Predisposition to particular behaviors is presumed to be an inherited trait. Behaviors seen today are likely to have an evolutionary explanation, as our current behaviors may be advantageous towards human survival and reproduction. As such, there are many studies to better understand the evolutionary explanations of behavior. In this essay, I will evaluate two studies that examine the evolutionary basis in human behaviors, specifically with respect to the behavioral emotion of disgust. 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 behavioral 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 behaviors are presumed to be genetically based, an evolutionary basis can be established if the behavior can be shown to enhance reproduction and survival. For example, if an organism must adapt to the environment with behavioral reflexes in order to assist in reproduction and survival, the genes associated with that behavior are preserved and perpetuated as they get passed on to the next generation. In 2006, Fessler comes up with a hypothesis that the behavioral emotion of disgust helps offset a woman’s suppressed immune system during pregnancy. He surveys 500 pregnant women between ages of 18-50 to rank repulsive scenes. He controls the study by identifying when women have morning sickness so that pregnancy trimesters and feelings of nausea can be evaluated separately with respect to disgust ranking. The results display that pregnant women in trimester one rank greater disgust in every scenario compared to women later in pregnancy. The food related scenes are rated more disgusting than non food-related ones. Fessler concludes that women in trimester one, when immune system is most suppressed, had the highest level of disgust because it will be advantageous for the early pregnant woman to be most cautious about food since both the woman and fetus will 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 capability 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 be more effective to measure disgust in the participants. Moreover, disgust rankings can be influenced by individual and cultural differences in food preferences. Despite these limitations, this study supports that disgust is an evolutionary behavior by protecting against disease and assisting in human reproduction. In 2004, Curtis et. al hypothesizes that the disgust emotion helps people avoid things which threaten health. His reasoning is that as disgust permits our ancestors to avoid situations which can lead to sickness, there will be better survival and reproduction to pass on this trait. He investigates patterns of people’s disgust responses with an online survey to 77,000 people from multiple countries. The participants are asked to rank their disgust level for 20 images where some appear potentially harmful to a person’s immune system. The results show that the disgust ranking was the highest with the images that threaten health, and that the disgust rankings are also higher in younger people and higher in females. Curtis et. al concludes that disgust is 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 childbearing 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 and strongest who survive the environmental challenges are most likely to give their offspring traits best suited to the environment. Natural selection results in adaptation which give the species advantages in survival and reproduction. Since behaviors are believed to be genetically inherited, evolutionary explanations of behaviors can be studied. The studies by Fessler in 2006 and Curtis et. al in 2004 give compelling evidence of the influence of evolution on the emotional behavior of disgust. Fessler’s study supports that disgust may be an evolutionary behavior that shields away disease in pregnancy and aids reproduction, which in turn allows the behavioral 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.