Inclusion criteria

Types of studies

This scoping review considered all study types.

Study selection

Following the search, all identified citations were imported into EndNote version 20, with duplicate studies removed and imported into Covidence. Following a pilot test, two independent reviewers (MT and HG) screened the titles and abstracts against the inclusion criteria previously outlined. Full texts of the potentially relevant studies were retrieved and reviewed in detail against the selection criteria by two independent reviewers (MT and HG), and a third reviewer (BF or ABM) was available at each stage to resolve any disagreements arising between the reviewers. Two disagreements were settled through discussions by the research team. Reasons for exclusion for those studies deemed not to meet the inclusion criteria are reported in Fig 1: PRISMA flow diagram.

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Fig 1. PRISMA flow diagram.

https://doi.org/10.1371/journal.pone.0346884.g001

Data extraction

We used a purpose-designed excel spreadsheet as a data extraction tool. The data extracted included details on the participants, study design (type of evidence), country, and key findings relevant to the review questions. As part of the data extraction we also included practices identified or recommended in the papers that encourage military personnel to participate in health research. The data extraction tool was pilot-tested with a small sample of studies, and their extracted data was compared and discussed for any inconsistencies or uncertainties. Based on this feedback, the tool was refined to improve clarity, consistency, and shared understanding among reviewers.

Data analysis

Using a narrative synthesis approach, findings were identified, summarised and explained within and across each included study. Through this synthesis, we were able to summarise key concepts and insights to support understanding the broader context and connections between different studies.

Study inclusion

Following the removal of duplicates, 219 studies were screened for eligibility. A total of 37 full text studies were retrieved and reviewed for inclusion in the scoping review by two independent reviewers (MT and HG), with 10 studies identified to be included in the review. Please see Fig 1 PRISMA flow diagram for the selection process and S4 File of included studies.

Characteristics of included studies

All ten studies were conducted in the United States, including five qualitative studies [23–27] and five quantitative studies [28–32]. Four of the qualitative studies used interviews [23–26] either through the phone or in person and one conducted focus groups [27]. The quantitative studies all used cross-sectional surveys [28–30,32,33], distributed online or via mail or face to face. Five of the studies focused on biomedical research, i.e., clinical trials [28,32], biobanking [29], and clinical research conducted during COVID-19 [31] or as part of Veteran Affairs (VA) medical research [30]. The other five studies were focused more broadly on health research with military personnel [23–27].

Most studies were conducted with veterans [23,25,27,29–31]. One study compared veterans’ views with non-veterans [32], another study examined the perspectives of National Guard and Reserve veterans, service members and military spouses [26] and a further study examined veterans and their caregivers’ perspectives [31]. Cook, et al. [24] examined the perspectives of current serving members. Several studies had a specific focus on understanding the perspectives of less researched military populations, including minority members [30], women [23], and transgender and gender-diverse veterans [25]. The characteristics of the included studies are further detailed in S2 File.

Reasons for participating: Altruism, personal benefit and moral obligation

Military personnel cited three motivations for taking part in health research: altruism, personal benefit and a moral obligation to help fellow military personnel. Six studies with current and former serving military personnel [23–26,28,30,32] described altruistic motivations, with participants viewing their participation to help other military personnel or their families. Female veterans, for example, valued their participation to represent and support fellow female military personnel [23,26].

Participating in military-based health research was also seen to reduce stigma towards the chronic and mental health conditions military personnel may experience [24]. Veterans involved in cancer research emphasised the importance of contributing to improve cancer treatments for other military personnel [28]. Campbell, et al. [32] have suggested that military personnel’s altruism may stem from the free health care they have received. Cook, et al. [24] have proposed that military personnel wanted to support researchers in undertaking their health studies.

The second reason cited for participating was a moral obligation, described as a duty to help military colleagues. What separated this reason from altruism more generally was that it was presented as an extension of the military role or a role-related duty. For transgender and gender-diverse veterans, research participation was seen as ‘mission orientated’, aimed at improving care for peers [25]. Cook, et al. [24] also described this duty to participate in health research to support colleagues in their study.

Finally, personal benefit, under various guises, was the third key motivator, cited in five studies [23,24,26,30,32].Current serving military personnel were particularly motivated to participate in health research that offered alternative treatments, especially as an alternative to medication use [24]. Another study, described this as ‘conditional altruism’, where both supporting others and securing a personal benefit were equally valued [24]. Similarly, veterans in the Campbell, et al. [32] rated improving their own health and accessing high‑quality medical care as equally important. Veteran participants also wanted to receive both overall research findings and their individual results to share with their doctor [27]. Connecting with other current serving military families through research was cited as an important benefit in another study [26].

