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Problem gambling and associated mental health concerns in elite athletes: a narrative review
  1. Jeffrey Lee Derevensky1,
  2. David McDuff2,
  3. Claudia L Reardon3,
  4. Brian Hainline4,
  5. Mary E Hitchcock5,
  6. Jeremie Richard1
  1. 1 Educational and Counseling Psychology, McGill University, Montreal, Quebec, Canada
  2. 2 Department of Psychiatry, University of Maryland School of Medicine, Baltimore, Maryland, USA
  3. 3 Department of Psychiatry, University of Wisconsin Madison School of Medicine and Public Health, Madison, Wisconsin, USA
  4. 4 National Collegiate Athletic Association (NCAA), Indianapolis, Indiana, USA
  5. 5 Ebling Library for the Health Sciences, University of Wisconsin-Madison, Madison, Wisconsin, USA
  1. Correspondence to Dr Jeffrey Lee Derevensky, Educational and Counseling Psychology, McGill University, Montreal, QC H3A1Y2, Canada; jeffrey.derevensky{at}mcgill.ca

Abstract

Opportunities to participate in gambling have dramatically changed during the past 20 years. Casinos have proliferated as have electronic gambling machines, lotteries, sports betting, and most recently online gambling. Gambling among the general population has moved from being perceived negatively to a socially acceptable pastime. As over 80% of individuals have reported gambling for money during their lifetime, governments recognise that regulating gambling—a multibillion dollar industry—is a significant source of revenue. While the vast majority of individuals engaged in some form of gambling have no or few gambling-related problems, an identifiable proportion of both adolescents and adults experience significant gambling-related problems. Elite athletes have not been immune to the lure of gambling nor its concomitant problems. Prevalence studies suggest higher rates of gambling problems among athletes than the general population. In this narrative review, we examine several risk factors associated with gambling problems among elite athletes and new forms of gambling that may be problematic for this population. Given the potential serious mental health and performance consequences associated with a gambling disorder for athletes, we aim to increase coaches’, athletic directors’ and health professionals’ knowledge concerning the importance of screening and treatment referrals.

  • psychiatry
  • psychology
  • disorder

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Introduction

While gambling is a relatively understudied topic in general, this is especially the case for gambling and gambling problems in elite athletes. Thus, this narrative review seeks to summarise the existing literature on gambling problems in elite athletes, including information on symptoms and diagnosis, prevalence, risk factors, differences across various demographic groups and levels of elite competition, forms of gambling, and management.

We searched five electronic databases (PubMed (MEDLINE), SPORTDiscus via EBSCO, PsycINFO via ProQuest, Scopus and Cochrane) from inception to November 2018. Search terms relating to mental health disorders, sports participation, the elite nature of participation and gambling were combined. Articles must have been available in English. To avoid missing relevant publications, references of the included studies were screened, as well as relevant reports. Elite athletes were defined as those competing at professional, Olympic or collegiate/university levels.

Symptoms and diagnosis

Currently, gambling disorder is viewed as a ‘non-substance-related disorder’ within the ‘substance-related and addictive disorders’ section of the Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5).1 As such, gambling disorder is conceptualised as a persistent and recurrent gambling behaviour leading to clinically significant impairment or distress experienced by the individual.1 To be clinically diagnosed with a gambling disorder, this behaviour may not be better explained by a manic episode. As such, according to the DSM-5,1 the individual with gambling disorder exhibits four or more of the symptoms (during a 12-month period) listed in box 1.

Box 1

Diagnostic criteria for a gambling disorder1

  • Needs to gamble with increasing amounts of money in order to achieve the desired level of excitement.

  • Is restless or irritable when attempting to cut down or stop gambling.

  • Has made repeated unsuccessful efforts to control, cut back or stop gambling.

  • Is often preoccupied with gambling (including persistent thoughts of reliving past gambling experiences, handicapping or planning the next gambling venture, and/or thinking of ways with which to gamble).

  • Often gambles when feeling distress (eg, helplessness, guilty, anxious, depressed).

  • After losing money gambling, often returns another day to get even (chasing one’s losses).

  • Lies to conceal the extent of involvement with gambling.

  • Has jeopardised or lost a significant relationship, job, educational or career opportunity because of gambling.

  • Relies on others to provide money to relieve desperate financial situations caused by gambling.

