Political Attitudes and Disease Threat: Regional Pathogen Stress Is Associated With Conservative Ideology Only for Older Individuals.

What environmental factors are associated with individual differences in political ideology, and do such associations change over time? We examine whether reductions in pathogen prevalence in U.S. states over the past 60 years are associated with reduced associations between parasite stress and conservatism. We report a positive association between infection levels and conservative ideology in the United States during the 1960s and 1970s. However, this correlation reduces from the 1980s onwards. These results suggest that the ecological influence of infectious diseases may be larger for older people who grew up (or whose parents grew up) during earlier time periods. We test this hypothesis by analyzing the political affiliation of 45,000 Facebook users, and find a positive association between self-reported political affiliation and regional pathogen stress for older (>40 years) but not younger individuals. It is concluded that the influence of environmental pathogen stress on ideology may have reduced over time.

Individual-Level Analyses of the Impact of Parasite Stress on Personality: Reduced Openness Only for Older Individuals.

The parasite stress hypothesis predicts that individuals living in regions with higher infectious disease rates will show lower openness, agreeableness, and extraversion, but higher conscientiousness. This article, using data from more than 250,000 U.S. Facebook users, reports tests of these predictions at the level of both U.S. states and individuals and evaluates criticisms of previous findings. State-level results for agreeableness and conscientiousness are consistent with previously reported cross-national findings, but others (a significant positive correlation with extraversion and no correlation with openness) are not. However, effects of parasite stress on conscientiousness and agreeableness are not found when analyses account for the data's hierarchical structure and include controls. We find that only openness is robustly related to parasite stress in these analyses, and we also find a significant interaction with age: Older, but not younger, inhabitants of areas of high parasite stress show lower openness. Interpretations of the findings are discussed.

Personality, Parasites, Political Attitudes, and Cooperation: A Model of How Infection Prevalence Influences Openness and Social Group Formation.

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 examine the evolution of cooperation and the formation of social groups within social networks, and 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.

Do assortative preferences contribute to assortative mating for adiposity?

Assortative mating for adiposity, whereby levels of adiposity in romantic partners tend to be positively correlated, has implications for population health due to the combined effects of partners' levels of adiposity on fertility and/or offspring health. Although assortative preferences for cues of adiposity, whereby leaner people are inherently more attracted to leaner individuals, have been proposed as a factor in assortative mating for adiposity, there have been no direct tests of this issue. Because of this, and because of recent work suggesting that facial cues of adiposity convey information about others' health that may be particularly important for mate preferences, we tested the contribution of assortative preferences for facial cues of adiposity to assortative mating for adiposity (assessed from body mass index, BMI) in a sample of romantic couples. Romantic partners' BMIs were positively correlated and this correlation was not due to the effects of age or relationship duration. However, although men and women with leaner partners showed stronger preferences for cues of low levels of adiposity, controlling for these preferences did not weaken the correlation between partners' BMIs. Indeed, own BMI and preferences were uncorrelated. These results suggest that assortative preferences for facial cues of adiposity contribute little (if at all) to assortative mating for adiposity.

Men's, but not women's, sociosexual orientation predicts couples' perceptions of sexually dimorphic cues in own-sex faces.

Previous research suggests that people's perceptions of own-sex individuals can change according to within-individual variation in their romantic partners' sexual strategies. For example, men are more likely to perceive other men's faces as looking particularly dominant during the fertile phase of their partner's menstrual cycle, when women tend to be more open to uncommitted sexual relationships. By contrast, little is known about how relatively stable between-individuals differences in partners' openness to uncommitted sexual relationships (i.e., their sociosexual orientation) predict perceptions of own-sex individuals. The revised Sociosexual Orientation Inventory (SOI-R) assesses individuals' openness to uncommitted sexual relationships and shows high test-retest reliability over long periods of time. Consequently, we tested whether the SOI-R scores of men and women in heterosexual romantic couples predicted their perceptions of own-sex faces displaying exaggerated sex-typical cues. Men's, but not women's, SOI-R was positively correlated with the extent to which both the man and woman within a couple ascribed high dominance and attractiveness to own-sex faces with exaggerated sex-typical cues. In other words, individuals in couples where the man reported being particularly open to uncommitted sexual relationships were more likely to ascribe dominance and attractiveness to own-sex individuals displaying a putative cue of good phenotypic condition. These findings suggest that both men's and women's perceptions of potential competitors for mates are sensitive to the male partner's sexual strategy. Such individual differences in perceptions may benefit men's ability to compete for extra-pair and/or replacement mates and benefit women's mate guarding behaviors.

