Poll workers staffing a table at a polling station in the US.

Does exposure to infectious disease influence the way you vote?

1 February 2024

The article at a glance

Exposure to higher levels of human infectious diseases is linked to conservative (Republican) voting tendencies in America but the association weakens over time, finds study co-authored by Professor David Stillwell of Cambridge Judge Business School.

Our decisions on how to vote are often crystallised long before we mark a cross at the ballot box, with personality, family upbringing and economic situation all playing a role. But what about our previous exposure to disease? It may not seem an obvious contributing factor but a new study examines the relationship between voting behaviour and the threat of infectious disease (parasite stress) within the local environment.

“There is a clear pattern of strong positive correlations between parasite stress and conservatism – that is, high levels of infection are associated with more conservative ideology,” finds the study.

The effects of parasite stress is strongest in voters over 40 years old

David Stillwell.
Professor David Stillwell

The study published in the journal Personality and Social Psychology Bulletin – entitled “Political Attitudes and Disease Threat: Regional Pathogen Stress is Associated with Conservative Ideology Only for Older Individuals” – is co-authored by David Stillwell, Professor of Computational Social Science at Cambridge Judge.

“We found there is a positive interactive between age and parasite stress”, David says. “The relation between parasite stress and conservative ideology became more positive in older participants. The effect of parasite stress is strongest in those near the age of 40 and older, and the smallest among those in their late 20s and younger.

“One possible explanation of the relation between environmental levels of parasite stress and social conservatism is that high levels of infection-related childhood mortality led to behaviour that maximises the chances of survival at least to reproduction age,” he says.

The avoidance of infection-related death and disability has been a dominant evolutionary driving force throughout human history. A behavioural immune system is a term that refers to a range of psychological mechanisms individuals can use to detect the potential presence of infectious parasites in the immediate environment, and to engage in behaviours that prevent contact with infected people. It has been suggested that conservative ideology may reflect one such adaptation.

Three ways that infectious diseases drive behavioural immune system responses


Prevalence-independence model

The sensitivity of the system is not responsive to change levels of infection in the environment, or at least not sensitive in a way observable over a timescale of mere decades.


Prevalence-dependence model

The system is sensitive to changes in prevailing infection levels, so that whenever levels are high it registers the increased threat level and responds by initiating generally conservative and infection-avoidant patterns of behaviour, while at the same time becoming more responsive to (and repelled by) infection-relevant stimuli of any kind.


Early-set model

The general sensitivity of the system is either inherited or becomes fixed during childhood. If the system is set during childhood, the setting could reflect either prevailing levels of infection at the time of childhood or could be influenced by parental / grandparental transmission of infection-relevant norms. This relationship will apply to both US citizens who live in their birth states, and the substantial minority who do not, because it depends on the environment experienced in their early years.

Previous research into the link between infection and ideology has overlooked older participants

“Most previous studies have been conducted in developed countries using young people, who have grown up and live in a world where levels of parasite stress are very low by evolutionary standards,” says David. “It follows that, in recent decades, levels of infectious disease may therefore have less of an impact on cognitive style and social attitudes such as conservatism due to the greatly reduced relevance of infection relative to other influences. We examined whether this was indeed the case and tested whether associations are stronger in people who are currently older, as they or their parents will have grown up when infection rates were higher.”

“Although it is possible that the behavioural immune system continues to be activated by the elevated presence of pathogen cues, it is also true that conservatives (eg Republicans) tend to be less concerned with the recent COVID pandemic.”

State-level pathogen stress and political data from Facebook analysed in 2 comprehensive studies


Cross sectional analysis of state-level ideology in the US and state-level pathogen stress

State-level measures of parasite stress (based on data reported by the US Centers for Disease Control between 1993 and 2007) were correlated with state-level measures of ideology collected for each year from 1960-2006. The measure of ideology is calculated by estimating the ideological position of members of congress and challengers for each congressional district using interest group ratings, then using voting records as a weighting factor. This analysis was then repeated using an additional measure of the parasite stress burden based on mortality, using data from US Centers for Disease Control (CDC).