Each of the nine DSM-5 diagnostic criteria are weighted equally despite differences in the severity of consequences.1 No single constellation of symptom endorsements from among the nine DSM-5 symptoms exists among disordered gamblers. However, it is not unusual for help-seeking disordered gamblers in treatment to endorse eight or nine of the criteria; the higher the number of endorsements, the more severe the disorder.2

Sports wagering

Sports wagering consists of betting on individual sporting events or contests, through fantasy sports (seasonal or daily), or on individual players or events through prop bets or inplay sports betting.3 4 It may occur among peers, through a bookmaker (individuals accepting illegal wagers or gaming operators where legal) or through online sites. Sports wagering has grown in popularity, with the total amount of money wagered yearly increasing.3 5–9 A US consumer survey revealed that 42% of adults report having placed a sports wager during their lifetime,10 with 4% doing so regularly.10

More jurisdictions are approving sports wagering legislation in the USA since the recent Supreme Court ruling (May 2018)11 permitting states to pass legislation that permits sports wagering. Currently, more than 20 US states have legislation permitting or about to permit regulated sports wagering. The prevalence of online sports wagering has increased in many jurisdictions, likely accounting for its significant growth.12 13 In 2017, over 12 million Europeans had an active online account with members of the European Gaming and Betting Association, equivalent to 2.4% of Europe’s total population.14 Online and mobile operations represented 20.7% of the total gambling activity, with sports wagering being the most popular form of online gambling (40.3%).14 Gambling operators continue to expand their online platforms to include mobile gambling, with research suggesting that mobile gambling is especially popular among problem gamblers.15

Results from Australia suggest that young male internet sports bettors are particularly vulnerable to gambling problems if they held positive attitudes towards gambling sponsors who imbedded promotions into their sports broadcasts.6 Since the 2018 US Supreme Court ruling, major international gambling operators have formed sports wagering partnerships with the professional sports leagues, including Major League Baseball, NBA, Women’s National Basketball Association and NHL.16 17

Collegiate and professional e-sports (competitive video-gaming) have become more organised and grown in popularity.18 19 Since their establishment in the early 2000s, professional and club e-sports have seen a rapid growth in both participation and viewership,20 21 with some universities offering scholarships for top players.19 Gambling operators have capitalised on this market by offering opportunities to wager on specific player and/or teams.

Fantasy sports wagering

Fantasy sports, representing one form of sports wagering, are typically online activities where participants assemble virtual teams of real players. These teams then compete against other teams based on the statistical performance of the players. In a sense, individuals become managers, drafting, trading or dropping players during a season. Team performance is typically converted into points, with the individual having the most points being declared the winner. While fantasy sports wagering initially began as a competition among friends, it has evolved into an online competition with individuals wagering money that their team and players will be superior to others. Originally based on a season, online gambling operators developed daily fantasy sports (DFS) to increase the frequency of betting. DFS contests are played across shorter periods of time (eg, a week), rather than an entire season. Recent reports from the Fantasy Sports Trade Association22 estimate that 57.4 million people participated in fantasy sports wagering in the USA and Canada, an increase from 13.5 million in 2004. In Australia, there are an estimated 1.65 million fantasy sports players despite the ability to legally gamble on non-fantasy sports,23 with participation in some form of DFS doubling from 31% in 2012 to 64% in 2015 (39% of fantasy sports players primarily using a mobile device to participate).22 The widespread participation in DFS has become a cultural phenomenon and may have been increased by the nearly $206 million spent on advertising by the two largest DFS companies in 2015.24 25

Although there is still debate concerning whether fantasy sports wagering should be considered ‘gambling’ versus ‘skill’, DFS players have been characterised by high gambling frequency and gambling-related problems.26 Nower and colleagues27 reported that 22.4% of adults engaged in DFS, with most being between the ages of 25 and 34 (61%), married or living with a partner (62.7%), and having a college or postgraduate degree (46%). Overall, 95% of DFS players were reported to be high-frequency gamblers in general and were identified as high-risk problem gamblers.

In a study examining collegiate athlete participation in fantasy sports, Marchica and Derevensky28 reported that among fantasy sports players, 48.1% of men and 25% of women could be identified as at risk for a serious gambling problem or had already reached the clinical criteria for disordered gambling. In a study of over 7000 Ohio high school students, Marchica et al 29 reported that 13.5% reported wagering money on sports teams, 7.3% wagered money on fantasy sports, and 5.1% had bet on DFS. Among individuals who participated in fantasy sports more than once per month, 36% of men and 59% of women were considered at risk for a gambling problem.

