Keywords help to understand authors' self-assessment of how the concepts and studies are classified and related in this context. This clarifies which of these concepts have not been subject to sufficient analysis. Main contributors and journals show who has conducted most research and which publication has served to accumulate the majority of those studies. Co-occurrence in authors provides a structure of research communities, and co-occurrence in citations helps to understand the intellectual framework. Finally, maps provide a clearer picture of what has been done, the communities involved, and also points to possible lines of future research (Danvila-del-Valle et al., 2019).