Reliable participation and sustained retention rates are crucial in longitudinal studies involving human subjects and biomaterials. Understanding the decision to enroll is an essential step to develop adequate strategies promoting long-term participation. Semi-structured interviews were implemented with newly recruited and long-term participants randomly drawn from two ongoing longitudinal studies with a biobank component in Germany. Iterative qualitative content analysis was applied to the transcribed interviews. Participants (n = 31) expressed their decision to enroll or remain in the study as the result of the complex interplay of individual factors, institutional cues, study-related features, and societal dynamics. Different forms of trust were identified as central within the elements used to explain participation and could be compared to Dibben, Morris, and Lean's dynamic model of interpersonal trust. Given these high levels of trust, an investigation of the morality of the trustful relationship at stake between participants and research(ers) is warranted.

As a cornerstone of public health, epidemiology has lately undergone substantial changes enabled by, among other factors, the use of biobank infrastructures. In biobank-related research, the return of results to participants constitutes an important and complex ethical question. In this study, we qualitatively investigated how individuals perceive the results returned following their participation in cohort studies with biobanks. In our semi-structured interviews with 31 participants of two such German studies, we observed that some participants overestimate the nature of the personal information they will receive from the study. Although this misestimation does not seem to jeopardize the validity of the consent provided at recruitment, it may still represent a threat for participants’ trust in research and thus their long-term commitment, crucial for such studies. We argue that such misestimation may have ethical consequences on the principles guiding the reflection on the return of results in biobank research, i.e. autonomy, beneficence, non-maleficence and reciprocity. We suggest that shifting from the idea of directly benefiting participants through the return of research results could help focusing on benefiting society as a whole, thereby increasing research trustworthiness of population-based studies using biobanks.