Female rats express habitual behavior earlier in operant training than males.

Habitual behavior can be advantageous by increasing the availability of cognitive resources for use in other tasks. However, habitual behaviors are problematic when they are coopted to prolong the maladaptive responding present in several psychopathologies such as substance abuse, dysregulated fear responding in posttraumatic stress disorder, and obsessive-compulsive disorder. Although sex differences exist in the occurrence or progression of these psychopathologies, there are no studies that compare the development of habitual behavior systematically in male and female animals. In the present study, male and female rats were identically trained on a variable interval 30-s (VI 30-s) schedule of reinforcement to nose-poke for sucrose pellet reinforcers. Subsequently, the sucrose was devalued in one half of the animals by pairing its presentation with injections of lithium chloride (LiCl) to induce nausea, thus conditioning a taste aversion. Habitual behavior was operationalized as continued operant responding in an extinction test following devaluation of the sucrose reinforcer. Successful devaluation was confirmed with both a consumption and reacquisition test. Given identical training to 240 sucrose pellets, female rats demonstrate habitual behavior whereas male rats remain goal-directed. Additionally, females are habitual after 200 or 160 reinforcers earned on a VI 30-s schedule, but remain goal-directed at 120 and 80 reinforcers on this schedule. These data suggest that behavioral flexibility may be compromised in female rats compared to males due to accelerated habit formation in females. These results are important because sex differences are present in several psychopathologies, which may be related to differences in the development of habitual behavior.