The Behavioural Immune System: What is it?

Think about your immune system. What comes to mind? Personally, I think of white blood cells and antibodies. Those tiny elements are part of the complex system that defends our body from disease, but it’s costly protection.[1] A fever increases the body’s energy consumption by 13%, while fighting a pathogen leaves the body vulnerable to new infections.[2] That’s why our immune system is not the first line of defence—that role goes to our mind.[3]

“Stop, pathogens!” Sometimes our physiological immune system might not be enough to protect us from illness. Taken from Sleep Score

The behavioural immune system (BIS) is defined as a group of psychological mechanisms that are designed to promote disease-avoidance.[3] After hundreds of generations, evolution has developed the BIS to guide our daily decisions.[4] In other words, our brain comes pre-programmed to encourage behaviours that will help us not catch a cold—or worse. 

You might be wondering how this BIS thing works and how certain events (ahem, COVID-19) influence it. We break it down into its main components so you can see how our unconscious minds defend us from disease.

How does the behavioural immune system work?

The BIS is an unconscious mechanism made up of different levels of behaviour that reduce the risk of contracting an illness.[4] What are the levels? Well, we can distinguish them among reactive and proactive behaviours.[1] 

Reactive responses kick in when we encounter situations that are likely to get us sick. Think open wounds and bodily fluids.[4] This reaction has three parts: affect, behaviour, and cognition,[10] also known as the ABC model.[5] When we encounter a risky situation, our mind reacts with disgust and thoughts of contamination,[3,4] and we get the instinct to get away from the source of infection.[1] Think about how uncomfortable you feel when taking out the trash, for example. 

The ABC model of attitudes. Taken from KPUPressbooks

Proactive responses are unprompted. They’re not about avoiding sources of infection; instead, they work on preventing us from coming across them altogether. Washing our hands, showering, and other hygiene behaviours are proactive responses.[1] 

The BIS is constantly working together with the physiological immune system. “Disease cues” trigger the mental defenses,[2] which in turn command the body to heighten its immune response. The opposite is also true: the BIS can come after the physiological immune system to protect the body from unhealthy situations when we’re already sick.[2] That’s why we feel more paranoid about our health just after we experience illness.[2] 

Is Prejudice an Immune Response?

One of the main problems with BIS is that it works to avoid perceived disease threats.[4] It works under the “smoke detector principle”; just as a smoke alarm can be triggered by non-threatening airborne substances, so can we be averse to things that aren’t actually harmful to our health, like physical deformities or odd behaviours.[1,6] This means that disfigurements, disabilities, obesity, and elderly appearance can all be interpreted as pathogen cues[1] since those were indicators of disease for our ancestors. 

When you’ve been staying healthy but people still avoid you because you’re old :/
Taken from EastleighCareHomes

The sensitivity of the BIS varies across people,[4] with differences coming from genetic, developmental, and sociocultural factors.[7] People with more sensitive systems might be more averse to those perceived risks, leading to prejudice.[2] People who perceive themselves as being more vulnerable are also more sensitive.[6] This is why high-risk individuals have been more likely to follow social distancing and isolation guidelines during the COVID-19 pandemic.[7] 

BIS-based prejudice also relates to comparing our differences with those of outgroup members.[1] Simply put, we are less likely to trust people from different sociocultural backgrounds because we think they’re less likely to follow the social norms that protect us from infection.[8] Plus, germs from other social groups can be deadly.[9] This reaction was passed down to us from our ancestors, but it still affects our behaviour nowadays, even when diversity is ubiquitous in many places.

Differing responses to disease threats. Taken from ABC

The other big social consequence of BIS is the instinct to acheive ingroup cohesion.[9] This basically means that we pay more regard to social norms as a means of protection.[1,10] Research shows that people conform more easily to majority views when diseases are salient.[9] They are also more likely to be morally vigilant

The Cultural Immune System

If prejudice can be linked to the BIS, how about ethnocentrism and xenophobia?[8] Well, BIS-sensitive people might treat immigrants negatively because they see potential sources of infection in them.[4] Now, we’re not saying this to justify discrimination as a health behaviour. We’re saying it could explain why some people support so-called “protective policies”.[4] This explains why women become slightly more ethnocentric during the first trimester of pregnancy, when the fetus is more vulnerable to pathogens.[9] 

Going further, we can draw links between the BIS and cultural cognition. Countries with higher pathogen levels tend to be less extroverted[6] since extended social contact can spread disease. Naturally, these cultures are less curious and risk-seeking.[6] That way, they prevent the onset of illnesses. This can be valuable when an epidemic can ruin your community.

