Discussion In young adults, among whom the majority of identified studies were conducted, with whom they socialize and how they socialize appears to be associated with alcohol consumption; this was observed across methodologies and settings. We still know very little about the relationship of social networks to drinking in older age groups, and in populations most impacted by alcohol. As social networks appear to play a role in the consumption of alcohol in young adulthood, interventions that utilize social networks to help reduce harmful alcohol consumption should be considered.


Results
Most studies were conducted among young adults (mean age<30), in university settings or follow up visits with adolescent networks moving into adulthood. The objectives and methods of the included studies were heterogeneous. All included studies reported a statistically significant association between a social network characteristic and an alcohol consumption- PLOS

Introduction
Alcohol consumption is prevalent worldwide, with more than 2.4 billion people (33% of the global population) being current drinkers [1]. In the US, the prevalence of two forms of excessive alcohol consumption, high-risk drinking and alcohol use disorder, have increased substantially in adults over the past decade, such that 1 in every 8 adults report past-year high-risk drinking [2] and the prevalence of lifetime alcohol use disorder is high [3]. Alcohol use is also a leading cause of global disease burden and health loss [1]. Risk of all-cause mortality is positively associated with level of alcohol consumption, such that any level of consumption is potentially harmful [1]. These recent findings are consistent with the well-demonstrated relationship of excessive alcohol consumption to numerous adverse health consequences [2][3][4][5], and to increased morbidity and mortality worldwide [6][7][8]. Excessive alcohol consumption additionally places psychological and financial burdens not only on those who engage in these behaviors, but also their families, friends, coworkers and society as a whole [9,10]. Many studies have identified individual-level determinants of alcohol consumption [2,3,11], but. these studies have limited their focus to psychological or other individual characteristics of alcohol users. Socio-ecological models [12] point to larger social units, ranging from networks to institutional factors, as potential drivers of alcohol use. In this regard, one step beyond looking at individual risk factors is to consider the influence of social networks on alcohol-related outcomes. Social network analysis can be used to show how peer drinking behavior and patterns of relationships that connect social actors influence an individuals drinking behavior. That alcohol consumption can both influence choice of relationships (e.g. selecting drinking buddies as friends) and be influenced by them (e.g. being pressured by peers to drink alcohol), suggests that this is an area ripe for investigation.
Social network analysis is the term applied to a set of theories and methods used to study social interactions between individuals, and how these social interactions influence various outcomes [13]. A fundamental tenet in social network analysis is that it incorporates information about relationships between members of a shared social network. The most comprehensive approach that researchers use to collect data on social networks is termed a sociometric network approach, which involves interviewing multiple members (ideally all members) of a social network [13]. When a social network of interest is complete (i.e. bounded), rosters can be used to facilitate selecting the sampling frame and identifying connections between social network members. Alternatively, social network data can be collected by interviewing an from the National Longitudinal Study of Adolescent to Adult Health (Add Health) [58]. This review identified eight studies that looked at alcohol-related outcomes, all of which found that adolescent friendship networks promoted drinking alcohol [57]. Various types of friendships were found to influence adolescents' drinking behavior, with friendships that mattered (based on levels of closeness or by being reciprocated), being more likely to exert influence. The included studies that used longitudinal data showed that individuals who have friends who drink or are linked to friendship networks where individuals drink are at increased risk for drinking themselves, initially and over time.
How social networks influence drinking behaviors could be substantially different among adults compared to adolescents [59]. For example, peer groups tend to expand and diversify from adolescence into adulthood [60]. Not only does one's peer group change, but so does the amount of time one spends with peers and how one interacts with them [61]. Perhaps, as a result, the influence of peers is believed to wane in later adulthood [62]. Looking at things beyond homophily, the impact of social network structure on alcohol might also be qualitatively different in adults compared to adolescents. For example, centrality is the most common social network analysis measure used in alcohol research among adolescents, and we know that more popular adolescents (i.e., those with more friends) have higher levels of alcohol use [57]. However, in adulthood, having more friends may lose its association with alcohol use as it becomes less important in reflecting perceived social norms. Also, centrality may operate differently in adolescents and in adults above the legal drinking age as such adults would not need to rely on peers to access alcohol. Individual studies have examined the relationship of social networks to alcohol use among adults. However, thus far, this literature has not yet been summarized in a systematic review. The lack of a review on this topic in adults represents an important gap in knowledge. The current paper aims to fill this gap by systematically identifying and describing empirical studies that used social network analysis to evaluate alcoholrelated outcomes among adults (persons who are 18 years and older). We then synthesize the findings of the identified studies that statistically measured the influences of social network characteristics on alcohol use in adults.

