Table 2. Overview of the preregistered hypotheses, related theories, and explanations.

#	Hypothesis	Related theory	Explanations
1	H1: Respondents’ willingness to participate in a survey is higher than their willingness to undergo a blood test.	Economic participation theory	A task with low burden, such as participating in a survey, would lead to a higher participation rate than a task with a higher burden, such as undergoing a blood test. This assumption is in line with the economic participation theory (Biner and Kidd, 1994 [25]; Porter and Whitcomb, 2003 [26]; Leeper, 2019 [27]), which suggests that individuals aim at minimizing their costs, while maximizing their gains.
2	H2: Respondents are more likely to participate in a survey or to undergo a blood test if the disease is less stigmatized.	Economic participation theory	HIV is expected to be more stigmatized than diabetes. Stigmatization involves psychological costs.
3	H3: Promising incentives will increase the willingness for both survey participation and undergoing a blood test.	Economic participation theory	Financial incentives are more likely to trigger economic motives for survey participation.
4	H4: Providing a cash incentive or a lottery cash incentive increases willingness to participate compared with voucher.	Economic participation theory	Direct compensation (compared with vouchers) makes payments more tangible and therefore is more likely to trigger economic motives for survey participation.
5	H5: Local universities as sponsors increase the likelihood of participation compared with governmental institutions or NGOsa.	Theory of contextual integrity	Universities are directly related to research and to a lesser extent to (1) other policy fields and (2) corruption. This assumption is in line with theory of contextual integrity (Nissenbaum, 2010 [24]), which suggests that data flaws are likely to be considered more appropriate if they align with perceptions of higher integrity.
6	H6: The higher the level of trust in the respective survey sponsor, the higher the willingness to participate.	Social exchange theory	Trust is considered important to facilitate reciprocal and altruistic behavior and therefore is conducive for social exchange (Dillman, 1978 [28]; Goyder et al, 2006 [29]).
7	H7: The more concerned respondents regarding the privacy of their blood samples, the less likely they are to be willing to undergo the blood test.	Theory of contextual integrity	Privacy concerns influence the sensitivity level of the requested data and define the data sharing context, as they influence several parameters, which are directly related to the appropriateness of a data flow.
8	H8: If a respondent is HIV positive or was diagnosed with diabetes, the person is less likely to participate.	Theory of contextual integrity	Being diagnosed with the respective disease makes a research topic more sensitive and affects the context of the data sharing request.

^aNGO: nongovernmental organization.