An Unconscious Biological Driver for Ethnic Hatred and Trump’s Popularity: Fear of Disease


What is the origin of individual differences in ideology and personality? According to the parasite stress hypothesis, the structure of a society and the values of individuals within it are both influenced by the prevalence of infectious disease within the society’s geographical region.

High levels of infection threat are associated with more ethnocentric and collectivist social structures and greater adherence to social norms, as well as with socially conservative political ideology and less open but more conscientious personalities.

Here we use an agent-based model to explore a specific opportunities-parasites trade-off (OPTO) hypothesis, according to which utility-maximizing agents place themselves at an optimal point on a trade-off between

(a) the gains that may be achieved through accessing the resources of geographically or socially distant out-group members through openness to out-group interaction, and

(b) the losses arising due to consequently increased risks of exotic infection to which immunity has not been developed.

…we show that the groups that spontaneously form exhibit greater local rather than global cooperative networks when levels of infection are high. It is suggested that the OPTO model offers a first step toward understanding the specific mechanisms through which environmental conditions may influence cognition, ideology, personality, and social organization.

  1. Introduction
    What gives rise to different cultures, attitudes, and politico-economic systems in different regions of the globe? What might cause such attitudes and systems to change over time, and what might speed up or slow down such change?

Here we adopt the framework of the parasite stress hypothesis = many core human values and cultural patterns at least partly reflect adaptive responses, by a behavioral immune system, to the threat of infectious disease.

…the parasite stress hypothesis offers one potential explanation for the differences within both individuals and societies that are often simply taken as a starting point by disciplines such as economics and psychology. Moreover, the hypothesis suggests mechanisms by which climate change may influence global cognition and social transitions.

…there has been almost no computational specification of the cognitive psychological mechanisms that might underpin the relation between parasite stress and values or of how such cognitive mechanisms might lead to the emergence of different (e.g., collectivist rather than individualist) social structures.

…We focus on the effects of parasite stress on political ideology, on personality, and on the consequences of individual differences in openness to out-group interactions for the structure of mutually cooperative social groups. Specifically, we seek to understand the mechanisms that may underlie earlier findings that high levels of parasitic infection within a region are associated with:

  • less open personalities,
  • greater xenophobia and in-group orientation
  • more conservative political voting patterns

Consistent with the more general parasite stress hypothesis, Brown et al. interpreted their results in terms of a simple quantitative opportunity-parasites trade-off hypothesis (OPTO). OPTO aims to quantify some of the benefits and costs to an individual of interacting (e.g., trading or mating) with individuals from different regions (members of out-groups). The model suggests that the optimal point on this trade-off depends on prevailing levels of infectious disease.

The OPTO model can be seen as a simple mathematical instantiation of one of the core features of the more general parasite stress hypothesis. In this article we illustrate, using an agent-based model, the operation of the OPTO mechanism and show how the attitudes and values of rational individuals might come to differ as a function of infection-related threat (parasite stress) within their environment.

We also use the model to show how evolved social groups may come to be more inward looking and xenophobic when prevailing levels of infection are high.

The Parasite Stress Hypothesis

According to the parasite stress hypothesis levels of parasitic infection within a region may drive variability at both individual and societal levels. The avoidance of death or disability through avoidance of infection is a major evolutionary driving force. For example, until times that are relatively recent in evolutionary terms, almost 50% of children failed to survive to reproductive age, with the majority of deaths being due to infectious diseases.

Local parasite types and hosts defense systems will typically be involved in a co-evolutionary arms race, and so at any given time a host’s immunological defenses will be specialized to be most effective against local parasite species. To the extent that a host defense system is specialized locally, contact with out-groups will be associated with an increased risk of exposure to infectious diseases against which there is no a priori immunity. The relevant infection-avoiding behaviors such as avoidance of strangers and high conscientiousness in, for example, food preparation, are assumed to arise from the operation of a “behavioral immune system”.

The behavioral immune system comprises adaptive psychological response mechanisms that are sensitive to cues that might predict presence of infectious pathogens and which respond with appropriate cognitive processing and affective reactions. Such a system is likely to be oversensitive—the cost of a false positive (avoiding non-existent infection) is small relative to the possible cost (catching avoidable infection.

In social terms, an important component of the parasite stress hypothesis is that high levels of infection may lead to ethnocentrism, xenophobia, distrust of different others, and conformitybecause such behaviors will reduce the likelihood of exposure to unfamiliar infections to which immunity has not been developed.

Direct causal evidence for the operation of a behavioral immune system, albeit over shorter timescales, can be found experimentally. For example, priming people with slides containing disease-salient cues leads to increased feelings of between-person avoidance (and a classical immune system response.

  1. Introduction

…the framework of the parasite stress hypothesis, according to which many core human values and cultural patterns at least partly reflect adaptive responses, by a behavioral immune system, to the threat of infectious disease ….Moreover, the hypothesis suggests mechanisms by which climate change may influence global cognition and social transitions.