Methods
This review was informed by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement. The PRISMA checklist [63,64] is shown in S1 Table.

Data sources and literature search
A literature search of 4 databases (PubMed/MEDLINE, EMBASE, PsycINFO and Web of Science, Sociological) was conducted in March 2019 to identify studies that used social network analysis to evaluate the effects of social network characteristics on alcohol use. Keyword and terms used in search strategies varied based on the database to account for distinct indexing criteria and are described in detail in S2 Table. To identify additional studies not found in the literature search, the reference lists of relevant review articles of social network analysis [13, 44,57] and included articles were reviewed. The literature search was conducted with guidance from an Education and Curriculum Librarian who serves as the Coordinator for Systematic Review Services at the New York University School of Medicine.

Screening
Duplicates were removed and screening of retrieved articles by title and abstract was conducted by two independent reviewers using Covidence systematic review software (Veritas Health Innovation, Melbourne, Australia. Available at www.covidence.org). Final inclusion was determined by two independent reviewers (JK & EG) screening the full-text of potentially eligible articles using the following criteria. Studies were eligible to be included in the review if they (1) were published in a peer-reviewed journal, (2) were written in English, (3) were conducted in human populations, (4) utilized a social network analysis design that included directed graphs (where there is directionality in the ties) and which data were collected beyond dyadic pairs (i.e., data linking participants were measured or inferred (e.g., known roommates were considered to be connected) and data were collected among alters for 3 or more connected individuals) and network measures were calculated based on this data, (5) evaluated an alcohol-related outcome, and 6) included a majority of participants that were adults (18 years or older). Studies were excluded if (1) the impact of social networks was assessed using simulations, (2) the study described qualitative research or was a case study. In the few cases on which opinions about inclusion differed, the reviewers met and reached consensus through discussion.

Quality assessment
We used a 12-item quality assessment tool (S3 Table) to evaluate study relevance and methodology. This tool was developed using modified sets of criteria from other quality assessment tools for assessing observational studies [65] and network studies [66]. The tool assessed description of: the research question, data collection procedures (data source(s), study setting, sample size, response rate, sample selection), measures used (exposure and outcomes), analysis of social network data, results and findings, strengths and limitations, and conclusions drawn.

Data extraction and synthesis
A data extraction form was developed to extract information on study objectives, study design and sampling approach, data collection method, setting and target population, participants, social network data collection procedures, outcome measurement, social network analysis methods used to analyze the data and calculated measures, statistical analyses used (as relevant to social network analysis), and key findings.

Identifying empirical studies
The literature search yielded 5,907 records for screening (5,477 after removing duplicates), and an additional 33 were included based on reference list review for a total of 5,510. The full text of 150 articles was reviewed to determine eligibility and 17 articles were judged potentially eligible and further assessed for quality and relevance (S4 Table). Scores on the quality assessment tool scale ranged from 78%-100% (mean = 95%). Based on these scores, all 17 articles were included in the review (Fig 1) [67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. Because the methodologies used in the included studies were heterogeneous, they did not lend themselves to meta-analysis. Therefore, a narrative synthesis of the studies will be conducted.

