Political debate concerning moralized issues is increasingly common in online social networks. However, moral psychology has yet to incorporate the study of social networks to investigate processes by which some moral ideas spread more rapidly or broadly than others. Here, we show that the expression of moral emotion is key for the spread of moral and political ideas in online social networks, a process we call "moral contagion." Using a large sample of social media communications about three polarizing moral/political issues (n = 563,312), we observed that the presence of moral-emotional words in messages increased their diffusion by a factor of 20% for each additional word. Furthermore, we found that moral contagion was bounded by group membership; moral-emotional language increased diffusion more strongly within liberal and conservative networks, and less between them. Our results highlight the importance of emotion in the social transmission of moral ideas and also demonstrate the utility of social network methods for studying morality. These findings offer insights into how people are exposed to moral and political ideas through social networks, thus expanding models of social influence and group polarization as people become increasingly immersed in social media networks.O ur sense of right and wrong shapes our daily interactions in a variety of domains such as political participation, consumer choices, and close relationships. What factors inform our intuitions about morality? Influential theories in psychology maintain that our moral sense is shaped by the social world (1), insofar as decades of research demonstrate that social communities influence moral development in children (2) and account for cross-cultural variation in moral beliefs (3). Furthermore, social information serves as input for cognitive and emotional processes in moral judgment and decision-making (4).Despite a broad consensus that morality is influenced by attitudes and norms transmitted by our social world, remarkably little work has examined how social networks transmit moral attitudes and norms. Most existing research takes a dyadic perspective to study the social transmission of morality; typically, one person (such as a child) is exposed to another's ideas (e.g., parent) through behavior or communication (1, 2). In society, the transmission of morality goes well beyond the dyad. Our moods, thoughts, and actions are shaped by the entire network of individuals with whom we share direct and indirect relationships (5). Thus, we often develop similar ideas and intuitions as others because we are socially connected to them (6). This phenomenon is often deemed social "contagion" because it mimics the spread of disease. We use a social contagion perspective to illuminate how morally tinged messages about political issues are transmitted through social networks.Research on the emotional underpinnings of morality provides a theoretical framework to understand the processes that may drive social contagion in the domain of morality. Emotions...

Numerous polls suggest that COVID-19 is a profoundly partisan issue in the United States. Using the geotracking data of 15 million smartphones per day, we found that US counties that voted for Donald Trump (Republican) over Hillary Clinton (Democrat) in the 2016 presidential election exhibited 14% less physical distancing between March and May 2020. Partisanship was more strongly associated with physical distancing than numerous other factors, including counties' COVID-19 cases, population density, median income, and racial and age demographics. Contrary to our predictions, the observed partisan gap strengthened over time and remained when stay-at-home orders were active. Additionally, county-level consumption of conservative media (Fox News) was related to reduced physical distancing. Finally, the observed partisan differences in distancing were associated with subsequently higher COVID-19 infection and fatality growth rates in pro-Trump counties. Taken together, these data suggest that US citizens' responses to COVID-19 are subject to a deep-and consequential-partisan divide.

In recent years, scientists have paid increasing attention to reproducibility. For example, the Reproducibility Project, a large-scale replication attempt of 100 studies published in top psychology journals found that only 39% could be unambiguously reproduced. There is a growing consensus among scientists that the lack of reproducibility in psychology and other fields stems from various methodological factors, including low statistical power, researcher's degrees of freedom, and an emphasis on publishing surprising positive results. However, there is a contentious debate about the extent to which failures to reproduce certain results might also reflect contextual differences (often termed "hidden moderators") between the original research and the replication attempt. Although psychologists have found extensive evidence that contextual factors alter behavior, some have argued that context is unlikely to influence the results of direct replications precisely because these studies use the same methods as those used in the original research. To help resolve this debate, we recoded the 100 original studies from the Reproducibility Project on the extent to which the research topic of each study was contextually sensitive. Results suggested that the contextual sensitivity of the research topic was associated with replication success, even after statistically adjusting for several methodological characteristics (e.g., statistical power, effect size). The association between contextual sensitivity and replication success did not differ across psychological subdisciplines. These results suggest that researchers, replicators, and consumers should be mindful of contextual factors that might influence a psychological process. We offer several guidelines for dealing with contextual sensitivity in reproducibility.replication | reproducibility | context | psychology | meta-science I n recent years, scientists have paid increasing attention to reproducibility. Unsuccessful attempts to replicate findings in genetics (1), pharmacology (2), oncology (3), biology (4), and economics (5) have given credence to previous speculation that most published research findings are false (6). Indeed, since the launch of the clinicaltrials.gov registry in 2000, which forced researchers to preregister their methods and outcome measures, the percentage of large heart-disease clinical trials reporting significant positive results plummeted from 57% to a mere 8% (7). The costs of such irreproducible preclinical research, estimated at $28 billion in the United States (8), are staggering. In a similar vein, psychologists have expressed growing concern regarding the reproducibility and validity of psychological research (e.g., refs. 9-14). This emphasis on reproducibility has produced a number of failures to replicate prominent studies, leading professional societies and government funding agencies such as the National Science Foundation to form subcommittees promoting more robust research practices (15).The Reproducibility Project in psychology has b...

With more than 3 billion users, online social networks represent an important venue for moral and political discourse and have been used to organize political revolutions, influence elections, and raise awareness of social issues. These examples rely on a common process to be effective: the ability to engage users and spread moralized content through online networks. Here, we review evidence that expressions of moral emotion play an important role in the spread of moralized content (a phenomenon we call moral contagion). Next, we propose a psychological model called the motivation, attention, and design (MAD) model to explain moral contagion. The MAD model posits that people have group-identity-based motivations to share moral-emotional content, that such content is especially likely to capture our attention, and that the design of social-media platforms amplifies our natural motivational and cognitive tendencies to spread such content. We review each component of the model (as well as interactions between components) and raise several novel, testable hypotheses that can spark progress on the scientific investigation of civic engagement and activism, political polarization, propaganda and disinformation, and other moralized behaviors in the digital age.

Author contributions: The 1 st through 4 th and last authors developed the research questions, oversaw the project, and contributed equally. The 1 st through 3 rd authors oversaw the Main Studies and Replication Studies, and the 4 th , 6 th , 7 th , and 8 th authors oversaw the Forecasting Study. The 1 st , 4 th , 5 th , 8 th , and 9 th authors conducted the primary analyses. The 10 th through 15 th authors conducted the Bayesian analyses. The first and 16 th authors conducted the multivariate meta-analysis.