Awareness and delivery of study information

Participants in the included studies nominated a lack of awareness of research opportunities as a barrier to participation [23]. Direct and personalised advertising was recommended to better engage military personnel [23–25,27]. Five studies [23–26,29] highlighted the need for tailored information, though preferred communication methods varied: veterans responded positively to mailed invitations with follow‑up phone calls in Littman, et al. [27], whereas electronic communication and face‑to‑face recruitment through support services were preferred in Davis, et al. [26] study.

Clear and concise language was essential. Participants wanted explanations of study purpose, procedures and inclusion of sensitive content, such as mental health, or combat-related questions [27]. Avoiding sensitive or distressing topics or collecting information that could pose a security risk was also important [26]. Veterans recommended clear statements that participation would not affect benefits or healthcare. Current serving participants in Cook, et al. [24] study highlighted understanding inclusion criteria; noting that clarity increased willingness to engage. Clear explanation of research terms such as clinical trial and attending to limited literacy in information provision was also emphasised [28].Only a small number of studies addressed consent models. Opt-in consent was strongly preferred amongst veteran participants in one study [29]. Another study highlighted the importance of support of unit commanders, while also recognising that coercion could bias participation [26]. In the Hillyer study [28], former serving military personnel were more likely to participate in research when their primary care physician recommended the study within a trusted relationship. However, as this study was focused on clinical trials, it is unclear whether this influence extend to other types of health research.

Convenience and incentives

For both current and former serving personnel, convenience and incentives were identified as important facilitators of research participation in four studies [23–25,27]. For female veterans, travel and time compensation supported participation [23], with most participants favouring immediate payment in cash, cheque, or debit card form [27]. When exploring the views of gender-diverse former serving military personnel, participants identified being paid and having a central location and accessible time availability as essential requirements to participate [25]. Relatedly, current serving military participants highlighted that being able to participate from home or in a location close to home and at a convenient time would influence their participation in health research [24]. Some veteran participants in Littman, et al. [27] study, preferred research to be conducted in VA health settings, which they felt signalled legitimacy and researcher understanding, though a few found these settings less comfortable. Participants also emphasised the importance of researchers being punctual and using their time efficiently as a sign of respect [27], and recommended sharing an agenda in advance for longer research visits [27]. The former serving and current serving participants in Davis, et al. [26] study almost exclusively preferred online surveys for data collection for the convenience of their research participation. Another related area considered was the strong support (77%) for the use of residual clinical samples in Kaufman, et al. [29] study, which requires no further demands on veteran participants after consent is obtained.

Taking diversity into account in study design and execution

Three studies with veterans found that acknowledging the heterogeneity of the military population when designing and executing health research was a positive influence on participation [23,25,30]. Gender-specific research on health was seen as acknowledging and valuing diversity and it recognised that most military health research undertaken to date has been with male participants [23]. The role research can play in increasing understanding of the social, political and health issues that affect gender-diverse population groups was a facilitator for participating in military health research [25]. Additionally, the inclusion of gender diversity within health research has the potential to promote equity within the force [25]. Ethnic diversity has also been proposed as an influential component of willingness to participate in health research among military personnel [30]. However, the authors study found that race was not associated with increased participation of veterans in health research, although minority and less educated veterans had more negative attitudes regarding research [30]. Overall several studies suggested the recognition of diversity in research design and implementation can support enrolment and help address health disparities and challenges faced by diverse groups of military personnel.

Trust in research and researchers

Trust and safety concerns were identified as a barrier, particularly for veterans’ participation in research [27]. The importance of potential participants trusting that the research would be impactful and lead to positive change for others was identified [27]. Some groups were proposed as less trusting, including women, African Americans [23], and transgender and gender-diverse populations [25]. Veteran participants also needed to trust that researchers would maintain their confidentiality, particularly in relation to studies focused on military experience and mental health [23,25]. The emphasis on confidentiality focused on ensuring research information remained separate from service, veteran and health records [25,27]. Those studies recommended that the separation of service and health information from research data be conveyed to participants in clear and simple ways that explain that not only will the research data not be shared, but researchers are unable to access service or health records without the specific consent of the participant and for a particular ethically approved purpose [27]. Working with military personnel when designing and conducting research can support the relevancy and acceptability of participants as well as support dissemination [27]. For longitudinal studies, the need to build a trusting researcher-participant relationship was recommended as more important for retention than increasing incentives [26]. Padala, et al. [31] examined the perspectives of veterans and their caregivers on participation in clinical research during the COVID −19 pandemic and found that most were contented to continue participating with additional screening and information provided by researchers. A small number of participants in Chrystal, et al. [23] study noted that they had participated in previous research and had not been provided with the study findings, which could contribute to distrust of researchers.

