Evolution is a theory initiated by Charles Darwin, primarily in his book, Origin of the Species. It contains fundamental principles including natural selection, survival of the fittest and competition. Natural Selection suggests that advantageous genes are passed down through to subsequent generations and in psychology, it is important to consider that it describes the mechanisms of behaviour (MoB) rather than the behaviours themselves.
A MoB is the observable response made to a situation and the unconscious processes underlying it. Survival of the fittest favours organisms that are best adapted to a certain environment.Evolution of the brain can be observed in both humans and animals. Some common trends include the appearance of a hindbrain amongst reptiles, the limbic system amongst mammals as well as the frontal lobe in higher mammals.
Fessler (2006) aimed to investigate whether disgust sensitivity in the first trimester of pregnancy was as elevated as predicted. He also hypothesised that disgust helps to compensate for the suppressed immune system of a pregnant mother. Data were collected using a Web-based questionnaire through the participation of 691 healthy, pregnant females between the ages of 18-50 years. 155 of which were in their first trimester, 183 in the second trimester, and 158 in the third trimester. Their mean age was 28.1 years. They were recruited through postings to pregnancy-related web sites. Participation was anonymous, and no compensation was offered.
On the Web-based questionnaire, the participants indicated their current level of nausea using a 16-point scale and answered questions to test their disgust sensitivity in eight different areas (e.g. food; contact with animals, body products, and dead animals; hygiene; contact with toilets). The results indicated disgust sensitivity related to food and body products in women in the first trimester was higher compared to those in the second and third trimesters. Due to greater vulnerability, food-borne diseases are particularly dangerous to women in the first trimester and therefore it was predicted that disgust sensitivity related to food would be increased. The results may also indicate that nausea and vomiting are evolved behaviour because they limit the likelihood that pregnant women will eat dangerous or contaminated food.
Fessler explained that natural selection may have helped human ancestors to be captious with food to compensate for their higher susceptibility to disease. This behaviour would also help humans avoid diseases that could harm unborn offspring, and thus, threaten the species. As the sample size of the study was large and the participants had a wide age range, it was able to be generalised to the wider public. It was well controlled as only 32 images were shown to each participant. Only healthy pregnant women were selected, thus eradicating any confounding factor of illness affecting responses. The data was quantitative as it was ranked via self-report, thus could be used for statistical analysis.
However, the findings cannot be solely based on evolution, as the environment could be an interfering variable. There is also cultural and emotional influences on women’s disgust. Since it was a quasi-experiment, the IV could not be manipulated. Data was ranked by self-report which is often unreliable due to individual differences in terms of food ‘likes’ and ‘dislikes’ and past food poisoning experiences will affect ‘rankings’.
Fessler’s study supports that disgust may be an evolutionary behaviour as it may assist reproduction of offspring and protection against disease, thus having a greater chance of surviving or living a longer life, having children passing on genes to their offspring. Furthermore, the study provided evidence for women possessing the most powerful disgust mechanism in the first trimester, keeping in line with the theory of evolution since it is at this point that the immune system is its weakest (in order to protect the unborn baby). This leads to the conclusion that the women with the most active disgust mechanisms, survived long enough to pass down their genes through generations and therefore kept this mechanism prevalent.
An online questionnaire (internet survey) was conducted by Curtis et al (2004), which aimed to research whether there were patterns within people’s disgust responses. In the survey, 77,000 participants for 165 countries were shown 20 images. For each image they were asked to rank their disgust, varying from disease relevant (using images with yellow and brown tones) and disease irrelevant images (using blue visual tones). However, the number of participants had decreased to 40,000, as the other 37,000 responses were disregarded since participants were accidentally exposed to the aim of the study having an impact on the validity of the participants’ responses. The findings of the survey confirmed that disgust reactions were mostly illicit for those images that did threaten the immune system.
The disgust reaction also decreased with age, meaning they were comparatively high in lower ages, and women had higher disgust reactions than men. This study validated the theory that disgust is an evolutionary behavior which plays a role in the protection of the immune system and did so through a financially economic online survey, further attaining higher reliability due to the large sample size. However, confirmation bias (Hayes, 2005) is present as it is difficult to empirically test some evolutionary based theories. A minor degree of gender bias is also present as a slightly larger percentage of females (50.3%) undertook the survey in comparison to males. Curtis et Al’s study is also criticized due to the underestimation of nurturing, cultural and environmental influences on the responses.