Macartan Humphreys

Despite decades of scholarship on protest effects, we know little about how bystanders, citizens who merely observe protests without participating, are affected by them. Understanding the impact of protest on bystanders is crucial as they constitute the silent majority whose latent support, normative beliefs, and concrete actions can make or break a movement's broader societal impact. We address this gap with a novel field experiment in Berlin, Germany, randomly routing pedestrians past (treatment) or away from control three large-scale Fridays for Future (FFF) climate strikes. Additionally, we conducted a one-month follow-up to assess the persistence of effects. We find no detectable impact on climate attitudes, vote intentions, or norm perceptions but a substantial increase in immediate donations to climate causes. These results suggest that protest is more likely to influence bystanders through immediate behavioral cues than through changes in attitudes or norms. Our findings challenge the prevailing assumption in both scholarship and public discourse that protests reshape public opinion in a direct and unmediated way, calling for renewed theorizing of protest effects that centers on observers' immediate behavioral activation rather than just opinion change mechanisms.

Objectives: Most low-income countries are characterized by poor health infrastructures and lack systems needed to timely detect and control disease outbreaks, such as the 2014-16 Ebola Viral Disease and COVID-19. In such contexts, a “One Health” approach, which involves investing in both human and animal health systems, plausibly improves local health outcomes by enabling early detection of zoonotic diseases before they are transmitted to humans, and by timely triggering a health system response needed to mitigate possible outbreaks. There is an urgent call to translate One Health into action and create inclusive and sustainable policies. There is, however, little direct evidence on the gains from One Health approaches. We contribute here by using a randomised intervention to assess the impact of a participatory community-based One Health program. Evidence before this study: A 2016 systematic review searched Scopus, PubMed, and ISI Web of Science using the term “One Health”, restricting publication date between 2003 and 2015.1 The search yielded 1,839 unique articles, but only four evaluated a One Health intervention using quantitative metrics. We performed the same search on 10 April 2023. Reflecting the burgeoning interest in One Health, we found an additional 9,715 unique articles. In total, only 17 articles, however, evaluated a One Health intervention implemented in real-world settings, utilizing quantitative metrics. Furthermore, these studies did not employ experimental methods to assess impact, relied on datasets with often only few observations, and focused exclusively on disease incidence. In the Supplementary Material (Appendix A), we provide a flowchart of the literature review and summarize these 17 related studies. Contribution: This study is the first cluster-randomised trial to assess the impact of a participatory community-based intervention establishing local health platforms employing a One Health approach. We evaluate the intervention at scale and explore the impact of the program on human health, but also on key intermediary outcomes like animal health and animal and human health behaviours. Design: Cluster-randomised control trial. Setting: 363 villages in rural Sierra Leone Participants: The Sierra Leone government and communities recruited, trained and installed Community Animal Health Workers (CAHWs) to work alongside Community Health Workers (CHWs) in 300 randomly selected rural villages in Sierra Leone. Another 63 villages were randomly selected as control sites and had CHWs exclusively. CAHWs provided essential animal health services, disseminated information regarding animal and human health best practices, and actively participated in surveillance efforts by reporting suspected disease symptoms to government supervisors. Main outcome measures: Survey based measures of human health, as well as key intermediary outcomes; including animal health, animal and human health-related behaviours, integration into public services, and household wealth. Results: In July and August 2017, the community-based One Health program successfully recruited, trained and installed CAHWs across 287 villages. Throughout the program's duration, spanning from July 2017 to July 2019, the CAHWs reported on 19,283 suspected disease-related events. Using survey data from over 2,500 respondents, collected in March and April 2020, we found no evidence for impacts on human health (-0.008 Standard Deviation Units (SDU), 95% CI -0.148, 0.133). The program did however significantly improve core intermediary outcomes, including animal health (0.164 SDU, 95% CI 0.017, 0.311), animal husbandry practices (0.255 SDU, 95% CI 0.087, 0.424), human health behaviours (0.187 SDU, 95% CI 0.025, 0.348), integration into public services (0.339 SDU, 95% CI 0.137, 0.541), and household wealth (0.163 SDU, 95% CI 0.053, 0.273). Conclusions: Participatory community-based One Health interventions can serve as a guide for policymakers that seek to strengthen the national health systems by improving disease surveillance and preventative practices that are expected to increase health security nationally and globally. More research is needed to understand how the magnitude, modality, and timing of the program and background conditions may shape program impact on human health. Trial registration: The trial was registered at the National Trail Registry (#21660), which is part of the ICTRP, and OSF (https://osf.io/9xfv3).