Inplay live action wagering

Technological advances have continued to change gambling practices. One of the more innovative forms of online sports wagering is inplay/prop sports bets, where individuals can wager on specific events (eg, which team or player will have the most points in a given time frame) or players (eg, who will score first) within a sporting event. Inplay betting has been instrumental in the growth ofsports wagering.30 31 The structural characteristics of inplay sports betting have changed the mechanics of sports wagering. Factors such as bet frequency (the number of bets placed within a given time period), event frequency (the number of gambling events available to bet on in a given period) and payout frequency (the frequency of payouts) have greatly impacted individual gambling behaviours.32 33

Inplay betting has also allowed athletes potentially greater control over the outcome of certain events (eg, individuals can bet against themselves in a relatively anonymous manner using different online accounts). Inplay betting has significant implications for the integrity of sporting events and could be more harmful than other forms of gambling given the associated inherent structural characteristics. Out of 338 online gambling websites assessed, 26% offered at least one inplay betting feature,33 and inplay betting features were accessible via a mobile device in 87.5% of the websites visited.33 While the UK Gambling Commission34 concluded that individuals who make inplay bets are at a greater risk of harm, they failed to enact legislation prohibiting or restricting this type of wager.

Advertising

Concerns have been reported for vulnerable populations (ie, youth, problem gamblers) who seem to engage in more online gambling when watching betting commercials.35 On-screen displays of odds integrated into the narration accompanied by messages highlighting the ease of placing a wager, risk-free bonuses and the use of attractive hostesses have been found to be associated with increased wagering.36 Australian researchers reported that 20% of children ages 5–12 associated their favourite team with a corporate gambling sponsor.37 Lopez-Gonzalez et al,31 38 39 analysing a sample of British and Spanish sports betting advertisements, found inplay wagering in almost half of all advertisements. While stricter advertising regulations have become more commonplace in certain jurisdictions, in the USA major media networks have proposed developing gambling-related shows to accompany sports events.40

Prevalence and risk factors associated with problem/disordered gambling among college students

A number of large-scale epidemiological studies suggest that 75%–80% of the general population of college students report having gambled within the past year,41–43 with 6% of college students being identified as disordered gamblers and an additional 10% being problem gamblers (not reaching the clinical criteria for disordered gambler).44 Blinn-Pike et al’s43 earlier meta-analysis suggested that 7.89% of college students are disordered gamblers (prevalence data from studies ranged from 5.37% to 10.41%). As a result, millions of college students worldwide are experiencing at least some gambling-related problems.

College students have been found to engage in virtually all forms of regulated and non-regulated gambling, including lotteries, card games, sports wagering, fantasy sports wagering, casino gambling, online gambling and wagering on e-sports.45 Similar to the general adult population, male college students appear to be more likely than female college students to experience gambling-related problems, with gender being one of the strongest predictors of gambling interest, participation and disorders according to international research.9 24 29 41 46–48 Male college students tend to prefer more competitive and skill-based forms of gambling,49 whereas female college students prefer more luck-based/non-strategic forms of gambling (eg, slot machines and lotteries).50–52

Online gambling and sports wagering have both been identified as significant risk factors for increased involvement in gambling among college students28 45 53 54 and non-college adults.6 7 54 Other risk factors for gambling include impulsivity, maladaptive coping strategies, high sensation-seeking, risk-taking and social anxiety.55–58

To better understand the motivations for gambling among college students, Neighbors et al 59 asked college student gamblers (N=184) to articulate the reasons underlying their gambling behaviours. The primary motivations included winning money (42.7%), enjoyment/fun (23%), social reasons (11.2%), excitement (7.3%), winning (3.9%) and competition (3.4%).

