Plain Language Summaries: A Systematic Review of Theory, Guidelines, and Empirical Research

Background: Plain language summaries (PLS) have been introduced to communicate research in an understandable way to a non-expert audience. Guidelines for writing PLS have been developed and empirical research on PLS has been conducted, but terminology and research approaches in this comparatively young field vary considerably. This prompted us to review the current state of the art on the theoretical and empirical literature on PLS. Main Objectives: Main objectives were (1) to develop a conceptual framework for PLS theory, and (2) to synthesize empirical evidence on PLS criteria. Major research questions were how theoretical considerations on PLS can be conceptualized and what the empirical evidence of their effects on defined outcomes is. Methods: We searched Web of Science, PubMed, PsycInfo, and PSYNDEX (last search 07/2021). In our review, we included empirical investigations of PLS, reports on PLS development, PLS guidelines, and theoretical articles referring to PLS. A conceptual framework was developed through content analysis. Empirical studies investigating effects of PLS criteria on defined outcomes were narratively synthesized. Results: We identified 7,714 records, of which 90 articles met the inclusion criteria. All articles were used to develop a conceptual framework for PLS which comprises six categories each for PLS aims and PLS characteristics. Thirty-three articles empirically investigated effects of PLS on several outcomes, but study designs were too heterogeneous to identify definite criteria for high-quality PLS. Insular studies identified effects of various criteria on accessibility, understanding, knowledge, communication of research, and empowerment. For most criteria we identified from PLS writing guidelines, we found no empirical evidence. Conclusion and Implications: Considerable work to establish and investigate PLS has been done, but empirical evidence on criteria for high-quality PLS is scarce. The conceptual framework developed in this review may provide a valuable starting point for future guideline developers and PLS researchers.