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The Compensatory Prophylaxis Hypothesis
The Compensatory Prophylaxis Hypothesis (CPH) states that women are more likely to show disgust and pathogen avoidance in the luteal phase of their menstrual cycle when the increase in the progesterone represses their immune system. In this case, disgust sensitivity is hypothesized as a compensatory mechanism may contribute to avoiding pathogens. The CPH is a psychological response that contributes to behavioral avoidance of pathogens. Additionally, disgust sensitivity has per se investigated in the three domains of pathogen, sexual, and moral disgust.

Progesterone
Progesterone plays multiple roles in women’s bodies and helps regulate their reproductive systems. Additionally, as a sex hormone that is primarily produced in the luteal phase, it can affect reproductive immune modulation. The reproductive immunomodulation mechanism driven by progesterone leads to restraint on natural killer (NK) cells’ activity. Another widely accepted hypothesis discusses the connection between the surge of progesterone during pregnancy and its impacts on compensatory prophylaxis.

Pregnancy
Similar to the spike of progesterone observed during the luteal period, an increase in the concentration of progesterone has been noticed close to the end of pregnancy. Hence, it has been hypothesized that progesterone spikes could be correlated with feelings of disgust during this period in pregnant women. In the meantime, pregnant women report feelings of disgust and nausea more often during the first trimester of their pregnancy while the surge in progesterone levels happens close to the end of the pregnancy. This can potentially raise doubts about the connection between higher progesterone levels and disgust feelings. In other words, the hypothesized connection could be stronger if higher progesterone concentrations and increased feelings of disgust would both happen close to the end of pregnancy. Another observation that cast doubts on progesterone being the driving factor in the increased disgust feelings during pregnancy has been presented by social neuroendocrine research. In these studies, the higher concentration of progesterone is even connected to reduced social anxiety which can lead to increased willingness for social affiliation. All these would indicate the possibility of other reproductive immunomodulation factors driving pathogen disgust during the menstrual cycle.

Evolutionary significance
An evolutionary explanation for the modifications resulting in the weakening of the immune system during the luteal phase and pregnancy is protecting the embryo from possible attacks of the immune system against the half-foreign blastocyst. This protection enables the embryo to complete the implementation process. On the other hand, the weakened immune system in woman makes her (and the future embryo) more vulnerable to infections. For this reason, a mechanism that contributes to avoidance behavior towards pathogens could be preferred by natural selection.

Disgust Sensitivity
Feelings of disgust can be manifested in a variety of domains but most common ones include pathogens, sexual, and moral disgust.

Pathogen Disgust
Pathogen disgust can drive the feeling of avoidance of infection factors and increased hygienic behavior. Other observations have also shown an increased cleaning behavior in some women during the luteal phase as well as a domain of OCD behavior in which they showed an increased desire for washing and cleaning activities that can reduce the possibility of infections. Similarly, it was predicted that women show a decreased craving for meat, a major pathogenic food, in their diet during the luteal phase. Although the effect of the cycle phase on meat consumption was not supported in the studies, observations showed decreased consumption of meat intake in sexually active women. On the contrary, while some research disagrees with the connection between progesterone concentrations and pathogen disgust, a comprehensive study conducted by Karoline (2021), has shown the link between increased progesterone and increased pathogen disgust.

Sexual Disgust
Sexual disgust can be defined as a behavior in which the person avoids from participation in suboptimal sexual activities. In this definition, suboptimal sexual activity is considered an activity that could not have optimal fitness impacts. Examples of such activities include incest, age-disparate unions, and bestiality. Since sexual disgust aims to prevent sexual activities with less optimal outcomes, sexual sensitivity is expected to vary throughout the menstrual cycle regarding to the variation of conceptus. Some findings consistent with this prediction, and they show that women have an increased tendency to refrain from activities that could expose them to sexual assault. On the other hand, there is research questioning the increased sexual disgust in the preovulatory period. These studies argued that there is not sufficient evidence that feelings of sexual disgust can be driven by hormones.

Moral Disgust
Moral disgust can be defined as the negative feeling when social norms are violated. Also, studies show that moral disgust could happen even when less serious social values are disregarded such as gossip, lesser acts of aggression, or some feeling of disgust towards pathogen or sexual acts, which are known as taboos, could be considered morally inappropriate and need to be punished. Therefore, in the same vein as the pathogen and the sexual disgust, moral disgust may increase in the luteal phase and women’s moral punishments for moral violations might increase. These norms violations could be incest, bestiality, purity violation, or even have different race, sex, religion (xenophobia). On the other hand, some studies argue that there is no enough evidence for women’s cyclical moral disgust.

On the whole, a recent study stated that there are some methodological limitations in the study of CHP. One of the limitations are the estimation related to the cycle phase (e.g., the length or the day of the cycle). It is suggested that for testing CHP, ovulatory tests are necessary. The other limitations are utilizing only one method for the disgust measurement (i.e., just using questionnaires and not using pictures). Additionally, the design of some studies’ method is based on between-subject design which is problematic. Research design on CPH need to measure each woman’s disgust sensitivity repeatedly (between her phases) instead of comparing females with each other, or doing a single measurement.

Disgust measurement
The most respected and trusted methods of disgust sensitivity measuring the pathogen and moral domains are Three Domain Disgust Scale (TDDS), and a measurement based on pathogenic relevant pictures (image-based). A recent study, in addition to disgust assessment of Pathogen and Moral domains, used Contamination Obsessions and Washing Compulsions Subscale of Padua Inventory – Washington State University Revision to measure the contamination sensitivity.