Study selection and characteristics
The search strategy yielded 6,108 studies. After screening the titles and abstracts, 49 studies were selected for full-text screening. A total of 24 full-texts met the inclusion criteria (20 were published16,24-42 while 4 were pre-prints).14,43-45 Eight of them included more than one randomised controlled trial study (RCT), yielding 36 RCTs in total. No studies with other design (such as quasi-experimental studies) fulfilled the inclusion criteria. The majority of RCTs (32/36) targeted COVID-19, while 4 RCTs targeted multiple respiratory infections, including (but not limited to) influenza. The characteristics of included RCTs are described in more detail in Table S1 and S2. Sixteen (21 RCTs)14,24,28,29, Study 1,31-33,34, Study 1,35-38,40,42,44,45 of the 24 full-texts (36 RCTs) provided sufficient information to be included in the final quantitative analysis (Fig. 1).
Fig. 1: PRISMA Flow Diagram46 |* First 100 references from each database and search engine have been kept sorted by relevance
Effectiveness of prosocial messages
Twenty-six RCTs from high-income countries (USA, Denmark, UK, Japan, Germany, Turkey) reported positive effects regarding the included messages focusing on “protect-others” principle16,24,26, Study 1 & 2,27-29,32,34, Study 1 & 2,35-39,41,42,44, one RCT (from USA) reported negative effects25 and nine RCTs (from USA, Italy, France and multiple countries: Spain, Chile, Colombia) reported no difference14,26, Study 3,30,31,33,34, Study 3,40,43,45. None were in low or middle-income countries. Seventeen of the RCTs reporting positive effects regarding the included messages focusing on the "protect-others" principle were conducted in the USA, five in Europe, and four in Asia (see Supplementary file_1 pp.56-57). The main outcomes that positively affected by the messages focusing on protecting others were social distancing intentions (e.g., motivation to adhere to physical distancing, persuasiveness to self-isolate, avoid social gathering, stay home and keep a physical distance with others), mask wearing intentions, hand washing intentions, diverse-behavioural intentions (intentions that could not be categorized into specific groups, such as contact-avoidance intentions, protective behaviour willingness) and actual behaviours (see Supplementary file_1 pp.55-56).
Behavioural Tools and Mechanisms of Action (MoA) ontology evidence
Six of the nine MINDSPACE contextual influencers were identified across 102 intervention arms. On average, each intervention arm adopted 2.52 MINDSPACE contextual influencers. The four most common MINDSPACE contextual influencers were “Messenger” (n=33), “Salience” (n=96), “Affect” (n=42), “Ego” (n=43) (Table S3). The most often applied contextual influencers across the intervention groups focused on protecting others were “Salience” (n=53) and “Ego” (n=32). “Affect” was also present, however, less frequently (n=20).
Twenty-eight of the 93 BCTs were identified across 102 intervention arms. On average, each intervention arm adopted 4.03 behaviour change techniques. The seven most common BCTs identified were “Instruction on how to perform a behaviour” (n=96), “Information about health consequences” (n=84), “Salience of consequences” (n=69), “Information about social and environmental consequences” (n=28), “Credible source” (n=34), “Prompts/cues” (n=18) and “Avoidance/reducing exposure to cues for the behaviour” (n=26) (Table S4). Twenty-five of the 28 BCTs identified across the intervention arms focused on protecting others, with the following as the most frequently applied: “Instruction on how to perform a behaviour” (n=54), “Information about health consequences” (n=51), “Salience of consequences” (n=44), “Information about social and environmental consequences” (n=18), “Credible source” (n=14) and “Avoidance/reducing exposure to cues for the behaviour” (n=19).
MoA Ontology applied to 21 of the 24 full-text papers, covering 30 MoA subcategories. On average, each study adopted 3.48 MoAs, with “Behavioural intention” being the most common, followed by “Belief about one's social environment” and “Mental disposition” (Table S5; Table S6).
Cross-country Analysis
Due to the lack of data and the inability to perform a meta-analysis to assess the impact of each MINDSPACE contextual influencer and BCT across the USA and European countries, we calculated the Effective Ratio (ER) to strengthen our narrative analysis. In RCTs conducted in the USA, we evaluate the impact of five MINDSPACE contextual influencers (Fig. 2) and nine BCTs (Fig. 3). The most common MoAs were “Behavioural intention” and “Belief about message” followed by “Belief about consequences of behaviour” and “Willingness to comply”. In RCTs conducted in European countries, we evaluate the impact of four MINDSPACE contextual influencers (Fig. 2) and four BCTs (Fig. 3). The most common MoA was “Behavioural intention”, followed by “Motivation”, “Emotion process”, “Evaluative belief about behaviour” and “Belief about control over behaviour”. In RCTs conducted in Japan and Turkey, ER values could not be estimated for either MINDSPACE contextual influencers or BCTs, as there were no ineffective results.
