The selective advantage of morning sickness

By motivating avoidance of contaminants, the experience of disgust guards against disease. Because behavioral prophylaxis entails time, energy, and opportunity costs, Fessler and Navarrete [Evol. Hum. Behav. 24 (2003) 406–417] hypothesized that disgust sensitivity is adjusted as a function of immunocompetence. Changes in immune functioning over the course of pregnancy offer an opportunity to test this notion. Relative to later stages, the first trimester of pregnancy involves substantial suppression of the maternal immune response, and both maternal and fetal vulnerability to pathogens are greatest during this phase; food-borne illnesses, in particular, pose a threat during the first trimester. Using a Web-based survey of 496 pregnant women, we compared participants in the first trimester with those in later stages of pregnancy. Results reveal heightened disgust sensitivity in the first trimester, notably including disgust sensitivity in the food domain. This pattern is not simply a consequence of elevated nausea during the first trimester, as, although disgust sensitivity and current level of nausea are correlated, first trimester women remain more easily disgusted in the food domain even after controlling for the greater incidence of nausea. These results provide preliminary support for the hypothesis that disgust sensitivity varies during pregnancy in a manner that compensates for maternal and fetal vulnerability to disease.