To combat the high cost of textbooks, open (digitally free) textbooks have recently entered the textbook market. Griggs and Jackson (2017) reviewed the open introductory psychology textbooks presently available to provide interested teachers with essential information about these texts and how they compare with traditional (commercial) introductory textbooks. They did not, however, include any discussion of the research that has examined the effects of open introductory psychology textbooks and other open educational resources versus traditional introductory textbooks on students' course performance (e.g., course grades). The present study provides a review of this research. The review indicated that no firm conclusions can be drawn not just because there are a limited number of studies with seemingly conflicting findings but more importantly because of numerous uncontrolled relevant variables in all of the studies. To aid researchers who want to conduct future studies on this topic and reviewers who will evaluate these studies, we discuss these variables and the control issues they create.