Figure 2: ERs of MINDSPACE contextual influencers; ER=effective ratio|USA: The highest ER was observed for interventions focusing on “Norms” (ER=6), followed by “Affect” (ER=3.33), “Salience” (ER=2.27), “Ego” (ER=2) and “Messenger” (ER=0.57). For four MINDSPACE contextual influencers, ER values could not be estimated due to the absence of ineffective results. European countries: The highest ER was observed for interventions focusing on “Salience” (ER=7.5), followed by “Affect” (ER=6), “Ego” (ER=6) and “Norms” (ER=1). For five MINDSPACE contextual influencers, ER values could not be estimated due to the absence of ineffective results.
Figure 3: ERs of behaviour change techniques (within letters); ER=effective ratio.|USA: The highest ER in interventions pertained to “Avoidance/reducing exposure to cues for the behaviour” (ER=8), followed by “Salience of consequences” (ER=4.5), “Information about health consequences” (ER=4.25), “Instruction on how to perform a behaviour” (ER=3), “Information about others’ approval” (ER=2), “Information about social and environmental consequences” (ER=1.5), “Credible source” (ER=1.25), “Social support (practical)” (ER=1) and “Prompts/cues” (ER=0.29). For six BCTs, ER values could not be estimated due to the absence of ineffective results. European countries: The highest ER in interventions pertained to “Instruction on how to perform a behaviour” (ER=9), followed by “Salience of consequences” (ER=4.5), “Information about health consequences” (ER=4) and “Prompts/cues” (ER=1). For 19 BCTs, ER values could not be estimated due to the absence of ineffective results.
Populations’ characteristics affected by prosocial messages
Seventeen studies either lacked analysis on the impact of messages about protecting others on demographics or found no significant differences in demographic characteristics. However, six studies showed significant predictors of personal protective behavioural intentions and actual behaviours regarding respiratory infections. Such predictors were the age, gender, employment status, political orientation, race, region, education and health condition. The studies indicated that women, older people, those having less secure employment, more religious people, liberals or left leaning and those who are in worse health conditions intend to respect protective behaviours.
In particular, Browning, et al. 44 implied that older age, non-male gender identity, and less secure employment predicted higher intentions for COVID-19 preventative procedures. Conversely, identifying as White or White/Indigenous predicted fewer intentions for these procedures. Capraro and Barcelo 28 found that females, left-leaning individuals, and older individuals are more inclined to wear face coverings. However, the gender difference diminishes where wearing face coverings is mandatory, indicating a stronger impact on men. The study also proposes that messages emphasizing “your community” may be more effective for right-leaning individuals. According to Everett, et al. 14, older people and more religious people reported stronger behavioural intentions, while people who self-identified as White, male and conservative reported weaker behavioural intentions and felt less personally responsible for preventing the disease spread. Similarly, Hacquin, et al. 45 found that women and older people intend to respect protective behaviours and wash their hands more than men and younger people. In addition, education had a negative effect on people’s behavioural intentions regarding protective behaviours and handwashing. Pink, et al. 26 suggested that after seeing any message outlining key behaviours, women tend to increase their intentions to comply more than men. Additionally, the messages were more convincing to individuals leaning more liberal. Falco and Zaccagni 27 found that individuals in poorer health conditions and at higher infection risks are the most affected by the messages focusing on protecting others (families). Conversely, those in better health, facing lower infection risks, and frequently leaving their homes are less impacted by such messages.
Risk of bias
Agreement between the two independent raters in coding the risk of bias criteria was high (91,7%). Overall, we noted a high level of some concerns, which was the result of insufficient reporting of an appropriate analysis used to estimate the effect of assignment to the intervention. An intention-to-treat (ITT) analysis that includes all randomized participants was lacking, resulting in some concerns in all studies. Further, some of studies did not publish a protocol or register the studies on trial registries, making it difficult to assess reporting bias, and this led to downgrading of the evidence quality for the large majority of the included intervention types. The studies were assessed as low risk of bias for the domains “bias arising from the randomization process”, “bias due to missing outcome data” and “bias in measurement of the outcome”. Only one study assessed as high risk of bias due to missing outcome data (Fig. 4; Fig. 5; Table S7).
Fig. 4: Risk of bias assessment. Traffic-light plot of the domain-level judgements.
Fig. 5: Risk of bias assessment. Summary plot of the domain-level judgements.
