14.5 Evolved Mechanisms for Dealing with Disease
The immune system is a complex and amazing feat of nature. Despite its sensitivity to psychological and chronic stress, it can also adapt to changing environments and lead us to avoid diseases – without us even knowing it. Two major hypotheses have emerged in recent decades to help us understand how human behavior impacts and is impacted by evolving alongside pathogens and other environmental risk factors.
Hygiene Hypothesis
According to the Hygiene Hypothesis, early life exposure to certain microorganisms in one’s environment may confer an advantage in immune development. (Remember that our genetics and biology combine with our environment to result in an expressed phenotype.) In small doses, these exposures can lead to an immune tolerance to certain pathogens, allergens, and other parasites. As a result, children are slowly inoculated to their environments, and they can pass on this state to future children.
A study conducted in Finland and Russian Karelia by Kondrashova, Seiskari, Illonen, Knip, and Hyöty (2012) sought to understand the potential protection inferred by interaction with microorganisms in one’s environment. Concerned about the increase in autoimmune and allergic diseases, the researchers looked at environmental and lifestyle factors for children in Finland (high disease rates) and Russian Karelia (lower disease rates). Astonishingly, they found that children in Finland, exposed to fewer microbial infections, also exhibited higher ratees of certain autoimmune and allergy related diseases. In contrast, children in Russian Karelia who had been exposed to several microbial infections presented with significantly fewer autoimmune and allergy related diseases – as much six times lower.
Despite this evidence, the Hygiene Hypothesis should not be taken to mean that there is an advantage to a lack of cleanliness, attention to hygiene, or care in the face of major pathogens (and pandemics). Rather, it helps us understand how small exposures like a seasonal cold or playing outside can boost and support immune function as we age.
Evolution of Disgust
Our behavior is a key component of our immune system. We can take risks that put us directly in the path of epidemics and chronic stress. We also risk our health on smaller scales – eating raw fish as a delicacy, for example. Yet our ability to be disgusted and our cultural norms around hygiene and sanitation, may be one of our most important behavioral adaptations.
Valerie Curtis, Mícheál de Barra, and Robert Aunger (2011) propose that disgust specifically evolved to encourage disease avoidance. Given the importance of social inclusion, cooperation, and reciprocity in human evolution, it makes sense. First, the immediate disgust reaction, which occurs outside our executive decision-making, is most frequently triggered by rot, decomposition, noxious smells, and bitter or sour tastes – all things that regularly occur when pathogens and infectious microbes are present. The disgust response also becomes strongest during physiological vulnerable states like pregnancy, infancy, and our post-reproductive lives.
However, according to the authors, disgust’s adaptive value doesn’t stop there. Our cooperative nature leads us to be concerned about the potential for harming others. It is from here that they suggest “cooperative hygiene” evolved via social learning. Whether in hopes of future reciprocation or as the result of our desire to alloparent, disgust triggers a collective action to avoid disease as a group.
Disgust continues to be studied as an adaptation for disease avoidance. The following SciShow video describes two more recent research papers on the topic of disgust. The first study sought to unpack categories of disgust and the sociodemographic features that shape them. The second study considers the phylogeny of disgust through controlled experiments in a captive bonobo population. Note how age in the first study is associated with perceptions of disgust regarding everything but hygiene.