Sampling and study design
Sample selection procedures included the use of respondent-driven sampling, recruitment of peer groups, and the collection of complete sociometric social networks. The studies that used more robust sociometric social network sampling procedures [67-72, 77, 78, 81, 82] either collected data on complete (i.e. bounded) networks [77,81] or used complete rosters but limited the number of peers that an individual was able to nominate [67][68][69]71]. Two of the studies relied on peer nominations or recruitment without the availability of rosters [72,74,82]. Sample sizes ranged from 34 [81] to 12,067 [82]. Of the studies, 11 used cross-sectional data [67-70, 72-74, 77-80, 83], 5 used a longitudinal study design [71,75,76,81,82], and one was a  Using social network analysis to examine alcohol use among adults: A systematic review      naturalistic observation study in a 'bar-lab' [80]. Details of the individual studies, including objectives, study period and design, setting, participant characteristics, data sources, social network measures, analytic methods, and major findings are presented in Table 2.  Ego density: the total number of ties between an ego's nominations divided by the total number of possible ties between the nominations Prestige: How many connections an ego has and how many connections the alters of the ego has, and so on Group integration: The extent to which an ego's outdegree nominations are in a bounded social network (e.g. a school), including within sub-networks (e.g. grades) Network density: The total number of observed connections divided by the maximum number of possible connections Transitivity: The extent to which the relation between two members in a shared social network that are connected by another member is transitive, or put more plainly, that friends of a person's friends are also his friends Closeness: The minimum number of ties needed to reach all the other individuals in the network Gender heterophily: An index of how many cross-gender relationships an ego nominates Using social network analysis to examine alcohol use among adults: A systematic review

Objectives
An objective of all the included studies was to assess how a social network characteristic was associated with alcohol consumption. Specifically, 12 studies assessed the association between characteristics of network members (e.g. peers' weekly alcohol consumption or peers' beliefs about alcohol) and alcohol consumption [68, 70, 71, 73-76, 78-80, 82, 83]. Eight studies assessed the association between characteristics of network structure, such as network size, network shape, network attributes, or position within a network, and alcohol consumption [67-69, 72, 77, 78, 81, 82].

Measures
The measures used in the included studies were heterogeneous. Table 3 provides definitions for the social network characteristics that the studies examined, which included: attributes of network members (e.g. peers' weekly alcohol consumption or peers' beliefs about alcohol), indegree/indegree centrality, betweenness centrality, outdegree/outdegree centrality, Table 3. Social network measures used in the identified studies and their definitions.

Social network measure Definition
Indegree/indegree centrality The number of alters that nominate a given ego The extent to which the relation between two members in a shared social network that are connected by another member is transitive, or put more plainly, that friends of a person's friends are also his friends Using social network analysis to examine alcohol use among adults: A systematic review reciprocity, prestige, group integration, density, transitivity, closeness, gender heterophily, effective size, cluster identification, and homophily. All the studies measured some form of alcohol consumption over a specific time period as the outcome. These included frequency of alcohol consumption, quantity of alcohol consumption, and frequency of binge drinking. Some studies used a single item to measure alcohol consumption, while others measured multiple forms of alcohol consumption using previously validated scales. One study looked at frequency of drug and/or alcohol use with a social network member in the past 6 months without distinguishing between drug and alcohol use [72]. The naturalistic observation study relied on observed counts of alcoholic drinks consumed [80], all the other studies relied on self-reported data, including one that compared perceived levels of alcohol consumption by peers to self-reported alcohol consumption by participants [79].

Statistical analyses
The statistical analyses used to assess social network characteristics were also heterogeneous, although the majority used some form of regression modeling correcting for nonindependence of observations/autocorrelation among network members. The longitudinal studies used Generalized Estimating Equations (GEE) to account for repeated measures. Overall, the methodologies described by the included studies were rigorous enough that the studies met inclusion criteria based on the quality assessment tool (mean = 95%).