Network Meta-analysis
Twenty-one RCTs with behavioural intention outcomes (social distancing, mask wearing, handwashing and diverse-behavioural intentions) were considered for quantitative analyses. None of the studies with actual behaviour outcomes met the eligibility criteria for inclusion in the meta-analysis. Nineteen RCTs (n=30 intervention arms (prosocial messages focused on public and loved ones; self-focused messages); n=19 comparator groups) were included in the class-effects model including multiple behavioural intention outcomes except from handwashing intentions outcome, due to inconsistency between direct and indirect evidence. There was a small increase in personal protective behavioural intentions for each intervention group (prosocial messages and self-focused messages) compared to the control group (no message, baseline message or self-protection message), based on effect estimates of standardised mean differences (SMDs) with 95% CrI. The marginally largest effect was observed for prosocial messages focused on loved ones compared to the control message (d=0.09, 95% CrI 0.06 to 0.14, CINeMA: Low). Additionally, the 95% Crl for the comparison between prosocial messages focused on loved ones versus control did not cross 0, indicating that a positive association between prosocial messages focused on loved ones compared to the control group exists in the population of interest. There was relatively low heterogeneity (SD= 0.07, 95% CrI 0.04 to 0.11), therefore there was limited variability between studies in the analysis. The mean value of the total residual deviance (69.52) was similar to the number of data points (65) in the analysis, which indicates a reasonable fit (Table 1; Table S14).
[Table 1 here]
Subgroup Network Meta-analyses by behavioural intention outcomes
There was no inconsistency identified between direct and indirect evidence for social distancing intentions (design-by-treatment interaction model: χ2(6)=3.704, p=0.717), mask wearing intentions (design-by-treatment interaction model: χ2(4)=4.585, p=0.333), diverse-behavioural interventions (design-by-treatment interaction model: χ2(3)=0.140, p=0.987). However, there was evidence of inconsistency for handwashing intentions (design-by-treatment interaction model: χ2(3)=9.115, p=0.028). Local testing of evidence loops identified a statistically significant difference between direct and indirect evidence for the comparison between prosocial public messages and prosocial loved ones messages (d=0.618, 95% CrI 0.04 to 1.19; p=0.03).
Effects on social distancing intentions
Thirteen RCTs were included in the random-effects NMA. There was a small increase in social distancing intentions for each intervention group compared to the control group, with the marginally largest effect to be observed for prosocial messages focused on loved ones (d=0.10, 95% CrI 0.04 to 0.16, CINeMA: Moderate) (Table S8; Figure S1; Table S15).
Effects on mask wearing intentions
Four RCTs were included in the random-effects NMA. There was a small increase in mask wearing intentions for each intervention group compared to the control group, with the largest effect to be observed for prosocial messages focused on public (d=0.16, 95% CrI 0.04 to 0.30, CINeMA: Moderate) (Table S9; Figure S2; Table S16).
Effects on handwashing intentions
Seven RCTs were included in the random-effects NMA. There was a small increase in handwashing intentions for each intervention group compared to the control group, with the largest effect to be observed for prosocial messages focused on loved ones (d=0.20, 95% CrI 0.15 to 0.52, CINeMA: Low) (Table S10; Figure S3; Table S17).
Effects on diverse-behavioural intentions
Nine RCTs were included in the random-effects NMA. There was a small increase in diverse-behavioural intentions for each intervention group compared to the control group, with the largest effect to be observed for prosocial messages focused on loved ones (d=0.17, 95% CrI 0.04 to 0.31, CINeMA: Low) (Table S11; Figure S4; Table S18).
Component network meta-analyses (CNMA) with Mindspace contextual influencers
Handwashing intention outcomes were excluded due to the inconsistency identified. The CNMA model had a reasonable fit (total residual deviance=57.96 from 59 data points) and low heterogeneity (SD=0.05, 95% CrI 0.01 to 0.10). Although limited evidence, a small increase in personal protective behavioural intentions for interventions that incorporated the “salience” (d=0.06, 95% CrI -0.04 to 0.16), “affect” (d=0.06, 95% CrI -0.04 to 0.15) and “ego” (d=0.05, 95% CrI -0.05 to 0.14) compared to interventions that did not include these contextual influencers was found (Table 2). To reduce potential heterogeneity, separate CNMA for self-focused messages was conducted (Table S12).
[Table 2 here]
Component network meta-analyses with BCTs
Handwashing intention outcomes were excluded due to the inconsistency identified. The CNMA model had a reasonable fit (total residual deviance=56.53 from 59 data points)) and low heterogeneity (SD= 0.03, 95% CrI 0.01 to 0.07). A small increase in personal protective behavioural intentions for interventions that incorporated the “information about health consequences” (d=0.12; 95% Crl 0.03 to 0.28) and the “avoidance/reducing exposure to cues for the behaviour” (d=0.19, 95% CrI 0.06, 0.32) compared to interventions that did not include these BCTs was found (Table 3). Separate CNMA for self-focused messages was conducted (Table S13).
[Table 3 here]