AbstractThe global under-supply of sperm and oocyte donors is a serious concern for assisted reproductive medicine. Research has explored self-selected populations of gamete donors and their ex-post rationalisations of why they chose to donate. However, such studies may not provide the necessary insight into why the majority of people do not donate. Utilising the unique open form responses of a large sample (n = 1035) of online survey respondents, we examine the reasons participants cite when asked: “Why haven’t you donated your sperm/eggs?.” We categorise these responses into four core themes (conditional willingness, barriers, unconsidered, and conscientious objector) and eleven lower-order themes. We find that, on average, women are more conditionally willing (8.2% difference; p = 0.008) to participate in gamete donation than men. We also find that women are more likely than men to justify their non-donation based on their reproductive history (21.3% difference; p = 0.000) or kin selection and inclusive fitness (5.7% difference; p = 0.008). However, compared to women, men are more likely to validate their non-donation based on sociocultural or social norms (6% difference; p = 0.000) or religion (1.7% difference; p = 0.030). That so many of our study participants report in-principal willingness for future participation in gamete donation speaks to the need for increased research on understanding non-donor population preferences, motivations, and behaviours.