Salivary cortisol and pathogen disgust predict men's preferences for feminine shape cues in women's faces.

Recent studies suggest that individuals who are particularly concerned about infectious diseases show stronger preferences for exaggerated sex-typical characteristics in potential mates' faces. However, these studies have generally investigated individual differences in women's mate preferences and relied on questionnaires to assess disease-related concerns. Here we show that men's scores on the pathogen disgust subscale of the Three Domains of Disgust Scale are positively correlated with their preferences for femininity in women's faces and that this relationship is independent of the possible effects of both sexual and moral disgust. We then show that men with higher trait (i.e., average) salivary cortisol, a biomarker for immunosuppression, have stronger preferences for femininity in women's faces. Finally, we show that pathogen disgust is correlated with partnered men's femininity ratings of both their actual and ideal romantic partner. Together, these findings suggest that disease-related factors are important for individual differences in men's mate preferences.

Hygiene and the world distribution of Alzheimer's disease: Epidemiological evidence for a relationship between microbial environment and age-adjusted disease burden.

BACKGROUND AND OBJECTIVES: Alzheimer's disease (AD) shares certain etiological features with autoimmunity. Prevalence of autoimmunity varies between populations in accordance with variation in environmental microbial diversity. Exposure to microorganisms may improve individuals' immunoregulation in ways that protect against autoimmunity, and we suggest that this may also be the case for AD. Here, we investigate whether differences in microbial diversity can explain patterns of age-adjusted AD rates between countries. METHODOLOGY: We use regression models to test whether pathogen prevalence, as a proxy for microbial diversity, across 192 countries can explain a significant amount of the variation in age-standardized AD disability-adjusted life-year (DALY) rates. We also review and assess the relationship between pathogen prevalence and AD rates in different world populations. RESULTS: Based on our analyses, it appears that hygiene is positively associated with AD risk. Countries with greater degree of sanitation and lower degree of pathogen prevalence have higher age-adjusted AD DALY rates. Countries with greater degree of urbanization and wealth exhibit higher age-adjusted AD DALY rates. CONCLUSIONS AND IMPLICATIONS: Variation in hygiene may partly explain global patterns in AD rates. Microorganism exposure may be inversely related to AD risk. These results may help predict AD burden in developing countries where microbial diversity is rapidly diminishing. Epidemiological forecasting is important for preparing for future healthcare needs and research prioritization.

The parasite-stress theory may be a general theory of culture and sociality.

In the target article, we presented the hypothesis that parasite-stress variation was a causal factor in the variation of in-group assortative sociality, cross-nationally and across the United States, which we indexed with variables that measured different aspects of the strength of family ties and religiosity. We presented evidence supportive of our hypothesis in the form of analyses that controlled for variation in freedom, wealth resources, and wealth inequality across nations and the states of the USA. Here, we respond to criticisms from commentators and attempt to clarify and expand the parasite-stress theory of sociality used to fuel our research presented in the target article.

Parasite-stress promotes in-group assortative sociality: the cases of strong family ties and heightened religiosity.

Throughout the world people differ in the magnitude with which they value strong family ties or heightened religiosity. We propose that this cross-cultural variation is a result of a contingent psychological adaptation that facilitates in-group assortative sociality in the face of high levels of parasite-stress while devaluing in-group assortative sociality in areas with low levels of parasite-stress. This is because in-group assortative sociality is more important for the avoidance of infection from novel parasites and for the management of infection in regions with high levels of parasite-stress compared with regions of low infectious disease stress. We examined this hypothesis by testing the predictions that there would be a positive association between parasite-stress and strength of family ties or religiosity. We conducted this study by comparing among nations and among states in the United States of America. We found for both the international and the interstate analyses that in-group assortative sociality was positively associated with parasite-stress. This was true when controlling for potentially confounding factors such as human freedom and economic development. The findings support the parasite-stress theory of sociality, that is, the proposal that parasite-stress is central to the evolution of social life in humans and other animals.

Parasite stress promotes homicide and child maltreatment.

