Figure 1.

Considerations for studying sex differences in drug self-administration. Studies of rodent drug self-administration should generally include animals of both sexes, with equal numbers of male and female animals run concurrently. It is not necessary to increase the planned sample size of the study unless preliminary analyses reveal trends or statistically significant results that suggest an effect of sex on the measure of interest. When sex differences are not revealed, data from males and females can be combined for final analysis and visualization. When sex differences are present, additional animals should be tested to attain the statistical power to detect an effect of sex, and all data analyses and visualization should include sex as a variable. All analyses involving sex (including those that were not statistically significant) should be clearly reported in the main body of any published reports on the dataset.