There has been a recent rapid increase in the number of primary studies comparing the impacts of flipped to traditional instruction in undergraduate chemistry courses. Across these studies, there are wide variations in flipped model design, implementation, and reported impacts. To investigate these variations, 28 primary peer-reviewed studies were systematically analyzed. There were three notable trends. First, compared to final exams, course GPA seems to be the more sensitive measure of significant gains in students' overall academic performance. Second, courses reporting significant gains in course GPA concertedly used: (1) an extrinsic motivational tool for students to complete pre- and in-class activities; (2) responsive mini-lecturing as an in-class instructional strategy; and (3) the optional flipped model feature of independent postclass problem solving. In stark contrast, studies reporting no difference in course GPA rarely incentivized student completion of pre- and in-class activities, and none used responsive mini-lecturing or postclass problem solving. It was difficult to determine robust trends in impacts on various student populations as impacts were seldomly disaggregated by descriptors such as sex, race/ethnicity, and income level. Third, although there was a clear trend of constructivism being used as the theoretical framework for flipped courses, extrinsic motivation potentially plays a key role in the model's impact. Instructor ability or desire to motivate students to engage with learning, however, was not addressed in most studies. These trends imply - more research is needed to determine impacts of flipped courses on diverse student populations and the role of instructor beliefs and ability to motivate students to engage with learning in a flipped course. Such research should be used to advance the theoretical understanding of how, why, and in what contexts flipped courses positively and significantly impact diverse students' academic performance.