Which face is more attractive? Taken from Pinterest

When you think about it, health preservation has also shaped cultural perceptions of partnership.[1] Symmetrical features, for instance, seem beautiful because they hint at strong genes.[11] This means that an attractive person could be a better choice if we want our kids to have a good immune system.[1] 

Even cuisine finds itself shaped by the BIS. Why do you think some cultures cook with so many herbs and spices? Certain ingredients have antibiotic properties, which makes them valuable in regions that are more prone to diseases.[6] Just as how you feel your mouth burning when you eat a spicy meal, so do the harmful germs that might be in your food. 

Many Chinese soups contain various herbs, berries, and roots as they are said to have medicinal effects. Taken from EatWhatTonight

The Bottom Line

The BIS is a defence mechanism against mind diseases. It comprises our natural rejection against sources of illness as well as hygiene in general. It has developed, like many other mental systems, through generations of adapting. It’s so ingrained in our brain that social behaviour (and even culture!) are moulded by our instinct to preserve health. Sometimes, it goes as far as to elicit feelings of dislike against others. So, even though the BIS serves a fundamental role in our survival as individuals and communities, we have to make sure it does not go unchecked.

There’s probably not a better time to say this. As of the date this article is published, we are going through a global pandemic. If you’re like me, then reading about the BIS will have made you think of how many actions we see nowadays are a result of it. As a matter of fact, I am writing these words as I sit in self-isolation, waiting for my tests results. Doing the research on the BIS and reading about diseases made me feel anxiety and uncertainty about my health and that of those around me. 

Our mind and body work together constantly to keep us safe. Be it a global health emergency or just a normal day in town, the BIS constantly shapes our actions and reminds us that it’s not cool to get sick. With all its strengths and flaws, it’s essential for our collective survival. Now that you know about it, perhaps you’ll channel it correctly.


Academic References

  1. Ackerman, J. M., Hill, S. E., & Murray, D. R. (2018). The behavioral immune system: Current concerns and future directions. Social and Personality Psychology Compass, 12(2).
  2. Miller, S. L., & Maner, J. K. (2011). Sick Body, Vigilant Mind. Psychological Science, 22(12), 1467–1471.
  3. Terrizzi, J. A., Shook, N. J., & Mcdaniel, M. A. (2013). The behavioral immune system and social conservatism: a meta-analysis. Evolution and Human Behavior, 34(2), 99–108.
  4. Aarøe, L., Petersen, M. B., & Arceneaux, K. (2017). The Behavioral Immune System Shapes Political Intuitions: Why and How Individual Differences in Disgust Sensitivity Underlie Opposition to Immigration. American Political Science Review, 111(2), 277–294.
  5. Eagly, A. H., & Chaiken, S. (1993). The psychology of attitudes. Harcourt Brace Jovanovich College Publishers.
  6. Schaller, M., & Park, J. H. (2011). The Behavioral Immune System (and Why It Matters). Current Directions in Psychological Science, 20(2), 99–103.
  7. Makhanova, A., & Shepherd, M. A. (2020). Behavioral immune system linked to responses to the threat of COVID-19. Personality and Individual Differences, 167, 110221.
  8. Murray, D. R., Kerry, N., & Gervais, W. M. (2017). On Disease and Deontology: Multiple Tests of the Influence of Disease Threat on Moral Vigilance. Social Psychological and Personality Science, 10(1), 44–52.
  9. Terrizzi, J. A., Shook, N. J., & Mcdaniel, M. A. (2013). The behavioral immune system and social conservatism: a meta-analysis. Evolution and Human Behavior, 34(2), 99–108.
  10. Murray, D. R., & Schaller, M. (2011). Threat(s) and conformity deconstructed: Perceived threat of infectious disease and its implications for conformist attitudes and behavior. European Journal of Social Psychology, 42(2), 180–188.
  11. Schaller, M., & Park, J. H. (2011). The Behavioral Immune System (and Why It Matters). Current Directions in Psychological Science, 20(2), 99–103.

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