Examination of the role of pathogen prevalence on individuals’ conservatism relating to their age

This used a much larger dataset. The measures of state-level median household income, income inequality (GINI) and total population were obtained from the 2012 American Community Survey. In addition, individual-level data from tens of thousands of users of the Facebook application myPersonality was utilised — including gender, age, current country and state of residence. Political ideology was measured through a free text field where users were able to enter ‘political views’. For the analysis the dataset was restricted to myPersonality users from the United States and the two-party system makes this simpler than in other countries. The authors were left with 44,298 respondents – of these 74.6% were liberal and 25.4% were conservative.

A mixed model approach was used to estimate the relationship between parasite and ideology. In all models the effects of income inequality, state population, urban-rural ratio, and median household income were controlled. It focused on infectious diseases that can be transmitted between humans (as opposed to animals) because according to parasite stress theory, conservative ideology should be driven by the threat of infection from other humans not from different species.

Even though the 2 studies involve different methodologies and different datasets they both identify time related changes in the association between parasite stress and ideology.

Three reasons why the link between disease and voter behaviour changes over time


Lifespan effect

Older people show a stronger relationship between levels of infection and ideology simply because they are older and therefore likely to have weaker immune systems.


Period effect

Changes in parasite stress levels affect everybody, irrespective of age or date of birth.


Cohort effect

Only people born at a particular time will show a relationship between ideology and levels of parasite stress.

Older people might show stronger associations between ideology and parasite stress because they are older (a lifespan effect) or because they were born at a different time (a cohort effect). Although Study 2 results can’t distinguish between these two interpretations, the results of Study 1 might appear to provide partial evidence against a lifespan effect and consistent with a cohort effect. In the United States the proportion of older individuals has increased over time.

How your political allegiances are influenced by the environment in which you’re born

Three new analyses were also conducted for age-specific groups. The research found an adult born in 1970 would have been aged between 37 and 42 at the time they completed the Facebook personality questionnaire, and it is this age group that showed the strongest relationship between ideology and parasite stress. However, an adult born in 1990 would have been aged between 17 and 22 at the time of data collection, even though this group showed a significant (although much smaller) ideology-infection link.

“Even the oldest group in the study was born into relatively low-parasite stress environment, suggesting that the environment their parents or grandparents were born into is more relevant”, the study finds.

What is the role of reproductive strategies in shaping conservative attitudes?

The potential role of reproductive strategies in shaping conservative attitudes should not be overlooked. One possibility is that pathogen prevalence influences sexual disgust, which in turn motivates the adoption of more conservative sexual behaviours and attitudes. An alternative, but related, explanation for the cross-sectional relation between measures of infection-related mortality (parasite stress) and a variety of social behaviours is life history theory. According to this explanation environmental risks shape reproductive strategies. When risks are high, “faster” strategies are more prevalent, promoting early sexual maturation, reducing parental investment, and increasing the number of offspring. The fast life history approach suggests that disease prevalence is an outcome of reproductive strategies, rather than a cause. The authors note their effects are found even with a measure of parasite stress that excludes sexually transmitted infections, and reproductive strategies support the view that rates of infectious diseases are a significant source of ecological pressure – what they are demonstrating is that the pressure changes over time.

Reaching adulthood during a recession can have huge impact on how you vote

The authors acknowledge the research does have limitations.

“Although we can include controls for state-level urbanisation, income inequality and median income it is possible that parasite stress is acting as a proxy for more general resource scarcity or other negative aspects of the environment”, the study finds. “It is, for example, already well established that events occurring at particular times in life can have long-lasting effects on voter preferences and that reaching adulthood when the general economic environment is poor (eg in recessions) can permanently influence narcissism and job preferences as well as attitudes to democracy and immigrants.”

“Our claim is about changes in the relation between parasite stress and relative conservatism rather than the relationship between parasite stress and absolute conservatism,” the study says. “Given the importance of infectious disease as an adaptive influence, with almost 50% of children failing to reach reproductive age for infection-related reasons until relatively recently in evolutionary history, it seems plausible that declining levels of parasite-related disease and mortality may be responsible for the infection-ideology association also reducing in recent decades.”

The study is co-authored by Gordon D A Brown, University of Warwick; Lukasz Walasek, University of Warwick; Timothy L Mullett, University of Warwick; Edika G Quispe-Torreblanca, University of Oxford; Corey L Fincher, University of Warwick; Michal Kosinski, Stanford Graduate School of Business; David Stillwell, University of Cambridge Judge Business School.