Romantic partners may experience four types of self-change within their relationships: expansion (adding positive traits), pruning (losing negative traits), adulteration (adding negative traits), and contraction (losing positive traits). A dyadic study was conducted examining two questions. First, were individuals biased or accurate in their perceptions of their partner’s self-change, or some degree of both? Second, to what extent were individual’s perceptions of self-change predictive of relationship quality, satisfaction, and commitment? One hundred and forty-one romantic couples were recruited through SONA who were currently in a romantic relationship for at least three months. To examine consistent gender differences within dating relationships, the final sample consisted of 130 heterosexual dyads (n = 260). Both partners completed surveys measuring self change, relationship quality, commitment, satisfaction, and intimacy. To test both research questions, path-analytic models of the actor-partner interdependence model (APIM) were tested using Mplus. To address accuracy and bias, participants’ ratings of their own self-change were used to predict their perceptions of their partner’s self-change. To address relationship outcomes, one model used both participants’ ratings of self-change to predict each individual relationship outcome while a separate model used a latent variable approach. Relationship length, intimacy, and positive emotions were explored as moderators in all models. Overall, both women and men were significantly biased in their perceptions of self-change regardless of the addition of moderators; they used their own ratings of self-change to infer their partner’s self-change. Women were more accurate in perceiving men’s expansion and contraction while men were accurate only in perceiving women’s expansion. Regarding relationship outcomes, both women’s and men’s expansion and contraction significantly predicted relationship quality in both the full model and latent variable model of relationship quality. Moreover, the self-change-relationship outcomes findings remained significant even with the addition of relationship length and intimacy as moderators. Taken together, these findings suggest a positivity bias within relationships as expansion and contraction reflect processes involving positive self-concept content. Individuals focus on positive traits within themselves, and changes to these traits may then influence their own relationship quality.