Although humans engage in committed, long-term pair bonds, many romantic relationships end because one partner no longer desires to be in the relationship. Much of the literature on romantic relationship psychology and behavior has focused on mechanisms related to maintaining a partner. Mate retention behavior functions to deter romantic partners from defection and fend off potential alternative partners. However, when individuals are in a relationship where the costs of remaining in the relationship outweigh the benefits of leaving the relationship, mate expulsion, not retention, may be the desired goal. The present thesis examines mate expulsion behavior and psychology with the goal of developing a measure of mate expulsion to parallel a widely-used measure of mate retention. In my pilot studies, participants (n= 103) nominated behaviors and tactics that people use to reduce commitment in and terminate relationships. I identified 168 unique mate expulsion behaviors from these nominations that fell in the following four categories:signaling a lack of commitment to their partner, signaling their availability to others potential partners, extracting oneself from shared relationship commitments or investments, and reducing dependency on one’s partner or relationship. A separate set of participants (n= 131) rated the frequency with which they had used the behaviors, or had seen their partner use the behaviors, in their actual past break-ups. This procedure reduced the list of utilized tactics to 51 mate expulsion behaviors. A third set of participants (n = 290) in relationships rated the frequency with which they engaged in the 51 behaviors in their current relationship and completed measures of relationships satisfaction and mate retention. As expected, mate expulsion was negatively related to relationship satisfaction. I conducted a preliminary factor analysis using these data, which revealed 7 clusters of mate expulsion behavior: For my thesis, I collected a larger, less gender-biased sample to 1) confirm the factor structure of the mate expulsion inventory and 2) examine the relationships between the mate expulsion inventory, mate retention, and relationship satisfaction. Participants (n = 410) completed the Couple Satisfaction Index-16, the Mate Retention Inventory Short-Form, and the Mate Expulsion Inventory. Mate expulsion was again negatively correlated with relationship satisfaction and a similar, small positive correlation was replicated between mate expulsion and mate retention. My hypothesized model for the confirmatory factor analysis was acceptable but not excellent. I attempted several modifications to improve the measures of fit. Ultimately, the best model included the removal of specific items and eliminating a latent variable. This thesis produced a concise list of mate expulsion behaviors and has expanded on the literature of mating psychology in respect to relationship termination. These results suggest human mating psychology may include mechanisms that function to terminate and maintain relationships.