Search strategy and selection criteria

Relevant articles published on or before January 30, 2020 were searched from the following databases: Scopus, Embase, Medline, CINAHL, Web of Science, ProQuest databases, EBSCOhost Research Databases, and PsycINFO. The start year of the publication was not specified to filter the search results. To supplement searches of the indexed literature, grey literature sources were searched in consultation with an Information Specialist (Additional File 1). The following search terms were used in all of the databases: “basic income”, “guaranteed annual income”, “guaranteed minimum income”, “minimum income”, “negative income tax”, “optimal income transfer*”, mincome, demogrant*, “citizen* income”, or “universal income”. We also searched Google, Google Scholar, Open Grey and Campbell Library along with websites that focused on BI interventions. Additional studies were identified through expert contacts and the reference list of included studies.

Studies had to meet all three of the following inclusion criteria: 1) The article had to report on a BI intervention or program, which provided guaranteed income unconditionally to low-income individuals. Studies describing selective interventions, such as the Bolsa Familia or universal interventions in regions such as the Netherlands, were excluded due to pre-defined conditional requirements to gaining income [34]. 2) The study had to report on the methods used to evaluate the impact of BI interventions. Reporting on the methods used to implement a basic income scheme was not sufficient for inclusion. 3) The article had to report primary data from an observational study (e.g., case-control, cohort), experimental study (e.g., randomized controlled trial or RCT), or protocol for future research. Reviews, opinions, commentaries and editorials, or literature that specifically focused on a subset of the population (e.g., children, seniors) were excluded.