Findings
All the included studies reported a statistically significant association between a social network characteristic and an alcohol-related outcome. The different types of social network members whose alcohol consumption (or in one study, their beliefs about alcohol [76]) was associated with participants' alcohol consumption, included: peers/friends [69,70,74,76,78,80,82,83], dorm mates [68,73,79], drug network members [75], and parents [76]. In one study, participants' alcohol consumption was associated with alcohol consumption of relatives' and friends' but not that of immediate neighbors or co-workers [82].
Social network members' drinking behaviors were associated with participants' drinking behaviors in multiple ways. For example, among first-year university students in Germany, participants' quantity of drinking was associated with friends' quantity of drinking but participants' frequency of drinking was only associated with friends who also identified the participant as a friend [71]. At a medium-sized university in the Northeast US, weekly volume of alcohol consumed among nominated peers was significantly associated with that of participants but alcohol problems (measured using the 24-item Brief-Young Adult Alcohol Consequences Questionnaire [84]) were not [68]. In a subsequent study that sampled the entire firstyear class at the same university, participant's binge drinking frequency was positively associated with both perceived and actual binge drinking frequency of important peers, and resistance to peer influence weakened the effect of perceived peer binge drinking on participant binge drinking, but did not interact with actual norms [70]. In a sub-sample of past-month drinkers from the previous study, the number of drinks consumed by your peers on their heaviest drinking occasion was associated with greater drinking quantities on one's own heaviest drinking occasion [78]. Among black men who have sex with men (MSM) in a community setting in South Africa, individuals whose social networks included a higher proportion of hazardous drinkers were more likely to be hazardous drinkers themselves [74]. In the naturalistic observation study, peer group alcohol consumption was the strongest predictor of participants' alcohol consumption [80]. Data from the Framingham Heart Study was used to show how the impact of peers drinking habits on one's own drinking behaviors diminished across degrees of separation, i.e. one's own friends influence drinking more than the friends of one's friends [82].
A few studies looked at peer effects in social networks by examining homophily [68,81,82]. One study showed that quantity of alcohol consumption clustered within nominated peers of a university residence hall, then used regression to show that participants' quantity of alcohol consumption was associated with nominated peers' quantity of alcohol consumption [68]. Two studies observed that homophily increased over time (i.e. social networks became more homophilous), and provided support for induction because the directionality of friendship nominations mattered after controlling for participants' previous alcohol consumption [81,82].
Certain studies assessed associations between other aspects of social network structure than homophily (sometimes in addition to looking at the association between social network members drinking behaviors and participants' drinking behaviors), such as network size, network shape, network attributes or position within a network, and alcohol-related outcomes [67,72,77]. For example, among university students in the US, outdegree was positively associated with number of heavy drinking days [67]. In the same study, betweenness centrality was positively associated with alcohol-related problems, with a stronger association among women [67]. In a subsequent study that sampled the entire first-year class at the same university, popularity (indegree) was positively associated with participants' alcohol consumption, binge drinking frequency, and alcohol-related problems [69]. In a sub-sample of past-month drinkers from the previous study, being present at other peers' heaviest drinking occasions was associated with greater drinking quantities on one's own heaviest drinking occasion [78]. Among university students in Belgium, indegree was positively associated with binge drinking, while gender heterophily and effective size were negatively associated [77]. Among young, MSM in a community setting in the US, transitivity was positively associated with frequency of alcohol and/or drug use [72]. Taken together, these findings suggest that characteristics of social networks and one's position in a social network also are associated with alcohol-related outcomes.

