Accordingly, this is one area where preclinical research is active, the new NIH policy is pertinent, and inclusion of both males and females is warranted. The expression and treatment of pain constitute one domain in which sex differences are apparent in both humans and laboratory animals, and preclinical research is playing a major role in analgesic drug development in response to the NIH “Helping to End Addiction Long-term” (HEAL) initiative. 1 and Table 1 for a study involving pain-related behavior in mice.

The data to illustrate this approach are shown in Fig. The factors that may contribute to an individual researcher’s prioritization of sex differences among all possible independent variables are beyond the scope of this brief article.

We acknowledge that there are many instances where sex may be a top priority however, as written, the NIH policy and associated “Four C’s” guidance recognize that this is not invariably the case. This article describes a general, multi-step approach to address The Four Cs when investigation of sex differences is not the primary aim, and sample sizes required to detect sex differences are not known. As guidance to address this policy, NIH has articulated “The Four Cs of Studying Sex to Strengthen Science” ( ): (1) Consider (design studies that take sex into account, or explain why it is not incorporated), (2) Collect (tabulate sex-based data), (3) Characterize (analyze sex-based data), and (4) Communicate (report and publish sex-based data). The National Institutes of Health (NIH) issued a policy in 2015 to promote inclusion of both male and female subjects in preclinical biomedical research.

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