In contrast to opt-in systems, relatively little is known about what influences whether or not people register a decision about organ donation in opt-out systems. We address this gap in the literature. Participants (N = 756) living in a country with opt-out consent (Wales, UK) provided information on demographics and blood donor status. Participants indicated whether they had opted-in or opted-out (i.e., active decision), or not registered a decision under the assumption of deemed consent. Subsequently, their negative emotional beliefs (or affective attitudes) towards organ donation were measured. Opting-in was associated with being younger, having donated blood and holding superstitious beliefs about donation (jinx). Disgust (ick factor) deterred opting-in, and bodily integrity concerns increased opting-out. Positive affective attitudes increased opting-in and deterred opting-out. Actively opting-in increases the likelihood of organs being transplanted, thus, modifying affective attitudes and targeting blood donors should enhance the number of donors available under opt-out systems.