Critical evaluation
Almost all (15 out of 17) of the included studies were conducted in young adults [67-74, 76-81, 83], mostly in university settings [67-71, 73, 76-81], with just 2 in community settings [72,74]. Two studies were conducted among adults whose mean age was greater than 30 [75,82]. Considering the ways that social networks are formed, it is possible that school settings, in which individuals live in close proximity to similar-aged peers who share a large number of commonalities, have a qualitatively different effect on the ways that social networks are formed and how information is transmitted through these networks than networks of older adults or those in community settings.
The variation in measures used, both of social network characteristics and alcohol consumption, makes it challenging to summarize the body of evidence for how social networks influence alcohol consumption in young adults, and vice versa. Also as a limitation, many of the studies were cross-sectional [67-70, 72-74, 77-79, 83]. Cross-sectional designs allow researchers to test for correlations between an individuals' alcohol consumption and that of their peers, but not to distinguish whether the correlations result from selection, confounding or induction (i.e. that the observed homophily with regards to alcohol consumption observed among social networks of adults is due to the tendency for people to befriend those similar to themselves, the effects of shared environments, or the spread of drinking behaviors within networks). Two studies examined directionality and found support for induction [81,82].
Some studies collected data on complete networks [77,81] or used complete lists of members in a social network (i.e. rosters) but limited the number of peers that participants were able to nominate [67-71, 73, 78, 79]. Other studies relied on peer nominations or recruitment without the availability of rosters [72, 74-76, 80, 82, 83]. The studies with less complete networks suffer from missing data that raise doubt about whether the results apply to the entire network, and whether the network metrics are accurate. Dyadic index-peer data can provide useful information, but is incomplete social network data because it limits researchers to investigation of the effects of peer consumption rather than social network position or other characteristics of social network structure. Such studies contribute only limited information about how social networks influence adult alcohol use. Furthermore, results that rely on incomplete social network data are likely to suffer from selection bias because peers who are closer to the participant (and thus, more likely to share similar characteristics) are more likely to be selected into the study. While many of the studies in adults were limited in the completeness of social network data collected; collectively, they featured a wide variety of types of social network members studied (e.g. peers/friends, classmates/dormmates, drug network members, relatives/parents, neighbors, co-workers). In a few of the studies [72,74,83], the nature of the relationships among participants was not even known, just that there was some sort of connection between them because one participant's recruitment was attributed to the other. This is reflective of how peer groups are more expansive and diverse in adulthood [60], but also more difficult to capture completely. In nearly all cases, these social network members were found to influence participant alcohol consumption. Lastly, many of the included studies were conducted among specific populations (e.g. fraternity brothers [81], injection drug users [75], men who have sex with men [72,74]), raising questions about the generalizability of the findings.

