Preclinical research is fundamental for the advancement of biomedical sciences and enhancing healthcare. Considering sex differences in all studies throughout the entire biomedical research pipeline is necessary to adequately inform clinical research and improve health outcomes. However, there is a paucity of information to date on sex differences in preclinical work. As of 2009, most (about 80 percent) rodent studies across 10 fields of biology were still conducted with only male animals. In 2016, the National Institutes of Health implemented a policy aimed to address this concern by requiring the consideration of sex as a biological variable in preclinical research grant applications. This perspective piece aims to (1) provide a brief history of female inclusion in biomedical research, (2) describe the importance of studying sex differences, (3) explain possible reasons for opposition of female inclusion, and (4) present potential additional solutions to reduce sex bias in preclinical research.