Researchers using the parasite-stress theory of human values have discovered many cross-cultural behavioural patterns that inform a range of scholarly disciplines. Here, we apply the theory to major categories of interpersonal violence, and the empirical findings are supportive. We hypothesize that the collectivism evoked by high parasite stress is a cause of adult-on-adult interpersonal violence. Across the US states, parasite stress and collectivism each positively predicts rates of men's and women's slaying of a romantic partner, as well as the rate of male-honour homicide and of the motivationally similar felony-related homicide. Of these four types of homicide, wealth inequality has an independent effect only on rates of male-honour and felony-related homicide. Parasite stress and collectivism also positively predict cross-national homicide rates. Child maltreatment by caretakers is caused, in part, by divestment in offspring of low phenotypic quality, and high parasite stress produces more such offspring than low parasite stress. Rates of each of two categories of the child maltreatment--lethal and non-lethal--across the US states are predicted positively by parasite stress, with wealth inequality and collectivism having limited effects. Parasite stress may be the strongest predictor of interpersonal violence to date.

Parasite prevalence and the worldwide distribution of cognitive ability.

In this study, we hypothesize that the worldwide distribution of cognitive ability is determined in part by variation in the intensity of infectious diseases. From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks. Using three measures of average national intelligence quotient (IQ), we found that the zero-order correlation between average IQ and parasite stress ranges from r=-0.76 to r=-0.82 (p<0.0001). These correlations are robust worldwide, as well as within five of six world regions. Infectious disease remains the most powerful predictor of average national IQ when temperature, distance from Africa, gross domestic product per capita and several measures of education are controlled for. These findings suggest that the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.

Does infectious disease cause global variation in the frequency of intrastate armed conflict and civil war?

Geographic and cross-national variation in the frequency of intrastate armed conflict and civil war is a subject of great interest. Previous theory on this variation has focused on the influence on human behaviour of climate, resource competition, national wealth, and cultural characteristics. We present the parasite-stress model of intrastate conflict, which unites previous work on the correlates of intrastate conflict by linking frequency of the outbreak of such conflict, including civil war, to the intensity of infectious disease across countries of the world. High intensity of infectious disease leads to the emergence of xenophobic and ethnocentric cultural norms. These cultures suffer greater poverty and deprivation due to the morbidity and mortality caused by disease, and as a result of decreased investment in public health and welfare. Resource competition among xenophobic and ethnocentric groups within a nation leads to increased frequency of civil war. We present support for the parasite-stress model with regression analyses. We find support for a direct effect of infectious disease on intrastate armed conflict, and support for an indirect effect of infectious disease on the incidence of civil war via its negative effect on national wealth. We consider the entanglements of feedback of conflict into further reduced wealth and increased incidence of disease, and discuss implications for international warfare and global patterns of wealth and imperialism.

Zoonotic and non-zoonotic diseases in relation to human personality and societal values: support for the parasite-stress model.

The parasite-stress model of human sociality proposes that humans' ontogenetic experiences with infectious diseases as well as their evolutionary historical interactions with these diseases exert causal influences on human psychology and social behavior. This model has been supported by cross-national relationships between parasite prevalence and human personality traits, and between parasite prevalence and societal values. Importantly, the parasite-stress model emphasizes the causal role of non-zoonotic parasites (which have the capacity for human-to-human transmission), rather than zoonotic parasites (which do not), but previous studies failed to distinguish between these conceptually distinct categories. The present investigation directly tested the differential predictive effects of zoonotic and non-zoonotic (both human-specific and multihost) parasite prevalence on personality traits and societal values. Supporting the parasite-stress model, cross-national differences in personality traits (unrestricted sexuality, extraversion, openness to experiences) and in societal values (individualism, collectivism, gender equality, democratization) are predicted specifically by non-zoonotic parasite prevalence.

On the adaptive origins and maladaptive consequences of human inbreeding: parasite prevalence, immune functioning, and consanguineous marriage.

We propose that consanguineous marriages arise adaptively in response to high parasite prevalence and function to maintain coadapted gene complexes and associated local adaptation that defend against local pathogens. Therefore, a greater prevalence of inbreeding by consanguineous marriage is expected in geographical regions that historically have had high levels of disease-causing parasites. Eventually such marriages may, under the contemporary high movement of people with modern transportation, jeopardize the immunity of those who practice inbreeding as this leads to an increased susceptibility to novel pathogens. Therefore, a greater frequency of inbreeding is expected to predict higher levels of contemporary mortality and morbidity from infectious diseases. This parasite model of human inbreeding was supported by an analysis involving 72 countries worldwide. We found that historically high levels of pathogen prevalence were related positively to the proportion of consanguineous marriages, and that a higher prevalence of such marriages was associated with higher contemporary mortality and morbidity due to pathogens. Our study addresses plausible alternative explanations. The results suggest that consanguineous marriage is an adaptive consequence of historical pathogen ecologies, but is maladaptive in contemporary disease ecologies.