Discussion
Through a systematic review, 17 studies were identified and evaluated to assess the evidence on whether social network characteristics were associated with drinking behaviors in adults. These studies measured and analyzed social networks in various ways. The heterogeneity of methods used make it difficult to generalize about how, specifically, social networks influence alcohol consumption and vice versa. However, in young adults, among whom the majority of studies were conducted, with whom they socialize and how they socialize appears to be associated with alcohol consumption across methodologies and settings.
This review identified a lack of research on social network characteristics and alcohol consumption among middle-aged or older adults. Therefore, there is limited evidence to infer how social networks are associated with alcohol consumption further into adulthood, although it is likely quite different for a number of reasons. First, social network dynamics differ throughout the lifespan [60]. For example, as adolescents enter into young adulthood, their social networks tend to grow in size, and the strength of peer dynamics remain strong, especially when young adults are located in settings with relatively complete (i.e. bounded) social networks, such as universities [62]. The influence of peers is believed to wane in later adulthood [62]. Second, drinking patterns also change during the life course [2,85]. For example, entering college [86] or meeting the legal minimal age to purchase alcohol are known to alter drinking behaviors and alcohol-related harms [87]. Third, alcohol use also has changing physiological effects as individuals age [88], with increasing health risks associated with aging [89][90][91]. Whether sociometric social network data would be particularly informative for examining alcohol use in older adults remains an open question, especially given that network effects might be waning [62]. Sorting out these questions remains an area that requires further research. Another important area that was not explored in the literature that we identified is a comparison of the impact of social networks on alcohol use between adults and adolescents. Using a life course perspective to explore the changing impacts of social networks on alcohol use could be further enlightening.
An important question is whether social network analysis-based peer effects on drinking are different in adults than in adolescents. No reviewed studies directly investigated this. One study [92] investigated the effects of adolescent social network characteristics on participants in the National Longitudinal Study of Adolescent to Adult Health (Add Health) [58] when they were adolescents and then re-examined the effects of the adolescent social network characteristics on participants when they had become young adults. One characteristic, adolescent group integration, showed a weaker effect on binge drinking in adulthood than adolescence, while another adolescent social network characteristic, prestige, showed an increased effect on adult binge drinking, suggesting that the lasting effects of adolescent social network analysisbased peer effects into adulthood varied depending on the characteristic considered. However, this study was not included in the main review because it did not meet inclusion criteria (the social network recruitment was done during adolescence, not adulthood). Comparing how the studies in the current review on adults differed from the studies in the previous review that included studies of adolescents [57]: in adolescents, the social network data collected was much more complete and many social network characteristics were able to be explored, yet peer effects were almost exclusively examined in schoolmates, with studies distinguishing further about friendship, closeness, reciprocity, and shared affiliations (e.g., sports or club activities). The relationships studied in the social network literature among adults were more extensive, and included peers/friends, classmates/dormmates, drug network members, relatives/parents, neighbors, co-workers. While having more expansive and diverse social networks as adults makes collecting complete social network data more costly and challenging, one of the benefits of collecting this information is that they will help us more fully understand how social networks characteristics affect alcohol consumption among adults.
Social network research methods are still relatively novel for studying alcohol use in adults, with 14 of the 17 included papers published since 2010. This literature is likely to continue growing as social network research is expanding [50,93], and methodologies are being refined [13]. This review demonstrates the adaptability of social network analysis to study alcohol consumption, which is a prime topic for social network research as drinking (unlike many other health-related behaviors) is often undertaken as a shared social activity. The social aspects of drinking are especially evident in teens and young adults, but not as clear as people get older. Hopefully, as this field grows, more studies will be conducted among older adults and in community settings, where sociometric network data is more challenging to collect, but potentially more informative.
This review and the study results have several limitations. First, only 17 studies were identified and deemed eligible to be included based on relevance and quality. Articles may have been missed because the use of social network methods was not indicated in the title or abstract and thus they would not have been identified during the literature search. However, for that reason, a broad search was conducted and ultimately a large number (5,510) of articles were screened based on title and abstract. Second, all studies included in the review reported at least one statistically significant association; this might be a reflection of publication bias. Third, the results of social network analysis studies are context-specific, and insights are likely to vary based on setting and the exposures and the outcomes that were measured. Furthermore, even findings within studies varied by whether the outcome was frequency of alcohol consumption, quantity of alcohol consumption, or binge drinking. It might be important to consider implementing some level of standardization across social network studies, and to prioritize assessing the replicability of findings in different settings.
Despite the limitations, this review identified numerous studies that have applied social network analysis to study alcohol consumption in (mostly young) adult populations. Social network analysis is a method that helps us better understand alcohol use in young adults because it not only addresses the actual drinking behaviors of their peers, but also uniquely addresses how various characteristics of social network structure (e.g. homophily, popularity, transitivity) are associated with individual alcohol use. In other words, how young adults socialize and who they socialize with appears to matter when alcohol consumption is considered. In this sense, social network analysis is a useful tool with the potential to explore the effects of social mixing patterns on alcohol consumption.

Implications for research and intervention
There are a few important take-away messages from this first review of the growing number of empirical studies that have used social network analysis to explore alcohol use in adults. First, peer alcohol use and other social network characteristics (e.g. network attributes or position within a network) were associated with adults' alcohol consumption across studies. Second, we identified a lack of research on the impact of social networks on alcohol use in middle-age and older adults, especially those residing in community settings (i.e. without explicit boundaries). Future empirical research should work to address these gaps in our understanding. Efforts should also be made to reduce heterogeneity in social network analysis studies (e.g. agreeing on standardized definitions of social network measures, assessing and reporting the results of them consistently) to facilitate generalizability. This review informs alcohol researchers, health service providers, and policymakers about how social networks have been studied, thus far, to better understand alcohol consumption. As social networks appear to play a role in the consumption of alcohol in young adulthood, this suggests potential for interventions that utilize social networks to help reduce the burden of harmful alcohol consumption [92]. Interventions that utilize social networks to promote behavior change are increasingly available [44], calling for studies of their feasibility and efficacy in reducing alcohol consumption in young adult populations.
Supporting information S1