Empirical studies have found that altruism and self-interest are the two primary motivations for enrollment in clinical trials. Some studies have shown that in some cases these two motivations are contingent upon each other, which complicates our understanding of motivation. In this study, we interviewed 27 people with Parkinson's disease about their willingness to enroll in a hypothetical clinical trial. Through inductive, grounded theory analysis of the interview transcripts, we find four different contingent relationships between altruism and self-interest. It is important for ethicists to be aware of these more complex motivations because some are ethically problematic and others not. Moreover, practitioners need to be aware of these contingent relationships so that they can understand the motivations of the research participants.