Table 1 outlines practices that can encourage military personnel to participate in health research.

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Table 1. Practices that can facilitate the informed participation of military personnel.

https://doi.org/10.1371/journal.pone.0346884.t001

Summary of major findings

This scoping review aimed to understand military personnel’s perspectives on the conduct of health research undertaken with military populations. The findings from this scoping review indicate that there are three main reasons that military personnel decide to participate in health research: firstly, altruism, the desire to help others; secondly, a moral obligation to assist military colleagues; and thirdly, to receive a perceived personal benefit. Research participants often reported these reasons overlapped to support their participation. This combination of motives is described by Cook, et al. [24] as conditional altruism, where a personal benefit and benefiting others were both required to participate. Conditional altruism embodies the concept of reciprocity, whereby carrying out an action (helping others through duty or desire to assist) is accompanied by a personal benefit received through participating in research. This scoping review also identified key factors that facilitate or act as barriers to participating in health research: firstly, enhancing awareness of research and tailoring the delivery of study information to military personnel; secondly, careful deliberation regarding participant convenience and incentives; thirdly, diversity considerations incorporated into the study design, execution and dissemination; and, finally, the enhancement of trust for participants in research and researchers. In the rest of this discussion, we describe the key factors that can facilitate or act as barriers to participation in research and draw attention to the need to align these with motivations.

Interpretation of motivations

Many of the included studies suggested that military personnel are willing to participate in research but are unaware of opportunities or find that the study does not align with the motivations to participate that we identified in this paper. Clear and comprehensive communication about the study’s purpose and potential benefits for participants and the broader military community is essential. Studies that do not appear to offer benefits for the participants or other military personnel are therefore unlikely to be attractive.

Recruitment strategies should be tailored to participants and should directly address research motivation [29]. Outreach by researchers and indirect approaches through trusted health practitioners, community figures [35] to share research opportunities, establishing or utilising research registers [36,37], and contacting potential participants through email and mail invites, military personnel service organisations and foundations, and community groups are all important strategies. Although not identified in the current scoping review, social media and Facebook advertisements may also be effective in recruiting veterans [38,39]. A tailored approach could be further enhanced through the application of codesign methodologies to ensure research and the development of materials reflect the targeted population within the diverse military community, including older veterans, women, those deployed, military families etc. Codesign approaches have successfully been used with military personnel for a wide variety of clinical and applied health research [39–44]. Another opportunity may be through the implementation of decision-making aids which allow potential participants to indicate their own preferences and address misconceptions about the possible benefits of participating [45]. Decision-making aids have been successfully used in clinical trials for individualised risk and benefit assessment and to provide participants with opportunity for shared decision-making regarding treatment [9].

Interpretation of contextual factors

Military personnel, especially when serving, can have demanding schedules, frequent changes in location and unique stressors, so accommodating their availability, minimising disruptions and supporting their participation through considerations such as the provision of childcare or online participation can enhance their willingness to participate. Veterans may also experience difficulties related to age, location, health and mobility that make their participation more difficult. Campbell, et al. [34] provide a helpful review of the barriers that current serving and veteran military populations face. Careful consideration of how the needs of groups of potential participants can be best met, particularly through input from targeted populations, would assist in supporting convenience. Offering an appropriate balance of convenience and incentives addresses the conditional altruism motivation. As participation becomes more difficult, the personal benefit from taking part becomes less meaningful, and the win-win approach of benefits to self and other [46] becomes harder to achieve. Providing appropriate incentives acknowledges participants’ time and effort and may foster a sense of being valued and respected by researchers.

As only one study specifically explored retention [26], factors related to retention remain less understood, although it may be that an ongoing continuation of the original research motivators – altruism, personal benefit and/or a moral obligation – is required for military personnel to remain in research studies. Retention is of critical importance for ensuring adequately powered quantitative health research [47]. In a study on financial reimbursement in clinical trial research, Novak, et al. [47] reported a modest increase (5%) in retention in longitudinal clinical research when monetary incentives were used. This increase in retention, particularly in smaller-scale studies, may support meaningful study findings by reducing underpowered studies [47]. However, the findings by Davis, et al. [26] also highlight that maintaining a trusting participant and researcher relationship is likely to be more important in retaining participants than incentives.

Integration with literature

Taking diversity into account in study design and execution is crucial for addressing the trust and confidentiality concerns of military personnel, particularly women, African American and transgender and gender-diverse populations. If potential participants do not see study participants like themselves in a study, they are unlikely to perceive personal benefits or benefits for other military personnel, thus undermining most of the motivations for taking part. Engaging diverse military populations in the research process, from design to execution, contributes to the development of culturally and gender-sensitive materials, ensures the research team reflects the study population’s diversity [48], and helps build trust. Intentional development of a diverse research team can also support recruitment and retention, and codesign approaches can support this team’s diversity [48].

