Introduction: It can be challenging for clinical trials to achieve external validity, meaning the studied effects within a trial are generalizable to other contexts or the broader population (Averitt et al., 2020; Carroll et al., 1999; Kennedy-Martin et al., 2015). However, the people with the lowest chance of inclusion are often the ones at greatest risk of disease and poor health (Howcutt et al., 2018). The current study used data from a survey that was built in response to anecdotal observations of changes in menstrual bleeding after being vaccinated for COVID-19 that recruited people who are often excluded from health research, including gender diverse people, minority ethnicities or racial identities in the U.S., postmenopausal people, and people with reproductive conditions. We analyzed survey items regarding how people heard about the study and why they chose to participate, identifying themes and categorizing motivations to participate in unpaid health research. Open-ended responses were compared and contrasted among demographic subgroups to more closely attend to the diversity of responses and to avoid an overfocus on the majority responses.Methods: The data came from a large, online mixed methods study of menstrual and other symptoms after receiving COVID-19 vaccines in 2021. Survey items analyzed here include demographics (age, gender, race, and ethnicity) and two open-ended text responses. The sample was limited to people in the U.S. who wrote in English (N = 22,737). The analysis approaches were conducted using computer-assisted qualitative data analysis software packages in R. Results: Our results show that people largely found the survey through trusted sources or online outlets. People’s motivations to participate included the following themes: vaccine effects and women’s/people’s health, personal experiences, and interest in and need for research. Our themes reflected multiple types of altruism (e.g., political, experiential, and community). Across each analysis, we found largely consistent results for these questions in each demographic subgroup. Conclusions: Based on our study, we recommend more responsive, flexible post-clinical trial research to reach historically marginalized groups. In order to do this, scientific communities need to build credibility and create online presences to engender trust with the groups with whom they seek to work.