  • Parasites, personality, and political ideology

… A small number of studies have examined the relationship between political ideology and parasite stress directly, while rather more have examined the relationship of parasite stress to collectivism. Collectivism, in contrast to individualism, is associated with an emphasis on strong distinctions between in-group and out-group members, developing and maintaining in-group relationships, suspicion and avoidance of out-group members, and conformity to local customs and social norms (e.g., food preparation and hygiene practices). These behavioral tendencies can all be interpreted as mechanisms to reduce the probability of exposure to infectious diseases. The collectivism–individualism dimension is very similar to a conservatism–liberalism ideological dimension, particularly when social rather than economic conservatism is concerned — social conservatism is associated with, for example, negative attitudes to out-groups and adherence to social norms.

  • Pathogen prevalence is positively associated with various measures of collectivism at the level of both countries and U.S. states.
  • At the level of the individual, endorsement of collectivist values is associated with measures of sensitivity to disgust and concern with infectious disease.
  • Similar positive associations are found between measures of political conservatism and disgust sensitivity,
  • with a range of related findings being consistent with the suggestion that social and religious conservatism may in part reflect strategies for avoidance of infectious disease. Thus, experimentally inducing disgust can increase prejudice toward outgroups
  • showing people primes related to cleanliness can influence political attitudes

Overall, these findings are consistent with the suggestion that some dimensions of political ideology are associated with pathogen-related variables (such as disgust).

  • High regional levels of parasite stress are associated with authoritarian societies and authoritarian attitudes at the level of individuals
  • It was found that state-level political ideology in the 1960s and 1970s was correlated with levels of parasitic infection, with greater conservatism in states with higher parasite stress, even after the introduction of controls for state-level inequality, per capita GDP, and levels of health spending per capita.
  • However, the correlation reduced over time and by the 1990s had virtually disappeared. The reduction in effect over time was interpreted as evidence that the importance of infection in determining political ideology reduces with improvements in general wealth and health, perhaps because of reduced infection-related child mortality with consequent relative increases in the adaptive importance of other behaviors.

At least in the developed West, it is at least possible that the (very recent, in evolutionary time) reduction in infection-related mortality is reducing the importance of parasite stress as an evolutionary force. The ideological position of U.S. elected politicians, as identified by their voting patterns, was also found to be more conservative in high-infection states.

  • The Opportunity-Parasites Trade-Off Hypothesis

… the possession of personality and ideological characteristics can be understood in part as reflecting optimal trade-off between the costs and benefits of interacting with geographically (and hence, by assumption, immunologically) distant individuals and members of out-groups.


Benefits include:

  • the possibility that inhabitants of more distant regions may have access to complementary natural resources (leading to the possibility of economic gains through trading), along with access to a more diverse mating pool and exposure to socially or economically efficient cultural practices.
  • Such benefits should, in themselves, lead to a relatively open, liberal, trusting, and cooperative attitude toward out-group members from different geographical regions.

According to the parasite stress hypothesis, however, there are also costs associated with out-group contact—primarily in terms of the risk of exposure to new and dangerous infectious agents because individuals from out-groups are likely to have different immunity and potentially carry unfamiliar infectious diseases .

Thus, a more socially conservative cognitive and social style may in part represent a response to a specific type of environmental threat, while left-leaning/liberal ideology may in contrast partly reflect an openness to new ideas and interaction with outgroups when infection-related threat is reduced.

Consistent with such a view, the liberal end of the political spectrum, involving acceptance of out-groups and dissimilar others possibly at the expense of maintaining in-group social relationships, is associated with novelty seeking and high scores on the personality dimension of openness .

…taking geographical units as the unit of analysis, the parasite stress approach offers one useful perspective on the origin of differences in openness as reflected in both personality and political ideology Here, we show—using an agent-based model—that rational, utility maximizing individuals will (given some simple assumptions about the mechanisms that underpin group formation) learn to interact preferentially with geographically local rather than geographically distant others when infection prevalence is high, and to interact preferentially with more distant others when infection prevalence is low.

Geographical distance is in the network treated as a proxy for immunological distance (thereby simplifying reality, in which immunologically different individuals may co-exist in a given region). Moreover, we show that groups of mutually cooperative individuals can form even in the absence of initial individual differences, and we demonstrate that the groups that form are more assortative (preferentially associating with nearby others) and xenophobic when levels of parasite stress are high.

Overall, …the development of cooperation with out-groups, in turn associated with more liberal ideology and open personality styles, may be facilitated when levels of infection are low. Conversely, if perceived levels of infection-related threat increase within a local region (e.g., as a result of climate change), a more conservative cognitive, ideological, and personality style may result—leading in turn to reduced levels of cooperation with out-groups and hence increased conflict and difficulty in effecting global change.

Indeed, the frequency of the outbreak of interstate conflicts and civil war has been attributed to the intensity of infectious disease across countries of the world …

…We have focused here on specifying a mechanism by which a key feature of the environment (parasitic stress) may affect a characteristic of the individual (openness), which in turn influences the social group structures that evolve…The possibility that (perceived or actual) levels of parasitic infection may be causally related to changes in cognition and ideology is of particular relevance in light of current evidence for global warming. ….


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