The findings from this scoping review highlight the importance of trust as a fundamental pillar in undertaking research. Without trust in research and researchers, potential participants are unlikely to engage in research and, if they do participate, they may not provide honest and accurate data. Enhancing trust between participants and researchers is particularly important for vulnerable and diverse populations. Trust is interrelated with concerns about confidentiality and privacy, and fear of exposure [23]. Participants who do engage in research are entering into a relationship with researchers whom they often do not know but are required to trust [49]. This places an onus on researchers to act ethically, uphold this trust and protect the wellbeing of research participants [49]. The review also found that participants need to understand researchers’ motivations and believe that researchers will uphold participants’ rights and safeguard their wellbeing.

The provision of transparent, adequate and accessible information to support informed consent, which outlines confidentiality, privacy and data protection measures, can also engender trust. Guillemin, et al. [50] argue the need for information statements to shift from protecting researchers and institutions to become a trust contract that explicitly establishes what the “participant can trust the researcher and the institution to do, and not to do” (p 292). Participants need to understand their rights and how their data is being used and protected. Safeguards to protect their personal information and maintain their privacy and confidentiality must also be clearly described. It is also vital that researchers provide a feedback loop by sharing the findings with research participants, as not receiving these contributes to distrust in research.

For military personnel, lack of trust in military institutions [51,52] more generally may also contribute to a lack of trust in research during and after service. Guillemin, et al. [50] propose that trust in both the researchers and in the institutions conducting the research is important. If there are reputational issues with the institution involved in or sponsoring the study, this can impact their willingness to participate. Potential participants need to have faith in the institution’s capacity and history in upholding their systems of ethics and integrity [50]. Finally, trust is fragile, easily lost [53] and very difficult to rebuild, and breaches of trust can have implications far beyond the individual research study. Participants who come to distrust researchers are less likely to take part in research in the future. Accordingly, there is a need to share research decision-making with diverse stakeholders at all stages of the process to ensure ethical accountability [54] and continued renegotiation of trust over time [53].

Although this review highlights the importance of co-designing research with participants, this is a delicate balance. An example of this is the participants in the Davis, et al. [26]6) study who recommended not including sensitive topics such as deployment experiences or mental health in research. Without being able to sensitively investigate these experiences it is difficult to understand and address some of the challenges that are faced by military personnel that have been the focus of multiple government inquiries such as the Royal Commission into Defence and Veteran Suicide in Australia [55], the U.S. Government Accountability office [56] examination of mental health services for veterans and the UK Armed forces and veterans mental health inquiry [57]. Nonetheless, the participants’ views in the Davis, et al study [26] highlight how important it is for researchers to appreciate military personnel’s concerns and to ensure that wellbeing supports are proactively provided. Human research ethics committees have a key role to play here as review bodies that are explicitly concerned with ensuring the interests of research participants are protected.

Limitations

All the included studies were from the US, despite many other countries having large current and forming serving military populations. This limits the generalisability of the findings in terms of differences in recruitment and ethics, as well as legislation and protocols focused on consent and incentives between jurisdictions. Different countries may have varying military cultures, health care systems, and research ethics, which could impact the factors influencing participation. Most of the included studies were with veteran populations. Therefore, some of these findings may not apply to current serving members. All the included studies were cross-sectional in design, which limits the understanding of how perspectives may change across the military lifecycle and as a result of participating in research. Many of the studies also included small sample sizes or were focused on one aspect, i.e., willingness to continue to participate in face-to-face health research during COVID-19 pandemic or consent models for biobank research. A key limitation is that these perspectives only include those who consented to take part in a study [29]; therefore, there is a bias towards the perspectives of military personnel willing to take part in research, which may not reflect the views of the broader military population [32]. Despite these limitations, there are several key messages that can be used by researchers in the design and implementation of health research with military personnel.

Future directions

Some findings require further examination through empirical studies. For example, Kaufman, et al. [29] found that there was support for using residual samples in research and, likewise, Littman, et al. [27] study participants felt that utilising existing data in military records was an appropriate way of reducing the burden on participants. A better understanding of military personnel’s perspectives on the secondary use of their data and linkage of their research data with other administrative health data sources would help address this gap. Given the challenges noted in some countries with the recruitment and retention of current serving military personnel, including deployment, frequent relocation, training activities and fixed work schedules [47], it is important to undertake more research, especially with this group, to understand the needs they face. Similarly, future research should examine the views of individuals who did not participate, to provide a more representative understanding of perspectives across the broader military population. Sub-studies need to be built into longitudinal studies to understand how perspectives may change across the military lifecycle and as a result of participating in research.