Problem/Disordered gambling among collegiate athletes

Collegiate athletes represent a significant subpopulation of college students. In the USA alone, over 485 000 college students participate in the National Collegiate Athletic Association (NCAA) sanctioned athletics yearly.60 Similar to their peers, collegiate athletes engage in interrelated high-risk behaviours, including heavy alcohol use, substance use and gambling.61–65 Studies of prevalence rates of problem gambling among collegiate athletes suggest that the rates of disordered gambling are significant, ranging from 2.9% to 15%,61 66–70 with men having considerably higher rates of problem or disordered gambling than women.61 71 72

Huang and colleagues69 70 reported that collegiate athletes who gamble were far more likely to engage in risky behaviours or experience mental health symptoms, including past month episodic heavy drinking, marijuana and other illicit drug use, eating disorders, cigarette use, and past year unprotected sex. In addition, collegiate athletes who gamble experience significantly more alcohol/drug-related problems compared with non-gamblers.69 70 Nearly all these risky behaviours increased as gambling severity increased. For example, past month heavy episodic drinking for non-gambling collegiate athletes was 51.7%, compared with 76.6% for social gamblers (occasional gamblers), 85.5% for problem gamblers (endorsing some gambling-related problems) and 82.3% for disordered gamblers. The same held true for marijuana use (7.5% for non-gamblers, 16.5% social gamblers, 30.2% problem gamblers and 35.2% disordered gamblers), drug use (3.8%, 6.6%, 11.0% and 18.7%, respectively), binge eating (2.0%, 2.7%, 4.9% and 8.3%, respectively), unprotected sex (34.2%, 45.4%, 57.3% and 59.1%, respectively) and cigarette use (7.4%, 11.5%, 17.0% and 29.7%, respectively). As in studies of general and treatment-seeking populations, the association between problem/disordered gambling and risky behaviours in collegiate athletes was generally higher in men than in women.69 70

Since 2004, the NCAA has been monitoring collegiate athletes’ gambling behaviours. In the most recent cross-sectional analyses examining data from 2004, 2008, 2012 and 2016 samples (almost 85 000 student athletes), men (55%) were found to be consistently engaged in all gambling activities at higher rates than women (38%) int 2016. Overall gambling participation rates were found to have decreased over the 12-year span possibly due to the education and prevention efforts by NCAA staff and member schools. During this span, the proportion of male collegiate athletes at risk for or meeting clinical criteria for disordered gambling similarly decreased from 4% in 2004 to 1.8% in 2016, while rates remained relatively constant among women at <1% across all years.72

Volberg and colleagues73 and the Australian Productivity Commission9 both have highlighted multiple prevalence studies indicating that gambling by collegiate athletes begins prior to college. In the 2016 study, Paskus and Derevensky60 reported that most male collegiate athletes who indicated past year gambling first reported gambling either before or during high school. Among women, most reported initial gambling in high school or in college.60

Gambling activities, sports wagering and problem gambling among male collegiate athletes

The overall reduction in gambling participation rates among male collegiate athletes from 2004 to 2016 is reflected across most types of gambling activities (table 1). Sports wagering among male collegiate athletes continued to be popular, with 24.4% reporting engaging in sports wagering during the past year and 8.9% reporting doing so monthly. Although rates of past year sports wagering did not increase in 2016 compared with 2012, rates of monthly engagement in sports wagering increased, although marginally. The activity that showed the largest decrease over time was playing cards for money, with 46.8% of men engaging in this activity over the past year in 2004 and 22.9% in 2016. Monthly engagement in card playing also decreased notably over the 12-year span. Furthermore, although rates of past year internet casino gambling increased from 2004 to 2008, it has steadily decreased from 2008 to 2016. Richard et al 72 suggested that the wording of the question on the survey resulted in low responses for internet gambling. College students may likely endorse participation in internet wagering only if they do so on a desktop computer, rather than mobile gambling (smartphone), although all such forms constitute wagering over the internet.

Table 1

NCAA male participation in gambling activities between 2004 and 20164 72

Despite NCAA regulations prohibiting collegiate athletes from engaging in gambling activities related to intercollegiate or professional sporting events, sports wagering still appears to be a frequent activity among this population. Specifically, wagering on the NFL (65% of all collegiate athletes who engage in sports wagering) was the most common target of sports wagering, followed by NCAA basketball (55%).72 As for wagering on college games, the rates of participation remain low, with the lowest level of engagement for betting on games involving one’s own team.4 60 72

Gambling activities, sports wagering and problem gambling among female collegiate athletes

While overall rates of gambling participation among female collegiate athletes have decreased from 2004 (49%), they have remained relatively constant between 2008 and 2016 (39% in 2008, 39% in 2012, 38% in 2016) (table 2).60 72 Playing cards for money and betting on games of personal skill showed the largest decline among women. A small decrease was also observable for past year engagement in internet casino games. Rates of engagement in sports wagering are on a continuous decrease, with the lowest rates of past year (4.9%) and monthly (0.5%) participation being reported in 2016 in spite of increased availability and accessibility to gambling venues.