Parasites, democratization, and the liberalization of values across contemporary countries.

The countries of the world vary in their position along the autocracy-democracy continuum of values. Traditionally, scholars explain this variation as based on resource distribution and disparity among nations. We provide a different framework for understanding the autocracy-democracy dimension and related value dimensions, one that is complementary (not alternative) to the research tradition, but more encompassing, involving both evolutionary (ultimate) and proximate causation of the values. We hypothesize that the variation in values pertaining to autocracy-democracy arises fundamentally out of human (Homo sapiens) species-typical psychological adaptation that manifests contingently, producing values and associated behaviours that functioned adaptively in human evolutionary history to cope with local levels of infectious diseases. We test this parasite hypothesis of democratization using publicly available data measuring democratization, collectivism-individualism, gender egalitarianism, property rights, sexual restrictiveness, and parasite prevalence across many countries of the world. Parasite prevalence across countries is based on a validated index of the severity of 22 important human diseases. We show that, as the hypothesis predicts, collectivism (hence, conservatism), autocracy, women's subordination relative to men's status, and women's sexual restrictiveness are values that positively covary, and that correspond with high prevalence of infectious disease. Apparently, the psychology of xenophobia and ethnocentrism links these values to avoidance and management of parasites. Also as predicted, we show that the antipoles of each of the above values--individualism (hence, liberalism), democracy, and women's rights, freedom and increased participation in casual sex--are a positively covarying set of values in countries with relatively low parasite stress. Beyond the cross-national support for the parasite hypothesis of democratization, it is consistent with the geographic location at high latitudes (and hence reduced parasite stress) of the early democratic transitions in Britain, France and the U.S.A. It, too, is consistent with the marked increase in the liberalization of social values in the West in the 1950s and 1960s (in part, the sexual revolution), regions that, a generation or two earlier, experienced dramatically reduced infectious diseases as a result of antibiotics, vaccinations, food- and water-safety practices, and increased sanitation. Moreover, we hypothesize that the generation and diffusion of innovations (in thought, action and technology) within and among regions, which is associated positively with democratization, is causally related to parasite stress. Finally, we hypothesize that past selection in the context of morbidity and mortality resulting from parasitic disease crafted many of the aspects of social psychology unique to humans.

Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religion diversity.

Why are religions far more numerous in the tropics compared with the temperate areas? We propose, as an answer, that more religions have emerged and are maintained in the tropics because, through localized coevolutionary races with hosts, infectious diseases select for three anticontagion behaviours: in-group assortative sociality; out-group avoidance; and limited dispersal. These behaviours, in turn, create intergroup boundaries that effectively fractionate, isolate and diversify an original culture leading to the genesis of two or more groups from one. Religion is one aspect of a group's culture that undergoes this process. If this argument is correct then, across the globe, religion diversity should correlate positively with infectious disease diversity, reflecting an evolutionary history of antagonistic coevolution between parasites and hosts and subsequent religion genesis. We present evidence that supports this model: for a global sample of traditional societies, societal range size is reduced in areas with more pathogens compared with areas with few pathogens, and in contemporary countries religion diversity is positively related to two measures of parasite stress.

Pathogen prevalence predicts human cross-cultural variability in individualism/collectivism.

Pathogenic diseases impose selection pressures on the social behaviour of host populations. In humans (Homo sapiens), many psychological phenomena appear to serve an antipathogen defence function. One broad implication is the existence of cross-cultural differences in human cognition and behaviour contingent upon the relative presence of pathogens in the local ecology. We focus specifically on one fundamental cultural variable: differences in individualistic versus collectivist values. We suggest that specific behavioural manifestations of collectivism (e.g. ethnocentrism, conformity) can inhibit the transmission of pathogens; and so we hypothesize that collectivism (compared with individualism) will more often characterize cultures in regions that have historically had higher prevalence of pathogens. Drawing on epidemiological data and the findings of worldwide cross-national surveys of individualism/collectivism, our results support this hypothesis: the regional prevalence of pathogens has a strong positive correlation with cultural indicators of collectivism and a strong negative correlation with individualism. The correlations remain significant even when controlling for potential confounding variables. These results help to explain the origin of a paradigmatic cross-cultural difference, and reveal previously undocumented consequences of pathogenic diseases on the variable nature of human societies.