A new study explores how pathogen stress—the prevalence of infectious diseases—may fuel racial bias in the United States, particularly anti-Black prejudice. By analyzing Google search data and measures of implicit and explicit bias, the researchers found that while pathogen stress correlates with increased bias, the role of disease avoidance motivations in this relationship is less clear. The findings, published in Evolutionary Psychological Science, shed light on how environmental conditions interact with psychological processes to shape social behaviors.
Past research has demonstrated that pathogen stress can influence social behaviors, fostering ethnocentrism and prejudice. These patterns are often attributed to the behavioral immune system, a set of evolved psychological mechanisms aimed at minimizing the risk of disease transmission by increasing wariness of outgroup members. However, while this general relationship has been established, the specific psychological pathways linking pathogen stress to bias remain unclear.
“I’ve been interested in what drives prejudice since I was an undergraduate,” said study author Hugh McGovern, an associate research fellow at Deakin University.
“As I was beginning my honours year, I came across the work of Mark Schaller and Steven Neuberg (among others) who shifted the type of explanation of prejudice from ‘how does prejudice arise in one’s mind given social conditions’ to ‘why might prejudice have arisen given recurrent social pressures during human evolution?’ This blew my mind, and I really liked the fact that it was providing a more comprehensive explanation of why we have this tendency toward social prejudices taking into account our evolutionary past, and not just recent history.”
“With diseases in particular it made intuitive sense to me that the types of threat other groups offer us (e.g., infectious disease) would mean that unfamiliar outgroups in particular would be considered a pathogenic risk, as through evolution we did not have immunity to the infectious diseases that unfamiliar outgroups carried,” McGovern explained. “As such, at least for most humans, it made sense to be wary of unfamiliar outgroups as any contact could actually mean my entire group gets killed if I catch their pathogens. This is what happened to many Indigenous groups in South America during European colonization, where things such as syphilis were responsible for entire populations being killed.”
“I also liked the fact that the evolutionary approach was able to explain prejudice arising across cultural contexts. I think this is important, because I think a lot of earlier explanations for prejudice were somewhat Anglo-American centric and focused on intergroup dynamics specific to Europe and North America. For me, this was a bit of a shortcoming and actually hinders the possibility of truly understanding intergroup dynamics and ultimately understanding why prejudice arises.”
To investigate the relationship between pathogen stress, disease avoidance motivations, and anti-Black bias in the United States, the researchers analyzed data from two primary sources: Google Trends and Project Implicit.
The first part of the analysis used Google Trends data collected between 2008 and 2012. The researchers identified searches containing an anti-Black racial slur as a proxy for racial animus in different geographic areas. These data provided a relative interest score for each U.S. state and metropolitan statistical area, with higher scores indicating a greater frequency of such searches. The researchers also created a measure of disease avoidance motivations by compiling a list of search terms related to disease concerns, such as “virus” and “sick.” These terms were aggregated to generate a relative score for disease-related concerns at the state and metropolitan levels.
In the second part of the study, the researchers analyzed data from Project Implicit, a platform that collects implicit and explicit bias measures from participants. For the 2020 data, they examined implicit racial bias using the Implicit Association Test (IAT), which measures automatic associations between racial groups and positive or negative words. Explicit bias was assessed through self-reported preferences for Black or White individuals on a Likert scale. Additionally, participants completed the Perceived Vulnerability to Disease scale, which includes subscales for germ aversion and perceived infectability, to measure individual differences in disease avoidance motivations.
The results partially supported the parasite stress theory, which posits that pathogen prevalence drives intergroup biases. However, the role of disease avoidance motivations in mediating this relationship was not consistent across measures and geographic levels.
Using Google Trends data, the researchers found that pathogen stress was a significant predictor of anti-Black bias at both state and metropolitan levels. Disease avoidance motivations, as measured through Google search terms, mediated the relationship between pathogen stress and anti-Black slur searches at the metropolitan level but not at the state level. This suggests that localized pathogen stress may exert a stronger influence on disease-related concerns and racial animus than broader state-level measures. Regions with higher pathogen stress were more likely to have elevated disease avoidance concerns, which in turn correlated with more frequent searches for anti-Black slurs.
“More pathogens in the environment increased anti-Black prejudice insofar as pathogens made people more worried about infectious disease,” McGovern told PsyPost. “This means that higher amounts of pathogens may not increase prejudice, if more pathogens don’t make people more worried about pathogens.”
In contrast, when examining the 2020 Project Implicit data, the researchers found that disease avoidance motivations, particularly germ aversion, were associated with explicit but not implicit anti-Black bias at the state level. However, these disease avoidance motivations did not mediate the relationship between pathogen stress and bias. The lack of mediation in this part of the analysis suggests that the link between pathogen stress and racial bias may operate through mechanisms other than disease avoidance motivations, such as external social norms or structural inequalities.
“Higher rates of infectious disease can increase prejudices against ethnic or racial groups that already experience a higher incidence of infectious disease—in the American context, Black Americans,” McGovern said. “This is pernicious because it means that socially and economically disadvantaged groups, who may lack easy access to healthcare, can suffer a ‘double whammy’: as infectious diseases increase, so too might prejudice against those who already bear the highest burden of these illnesses.”
Interestingly, the study also identified the role of education as a mitigating factor. Higher levels of education were associated with lower racial bias, aligning with prior research showing that education fosters intergroup tolerance. Additionally, geographic areas with greater population dissimilarity—regions marked by racial segregation—exhibited higher levels of anti-Black bias.
The study demonstrated that pathogen stress is indeed associated with anti-Black bias, supporting the broader parasite stress theory. However, the findings cast doubt on the universality of disease avoidance motivations as mediators in this relationship. These results underscore the need for further research to disentangle the mechanisms underlying prejudice and to explore interventions that address both environmental and social drivers of bias.
“The results were not super consistent and could depend on how prejudice was measured – whether it be the Google-based measure, explicit prejudice, or implicit prejudice,” McGovern noted. “It is therefore important more data is collected with large and ideally international samples.”
The researchers also emphasize that addressing structural inequalities, such as healthcare access and education disparities, may help mitigate the conditions that foster prejudice. By understanding the evolutionary and social origins of bias, policymakers and social scientists can develop targeted interventions to promote social cohesion.
“It may sound grandiose, but I’d like to get to a place where this type of research makes its way to the ears of those who may wish to derive effective interventions for group-based prejudices,” McGovern told PsyPost. “Many current interventions either do not work or can (in some cases) actually make social prejudices worse. If we have a fuller scientific understanding of why evolution sculpted our brains such that we often preference members of our own group, and exclude outsiders or outgroups, we may actually be able to derive more effective interventions.”
“More generally, we may be able to derive more out of the box interventions at the level of public policy. For example, if we want to decrease prejudice in society, it might help to start with trying to improve healthcare access in disadvantaged communities or try to limit the spread of infectious disease.”
“I think it is important that we don’t just guess our way to interventions for prejudice,” McGovern added. “If we actually want to create cohesive and harmonious societies, we need to have an understanding of why we have evolved the tendency toward social or group prejudices, so that we can get to the psychological roots of it and thus create effective social or policy interventions that truly make things better.”
The study, “What Role Do Disease Avoidance Motives Play in Prejudice? Assessing Implicit, Explicit, and Google Search Data,” was authored by Hugh McGovern, Lewis Nitschinsk, Brian A. O’Shea, Damian R. Murray, and Khandis R. Blake.