Table 2

NCAA women’s participation in various gambling activities between 2004 and 2016 (4, 72)

Compared with men, sports wagering appears to be much less frequent among female collegiate athletes.60 72 For the few women betting on sports, the most common target of sports betting in 2016 was wagering on NCAA men’s basketball (44%) and on the NFL (44%).60 72 Additionally, betting on college games appears to be a rare event for women, with extremely low rates in 2016. Finally, only 3.1% of women in the 2016 survey reported fantasy league participation.

Gambling among elite athletes (collegiate and non-collegiate)

Elite athletes may be particularly susceptible to developing gambling problems given their age, the availability and accessibility of gambling venues, the social acceptability and glamorisation of gambling, their access to money through loans, credit cards and scholarships, their general risk-taking, their perceived knowledge of sports and awareness of injuries that might inform sports wagering, and their self-perceived invulnerability,45 74 all of which suggest easy engagement in widespread gambling opportunities.73

A plethora of high-profile professional elite athletes have reportedly suffered from problematic gambling-related behaviours. Indeed, high-profile professional athletes in virtually all professional sports have readily admitted having gambling problems. Prominent gambling-related scandals within professional, collegiate and Olympic sports have been reported. Such problems include basketball point shaving scandals, rugby league and cricket betting scandals, and the 1919 Black Sox World Series gambling scandal, among others. Such activities have become commonplace among elite athletes worldwide,75–77 and sports wagering, through match fixing, has made the industry highly vulnerable to corruption.78 79 The sports betting market and athletes in particular have become susceptible to influence from organised crime syndicates.80

Grall-Bronnec and colleagues,81 in their assessment of 1236 European professional athletes, reported that 56.6% of athletes had gambled during the previous year, with the prevalence of problem gambling being 8.2%, and with betting on one’s team, betting online and gambling regularly being associated with gambling problems. Betting on one’s team was specifically associated with current or past gambling problems. They concluded that professional athletes are particularly susceptible to both gambling and problem gambling. In yet another study comparing current, former and non-athletes, former (13.0%) and current professional and elite athletes (7.0%) scored higher on gambling severity screens and were more likely to be identified as disordered gamblers when compared with non-athletes (3.0%).82 Expanding on this research, Weiss and Loubier83 found that former professional and elite athletes tend to participate in ‘skill-based’ forms of gambling, including sports betting and poker, while also being more likely to wager on the sports in which they participate.

Cultural/Ethnic considerations in problem/disordered gambling

Although substantial empirical evidence supports the finding that male collegiate athletes engage in gambling more frequently and have more gambling-related problems than female peers, less research has gone into investigating the prevalence and characteristics of gambling disorder within different ethnic groups.84 Several studies of non-athletes in the USA, Canada and Australia85–92 reported higher prevalence rates of disordered gambling among Native Americans,89 92 Asians,86 93 Blacks and Hispanics90 compared with Caucasians. Relevant to collegiate athletes, Nowak’s review44 concluded that although only a limited number of studies have investigated the gambling behaviours among minority groups, and such groups tend to have higher rates of problem gambling.

Mental health symptoms and disorders associated with problem/disordered gambling

Problem and disordered gambling typically result in multiple negative outcomes, including risky behaviours, underperformance in roles and mental health disorders.94–96 Problem/disordered gamblers often experience decreased academic and athletic performance, engage in socially isolating behaviours, experience difficulties in social relationships, and are at heightened risk of suicidal ideation and attempts.74 97–99 They generally experience more intense anxiety, feelings of helplessness, irritability and depression.100 101 Similarly, they are more prone to substance use disorders, behavioural problems, familial and financial hardships, and poor work and school performance.102 103 According to the US National Epidemiologic Survey on Alcohol and Related Conditions, disordered gamblers had six times greater risk of alcohol abuse and 4.4 times greater risk for substance use disorder than non-gamblers.104 Studies of general population samples of problem gamblers revealed higher rates of nicotine dependence (60%), alcohol and substance use disorders (58%), mood disorders (38%), and anxiety disorders (37%).94 105 106 It has been estimated that over 96% of individuals with a significant gambling disorder have one or more mental health disorder, with 64% having three or more mental health disorders.107

