The theoretical structure of the investment model
The investment model is a theory of relationship maintenance that Caryl Rusbult proposed in 1980 and then refined over more than thirty years. Building on social exchange theory, it added a factor of its own, investment size, and thereby brought to light mechanisms of relationship maintenance that satisfaction alone cannot explain. The central claim of the model is that commitment, meaning the intention to maintain a relationship, is determined as a function of three factors. (1) Satisfaction level: the degree to which the rewards obtained from the relationship exceed its costs and the person's own expectations. The higher this is, the stronger the commitment. (2) Quality of alternatives: how attractive the options outside the current relationship appear, whether another partner, life as a single person, or a network of friends. The lower this is, the stronger the commitment. (3) Investment size: the magnitude of the resources put into the relationship, including intrinsic investments such as time, emotional energy, and self-disclosure, and extrinsic investments such as shared property, mutual friends, and children. The larger this is, the stronger the commitment.
Empirical support for the model and its applications
The investment model is among the most strongly supported models in relationship psychology. The three factors are thought to account for a substantial share of the differences in commitment, and commitment in turn is considered a strong predictor of relationship stability, meaning whether the couple stays together. The model has been validated not only for romantic relationships but across a range of contexts, including friendship, workplace relationships, and commitment to an organization. It has also been applied to explaining why people remain in abusive relationships. In an abusive relationship, satisfaction is low, but the quality of alternatives is also low because of economic dependence and social isolation, while the investment size is large because of children, shared property, and the years already spent, so commitment persists despite the absence of satisfaction. Measurement is typically carried out with the Investment Model Scale (IMS), which comprises four subscales: satisfaction level, quality of alternatives, investment size, and commitment.
The investment model and compatibility assessment
The investment model looks at compatibility from an angle other than whether two personalities match: what it examines is the mechanism by which a relationship is held together. Applied to your own relationship, it gives you three things to look at. (1) Satisfaction: how much reward the two of you are likely to provide one another, given the combination of your traits. (2) Vulnerability to alternatives: people high in extraversion and high in openness have wide social networks and encounter attractive alternatives more often, while high conscientiousness is thought to soften that vulnerability. (3) Patterns of accumulating investment: couples high in conscientiousness are more likely to make joint investments deliberately, through saving, buying a home, and planning careers, so their investment size tends to grow over time. None of the three is settled at the point where two people meet. That shifts the question the model really asks: not whether the two of you are a good match, but how many reasons are currently holding the relationship together, and whether those reasons rest on satisfaction alone or are also carried by what you have already built together. Two relationships can both be continuing and still not be the same on the inside.