The purpose of this study was to bridge the existing gap in the literature by exploring the relationship between the Adlerian concept of social interest, expressed through cooperation and conflict, and perceived level of marital happiness. This study explored behaviors along a continuum of social interest from cooperation to conflict. Preexisting data were used from the longitudinal Marital Instability over the Life Course Project funded by the United States Department of Health and Human Services and the National Institute on Aging (Booth, Johnson, Amato, & Rogers, 2003). Data from telephone surveys were collected in 1980, 1983, 1988, 1992-1994, 1997, and 2000 from married individuals who were between the ages of 18 and 55 in 1980. The initial random sample was 2,033, but attrition took place for each of the subsequent waves, resulting in 762 respondents in 2000. Results of hierarchical regression analyses revealed statistically significant relationships in a positive direction between cooperation and marital happiness, and in a negative direction between conflict and marital happiness across all waves of data. Cooperation and conflict as indicators of social interest accounted for between 26% and 37% of the variance in marital happiness, except for the 1988 wave, which was an aberration on all measures and accounted for only 12% of the variance. This still exceeded the a priori effect size selected for the study, a standardized regression coefficient of |.10|. Implications for theory, research and practice include focus on the link between higher levels of social interest as demonstrated through cooperative behaviors and greater marital happiness, one between conflict and lower levels of marital happiness. Marriage and family therapists need to consider the underlying goals of conflict such as power that may reveal underdeveloped levels of social interest. Counselors need to focus on helping couples develop relational skills that include the social provisions needed like empathy, understanding, and support. Future research is needed to more clearly define behaviors along the continuum of social interest.