In 7 experiments, the causal effects of the availability of an attractive alternative (AA) relationship partner on current relationship thoughts and intentions were tested using confederates, mental simulations, and virtual reality. Men behaved consistent with traditional relationship-commitment theories, showing decreased willingness to tolerate their partner's transgressions after the availability of an AA was made salient. However, consistent with a motivated cognition approach to commitment and work on relational self-construals, women increased their tolerance when presented with the relationship threat of an alternative. Word-fragment and lexical decision data suggested that AAs may activate threat for women, and their ability to dampen threat accessibility is associated with prorelationship responses. Finally, this "if relationship is threatened, then defend the relationship" contingency was induced in men with an implementation intention induction.