Investment Model
Rusbult's Investment Model of Commitment
Caryl Rusbult (1980, 1983) proposed the investment model as an extension of social exchange theory that more fully explains why people commit to and maintain long-term relationships. The crucial innovation is the addition of investment size as a predictor of commitment — alongside satisfaction and quality of alternatives. The investment model explains phenomena that neither SET nor equity theory can fully account for: why people maintain commitment through low satisfaction, and why leaving seemingly poor-quality relationships is so psychologically difficult.
The Three Components of Commitment
Rusbult proposed that commitment is determined by three factors:
| Component | Definition | Effect on commitment |
|---|---|---|
| Satisfaction level | Rewards − Costs relative to CL (as in SET) | Higher satisfaction → higher commitment |
| Investment size | Resources put into the relationship — time, effort, shared memories, mutual friends, children, possessions | Greater investment → higher commitment |
| Quality of alternatives | The attractiveness of available alternatives (equivalent to CLalt in SET) | Better alternatives → lower commitment |
Commitment = Satisfaction + Investment − Quality of alternatives
Why Investment Size Matters
The crucial innovation is investment size — resources that have been committed to the relationship and cannot be retrieved if it ends. Investments include: time spent together, shared experiences and memories, mutual friendships formed through the relationship, children and co-parenting responsibilities, shared financial arrangements, possessions, and emotional vulnerability. As investments accumulate over the life of a relationship, leaving becomes increasingly costly — even if satisfaction declines, the high cost of losing the investment maintains commitment.
This explains a phenomenon neither SET nor equity theory can fully account for: why people remain in relationships with moderate or even low satisfaction for long periods — not because no better alternative exists (some do) but because walking away means writing off substantial irreversible investments. The psychological cost of 'sunk costs' in relationships operates similarly to sunk cost effects in economic decision-making.
Research Support
Le and Agnew's (2003) meta-analysis of 52 studies involving over 11,000 participants found that all three components of the investment model significantly predicted commitment, which in turn predicted relationship stability (whether the couple stayed together). The investment model predicted staying together even when controlling for satisfaction alone — demonstrating that investment size and quality of alternatives make independent contributions beyond what SET accounts for. The model has also been validated cross-culturally and applied to non-romantic contexts (workplace commitment, sports team loyalty, friendship maintenance).
Explaining Difficult Relationship Decisions
The investment model has important applied value. It explains why people remain in abusive relationships: high investment (years of shared history, children, shared social network, financial entanglement) + artificially lowered quality of alternatives (through abuser's isolation, financial control, and psychological manipulation) = high commitment despite zero or negative satisfaction. This is not irrationality — it is the commitment equation operating as the model predicts, under conditions deliberately manipulated by the abuser to prevent leaving.
Key Takeaways
- Rusbult's investment model (1980, 1983): Commitment = Satisfaction + Investment − Quality of alternatives.
- Investment size: irretrievable resources committed to the relationship — time, shared history, children, mutual friends, financial entanglement. Growing investment increases commitment independently of satisfaction.
- Investment model explains why people stay in low-satisfaction relationships — high investment raises the cost of leaving even when satisfaction declines.
- Le and Agnew (2003) meta-analysis: all three components predicted commitment; commitment predicted relationship stability across 52 studies, 11,000+ participants.
- Application to abusive relationships: abusers manipulate investment (entrenchment) and quality of alternatives (isolation, financial control) to sustain commitment despite very low satisfaction.
- Extends SET by adding sunk-cost logic — the psychological cost of writing off irreversible investments is an independent source of commitment not captured by profit/alternatives alone.