Major crises regularly lead to a public debate about whether the government should be able to restrict personal freedoms in the interest of public welfare. Using a factorial survey experiment administered to more than 10,000 online survey respondents in Germany during the COVID crisis, we assess under what conditions citizens are willing to sacrifice individual civil freedoms in the interest of public well-being. Despite highly visible protests against Covid policies, we document broad willingness to accept restrictions on rights alongside sharp heterogeneity with respect to vaccination status and party identification. The majority of citizens are vaccinated and strongly support limitations on freedoms in response to extreme conditions---especially when the vaccinated themselves are exempted from these limitations. The unvaccinated minority generally prefer no restrictions on freedoms regardless of the severity of the pandemic. These policy preferences in turn matter for reported trust in government, in opposite ways for vaccinated and unvaccinated citizens.

Suppose X and Y are binary exposure and outcome variables, and we have full knowledge of the distribution of Y, given application of X. We are interested in assessing whether an outcome in some case is due to the exposure. This “probability of causation” is of interest in comparative historical analysis where scholars use process tracing approaches to learn about causes of outcomes for single units by observing events along a causal path. The probability of causation is typically not identified, but bounds can be placed on it. Here, we provide a full characterization of the bounds that can be achieved in the ideal case that X and Y are connected by a causal chain of complete mediators, and we know the probabilistic structure of the full chain. Our results are largely negative. We show that, even in these very favorable conditions, the gains from positive evidence on mediators is modest.

As of early 2022 an estimated 50% of the global population is fully vaccinated for COVID-19 but the global distribution of vaccines is extremely unequal, over 85% vaccinated in the top 10 countries and below 3% in the bottom 10. Given that governments need to secure public support for investments in global vaccine sharing, it is important to understand the levels and drivers of public support for international vaccine solidarity. Using a factorial experiment administered to more than 10,000 online survey respondents in Germany, we show that global inequities are out of line with domestic German public opinion. Respondents are supportive of substantive funding amounts, on the order of the most generous contributions provided to date, though still below amounts that are likely needed for a successful global campaign. Public preferences appear largely to be driven by intrinsic concern for the welfare of global populations though are in part explained by material considerations – particularly risks of continued health threats from a failure to vaccinate globally. Strategic considerations are of more limited importance in shaping public opinion; in particular we see no evidence for free riding on contributions by other states. Finally, drawing on an additional survey experiment, we show that there is scope to use information campaigns highlighting international health externalities to augment public support for global campaigns.

Recent evidence suggests that vaccination hesitancy is too high in many countries to sustainably contain COVID-19. Using a factorial survey experiment administered to 20,500 online respondents in Germany, we assess the effectiveness of three strategies to increase vaccine uptake, namely, providing freedoms, financial remuneration, and vaccination at local doctors. Our results suggest that all three strategies can increase vaccination uptake on the order of two to three percentage points (PP) overall and five PP among the undecided. The combined effects could be as high as 13 PP for this group. The returns from different strategies vary across age groups, however, with older cohorts more responsive to local access and younger cohorts most responsive to enhanced freedoms for vaccinated citizens.

Widespread acceptance of COVID-19 vaccines is crucial for achieving sufficient immunization coverage to end the global pandemic, yet few studies have investigated COVID-19 vaccination attitudes in lower-income countries, where large-scale vaccination is just beginning. We analyze COVID-19 vaccine acceptance across 15 survey samples covering 10 low- and middle-income countries (LMICs) in Asia, Africa and South America, Russia (an upper-middle-income country) and the United States, including a total of 44,260 individuals. We find considerably higher willingness to take a COVID-19 vaccine in our LMIC samples (mean 80.3%; median 78%; range 30.1 percentage points) compared with the United States (mean 64.6%) and Russia (mean 30.4%). Vaccine acceptance in LMICs is primarily explained by an interest in personal protection against COVID-19, while concern about side effects is the most common reason for hesitancy. Health workers are the most trusted sources of guidance about COVID-19 vaccines. Evidence from this sample of LMICs suggests that prioritizing vaccine distribution to the Global South should yield high returns in advancing global immunization coverage. Vaccination campaigns should focus on translating the high levels of stated acceptance into actual uptake. Messages highlighting vaccine efficacy and safety, delivered by healthcare workers, could be effective for addressing any remaining hesitancy in the analyzed LMICs.

Survey evidence suggests that vaccination hesitancy is too high in many countries to achieve herd immunity against Covid-19. In this study, we assess the effectiveness of three strategies to increase vaccine uptake, namely, providing freedoms, financial remuneration, and vaccination at local doctors. We use a factorial survey experiment administered to 20,500 online respondents in Germany. Our results suggest that all three strategies can increase vaccination uptake on the order of 5 percentage points (PP) among the undecided. The combined effects could be as high as 13 PP for this group. The returns from different strategies vary across age groups, however, with older cohorts more responsive to local access and younger cohorts most responsive to enhanced freedoms for vaccinated citizens.