We thank our many volunteer translators, whose names are listed in the Appendix. We also thank Prolific for sponsoring the participants for the survey experiment and Aristeo Marras for data advice. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

We draw on genetics research to argue that complex psychological phenomena are most likely determined by a multitude of causes and that any individual cause is likely to have only a small effect. Building on this, we highlight the dangers of a publication culture that continues to demand large effects. First, it rewards inflated effects that are unlikely to be real and encourages practices likely to yield such effects. Second, it overlooks the small effects that are most likely to be real, hindering attempts to identify and understand the actual determinants of complex psychological phenomena. We then explain the theoretical and practical relevance of small effects, which can have substantial consequences, especially when considered at scale and over time. Finally, we suggest ways in which scholars can harness these insights to advance research and practices in psychology (i.e., leveraging the power of big data, machine learning, and crowdsourcing science; promoting rigorous preregistration, including prespecifying the smallest effect size of interest; contextualizing effects; changing cultural norms to reward accurate and meaningful effects rather than exaggerated and unreliable effects). Only once small effects are accepted as the norm, rather than the exception, can a reliable and reproducible cumulative psychological science be built.

The spread of COVID-19 within any given country or community at the onset of the pandemic depended in part on the sheltering-in-place rate of its citizens. The pandemic led us to revisit one of psychology's most fundamental and most basic questions in a high-stakes context: What determines human behavior? Adopting a Lewinian interactionist lens, we investigate the independent and joint effects of macro-level government policies and micro-level psychological factors-i.e., personality-on whether individuals sheltered-in-place. We analyzed data collected in late March and early April 2020 from 101,005 participants in 55 countries, a time period that coincided with the early and accelerating stage of the COVID-19 pandemic. This time period also contained substantial variation in the stringency of governmental policy towards shelteringin-place, both between countries and within each country over time. Analyses revealed that personality and the stringency of governmental policies independently predicted sheltering-inplace rates. Policy stringency was positively related to sheltering-in-place. For the personality dimensions, openness, conscientiousness, agreeableness, and neuroticism all predicted higher rates of sheltering-in-place, whereas extraversion was negatively related to staying at home. In addition, two personality traits-openness to experience and neuroticism-interacted with governmental policy to predict whether individuals sheltered-in-place; openness and neuroticism each had weaker effects on sheltering-in-place as governmental policies became stricter. Theoretically, the findings demonstrate that individual differences predict behavior (i.e., sheltering-in-place) even when governments take strong action targeting that behavior. Practically, they suggest that even if governments lift their shelter-in-place restrictions, some individuals will shelter-in-place less than others.

We conducted a large-scale survey covering 58 countries and over 100,000 respondents at the onset of the COVID-19 pandemic—between March 20 and April 7—to explore how beliefs about citizens’ and government’s response to the COVID-19 pandemic affected mental well-being. Our analyses reveal three findings. First, many respondents indicate that their country’s citizens and government’s response was insufficient. Second, respondents’ perception of an insufficient public and government response and handling is associated with lower mental well-being. Third, we exploit time variation in country-level lockdown announcements, both around the world and through an event-study in the UK, and find that strong government actions—e.g., announcing a nationwide lockdown—were related to an improvement in respondents’ views of their fellow citizens and government, and to better mental well-being. These findings suggest that policy-makers may not only need to consider how their decisions affect the spread of COVID-19, but also how such choices influence the mental well-being of their population.