Predisposition to particular behaviours is presumed to be an inherited trait. Behaviours seen today would likely have an evolutionary explanation, as our current behaviours may have been advantageous towards human survival and reproduction. As such, there have been many studies to better understand the evolutionary explanations of behaviour. In this essay, I will evaluate two studies that examine the evolutionary basis in human behaviours, specifically with respect to the behavioural emotion of disgust. The first study by Fessler (2006) looked at pregnant women’s timing of pregnancy (and, in turn, immune status), nausea, and disgust level to different scenarios, in order to relate how disgust potentially helps women survive the pregnancy period to effectively produce offspring. The second study by Curtis et al. (2004) examined the disgust rankings of men and women of different ages from different countries to images that may or may not perceived to be dangerous to the immune system, in order to show how disgust can possibly help human survival by avoiding unhealthy situations.Evolution is the change of inherited traits in a species over a long period of time. The theory of evolution proposes that over time, organisms evolve through natural selection as their inherited physical or behavioural traits change to allow the best chance for survival and reproduction and passing on these traits. This is known as “survival of the fittest.” These advantageous alterations happen as the DNA of an organism gradually mutates and enables the organisms to survive and produce more offspring by adapting most suitably to their environments. Therefore, the organisms are then more likely to pass on these particular genes to their offspring. Since behaviours are presumed to be genetically based, an evolutionary basis can be established if the behaviour can be shown to enhance reproduction and survival. For example, if an organism must adapt to the environment with behavioural reflexes in order to assist in reproduction and survival, the genes associated with that behaviour are preserved and perpetuated as they get passed on to the next generation. Fessler (2006) hypothesized that the behavioural emotion of disgust helps counteract a woman’s suppressed immune system during pregnancy. He surveyed nearly 500 pregnant women between ages of 18-50 to rank repulsive scenes. He controlled the study by identifying when women had morning sickness so that trimester of pregnancy and feelings of nausea could be evaluated separately with respect to disgust ranking. The results showed that women in trimester one of pregnancy ranked disgust higher in all scenarios compared to women later in pregnancy. The food related scenes were rated more disgusting than non food-related ones. Fessler concluded that women in trimester one, when immune system is most suppressed, had the highest level of disgust because it would be advantageous for the early pregnant woman to be most cautious about food since both the woman and fetus would be vulnerable to foodborne diseases. This shows the protective role of disgust. The study’s strengths include generalizability with its large number of participants and wide age range, and its methodological strength was to evaluate timing of pregnancy and nausea and disgust independently. Limitations include the use of self reports which are not as reliable. A real life confrontation with repulsive objects may have been more effective to measure disgust in the participants. Moreover, disgust rankings can be affected by individual and cultural differences in food preferences. Despite these limitations, this study supports that disgust is an evolutionary behaviour by protecting against disease and aiding in human reproduction. Curtis et. al (2004) hypothesized that the disgust emotion helps people avoid things which threaten health. His reasoning was that as disgust allowed our ancestors to avoid situations which could lead to sickness, there would be better survival and reproduction to pass on this trait. He investigated patterns of people’s disgust responses with an online survey to 77,000 people from multiple countries. The participants were asked to rank their disgust level for 20 images where some appeared potentially harmful to a person’s immune system. The results showed that the disgust ranking was the highest with the images that threatened health, and that the disgust rankings were also higher in younger people and higher in females. Curtis et. al concluded that disgust was an important emotion that is advantageous to survival and also for successful reproduction. The strongest disgust response in women, and most notably younger women of child-bearing age, may be due to natural selection and adaptation. The strengths of this study include its generalization with its the large number of participants, and it is ecologically valid. Despite the limitations that online surveys may not be as reliable and different cultures may view the images differently, this study provides strong evidence that the emotion of disgust has a protective role and is passed on through genetics so that humans have better survival and reproduction. Darwin’s theory of evolution claims that the fittest who survive the environmental challenges are most likely to give their offspring traits best suited to the environment. Furthemore, natural selection results in adaptation which give the species advantages in survival and reproduction. Since behaviours are believed to be genetically inherited, evolutionary explanations of behaviours can be studied. The studies by Fessler (2006) and Curtis et. al (2004) provide strong evidence of the influence of evolution on the emotional behaviour of disgust. Specifically, Fessler’s study supports that disgust may be an evolutionary behaviour that protects against disease in pregnancy and aids reproduction, which in turn allows the behavioural trait of disgust to continuously be passed down generations. In addition, Curtis’s study also supports that disgust has an evolutionary basis that helps prevent sickness, which helps not only reproduction, but also survival of the human species.