Unlike other mental health disorders, including some other addictive disorders, a gambling disorder can go undetected for some time. It is often referred to as a ‘hidden addiction’,45 108 yet the consequences can be pervasive. Gambling begins early in most cultures, yet few educational institutions have gambling prevention programmes for youth.109–111

Management

Given the potential severity of problem/disordered gambling and its association with serious mental health symptoms and disorders, comprehensive and expanded screening, brief interventions, and referrals for more comprehensive treatment are warranted.112 113 More universal screening and follow-up with treatment referrals for individuals with a gambling disorder are especially important.114 Coaches and athletic directors remain in a unique position to help promote prevention programmes and assist athletes who are experiencing gambling problems.

Several studies point to the importance of coaches, athletic directors, college faculty and administrative personnel more closely addressing the issues related to gambling.61 71 As such, inservice training related to gambling disorders is warranted. In the 2016 NCAA survey, the three most effective ways to influence collegiate athletes not to wager on sports were by coaches, NCAA-imposed penalties and discussions with team mates.60 From a preventative perspective, more educational programmes are recommended.109 For collegiate athletes, it is important to allow those with a gambling problem to go on academic or athletic leave when recovery considerations predominate. Reasonable accommodations to attend off-campus treatment facilities or to take a medical leave may be warranted.

While there are no approved regulatory pharmacological interventions for disordered gamblers, multiple controlled trials of various medications (eg, naltrexone, nalmefene, fluvoxamine) have shown efficacy.115–119 A number of meta-analyses of studies examining the use of pharmacological and behavioural treatments for disordered gambling have reported positive results,120–123 in particular the use of cognitive behavioural treatments.121 124 Hodgins and colleagues124 have also suggested that brief motivational treatments may be promising, while others have argued for the use of self-help groups (eg, Gamblers Anonymous).125–127 While both pharmacological and behavioural treatments are helpful, they require further research to assess their effectiveness for specific types of gamblers. Treating all problem gamblers as a homogeneous group has significant limitations.45

Conclusions and recommendations

Elite athletes may be particularly at risk of a gambling disorder as a result of their over-representation of young men, their high degree of competitiveness, high levels of sensation-seeking, impulsivity, increased risk-taking behaviours, and their perceived knowledge and information concerning sports and the health of team mates. Reported prevalence rates of gambling disorders among elite athletes typically are higher than their non-athletic peers. Gambling disorders are typically accompanied by multiple social, psychological, mental health and financial issues, all of which impact the individuals’ mental health and athletic performance.

Only a few studies have assessed short-term and long-term effects of problem gambling among elite athletes. More research is necessary to determine the direct impact on athletic performance and whether athletes in certain sports are more prone to excessive gambling.60 While a gambling disorder is often a hidden addiction, the consequences can be devastating for elite athletes and the integrity of the sport.

What is already known

  • Gambling activities continue to grow and newer forms of gambling (eg, online gambling) dominate the market.

  • Disordered gamblers, those who experience significant gambling-related problems, most often have concomitant mental health, and psychological and psychiatric issues.

  • Elite athletes are not immune to the lure of gambling.

What are the new findings

  • Elite athletes are at a greater risk of gambling problems than the general population.

  • New forms of gambling (eg, online gambling, inplay gambling, prop bets) may be particularly problematic for elite athletes.

Acknowledgments

The authors thank the other participants in the 2018 IOC Consensus Meeting on Mental Health in Elite Athletes, including Cindy Miller Aron, David Baron, Antonia Baum, Abhinav Bindra, Richard Budgett, Niccolo Campriani, Joao Mauricio Castaldelli-Maia, Alan Currie, Lars Engebretsen, Ira Glick, Paul Filip Gorczynski, Vincent Gouttebarge, Michael Grandner, Doug Hyun Han, Margo Mountjoy, Aslihan Polat, Rosemary Purcell, Margot Putukian, Simon M Rice, Allen Sills, Torbjorn Soligard, Todd Stull, Leslie Swartz and Li Jing Zhu, for their input on the development and interpretation of this research.

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Footnotes

  • Contributors All authors have contributed to the preparation of this manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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