Positive outcomes from undergraduate research experiences (UREs) have resulted in calls to broaden and diversify participation in research. However, we have little understanding of what demographics are reported and considered in the analyses of student outcomes from UREs. Without this information, it is impossible to assess whether participation in UREs has been diversified and how outcomes may vary by participant identity. Through a comprehensive literature search, we systematically identified 147 peer-reviewed research articles on student participation in UREs in the natural sciences, published between 2014 and 2020. We coded each paper to document which student demographic variables are reported and considered in analyses. The majority (88%) of articles on UREs reported at least one demographic variable and 62% incorporate demographics into their analyses, but demographics beyond gender and race/ethnicity were infrequently considered. Articles on independent research apprenticeships included demographics in their analyses more frequently than studies on course-based undergraduate research experiences (CUREs). Trends in reporting and analyzing demographics did not change from 2014 to 2020. Future efforts to collect these data will help assess whether goals to diversify UREs are being met and inform how to design UREs to meet the needs of diverse student groups.