Abstract: Posttraumatic stress disorder is a kind of anxiety disorder which developed after severe trauma. Conditioned fear model is the most emblematical model of posttraumatic stress disorder. At the present time, the most effective therapy is the exposure therapy which uses extinction training to repress the conditioned fear memory. However, some of the PTSD patients were having relapses after the exposure therapy, these relapses were later named as the extinction return.
An experiment was designed to research for sex differences in the extinction return of conditioned fear memory. Forty normal students participated in the experiment, including 20 females and 20 males. Before the actual experiment, the participants had to attend the extinction training session; the participants were trained to consciously establish and extinguish the connection between neutral stimulus and repugnant stimulus. The experiment consisted of pre-exposure, acquisition, extinction, and test phases. The pre-exposure phase required the participants to understand the procedure. In the acquisition phase, the participants would acquire the conditioned fear response via the connection of the neutral stimulus and the repugnant stimulus. In the extinction, the neutral stimulus would be presented alone without the repugnant stimulus. Four hours later, test phase was to examine whether the extinction return would be found, and whether males or females performed differently on the acquisition and extinction of conditioned fear memory.
The results were as following: (1) The participants were observed to have obvious extinction return overall when they were tested after 4 hours later during the extinction phase. (2)The extinction return in females was much more significant than in males. (3)The females tended to acquire the conditioned fear memory more effectively and extinguish more slowly than males. But the difference is not significant.
The results of this study suggested that the extinction return was a general effect and sex difference does exist. This phenomenon might be found after each extinction effect. Since females seem to suffer from more extinction returns than males do, it is important for the consultants to extend the follow-up period for the female patients after the exposure therapy.

Key words: conditioned fear memory, extinction return, sex differences