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Why are some romantic relationships maintained for decades while others, seemingly similar, dissolve within months? The theories on this part of the specification share an economic or social-exchange logic: they conceptualise relationships as governed by an implicit weighing of what each partner puts in and gets out. This lesson examines three such theories in depth — social exchange theory (Thibaut & Kelley), equity theory (Walster et al.), and Rusbult's investment model — and links them to Duck's account of relationship breakdown, which the economic models help to explain. Throughout, the emphasis is on the distinction between a theory that explains why people are satisfied and a theory that explains why people stay committed, which are not the same thing.
Key Definition: Comparison Level (CL) is the standard against which a person judges the quality of their current relationship — essentially, what they believe they deserve and expect to receive. It is derived from past relationship experiences and from social and media norms. A relationship that exceeds the CL is judged satisfactory.
This lesson addresses the following points from the AQA A-Level Psychology (7182) specification, Paper 3, Section B: Relationships:
| Specification point | Coverage in this lesson |
|---|---|
| Social exchange theory | Thibaut & Kelley: rewards, costs, profit, CL and CL-alt; the four stages |
| Equity theory | Walster et al.: equity vs equality; over- and under-benefit; restoring equity |
| Rusbult's investment model of commitment | Satisfaction, comparison with alternatives, investment size; intrinsic/extrinsic investment |
| Satisfaction and commitment (links to maintenance and breakdown) | Comparison of the three theories; link to Duck's phase model |
These points are assessed through short-answer AO1 questions, AO2 application items (where a scenario stem is provided), and extended-writing essays up to 16 marks requiring balanced AO1 and AO3.
Social exchange theory (SET) is grounded in behaviourist assumptions: just as operant conditioning holds that behaviour is governed by its consequences, SET holds that people remain in relationships that are rewarding and leave those that are costly. The theory therefore treats the relationship as a kind of economic exchange in which each partner seeks to maximise rewards and minimise costs.
Rewards include companionship, emotional support, sex, financial security, and social status. Costs include time, effort, emotional stress, financial outlay, and the opportunity cost of foregone alternatives. The perceived value of any given reward or cost is subjective and varies between individuals and over time.
The outcome (or profit) of a relationship is conceptualised as:
Profit = Rewards − Costs
A positive outcome predicts satisfaction; a negative one predicts dissatisfaction. But — crucially — SET argues that people do not judge outcomes in absolute terms. They judge them against two comparison standards.
The comparison level is an internal benchmark of what a person feels they deserve from a relationship, shaped by:
If the current outcome exceeds the CL, the person feels satisfied; if it falls below the CL, dissatisfaction follows.
The CL-alt is the person's judgement of whether they could secure a better outcome elsewhere — in an alternative relationship or by being single.
The interaction of CL and CL-alt produces four possible situations. A relationship above the CL and above the CL-alt is satisfying and stable. A relationship below the CL but still above the CL-alt is unsatisfying yet stable ("the best of a bad set of options"). A relationship above the CL but below the CL-alt is satisfying but unstable, because something better seems available. A relationship below both standards is both unsatisfying and unstable, and is the most likely to end. This is why SET insists that satisfaction alone cannot predict whether a relationship persists — the perceived alternatives must also be taken into account.
Thibaut and Kelley proposed that exchange operates differently across four stages:
| Stage | Description |
|---|---|
| Sampling | The individual explores the potential rewards and costs of relationships, through direct experience or by observing others. |
| Bargaining | At the start of a relationship, partners exchange rewards and costs and "negotiate" what each will give and receive. |
| Commitment | Costs fall and rewards rise as exchanges become more predictable and mutually understood; the relationship stabilises. |
| Institutionalisation | Partners are firmly settled; the norms, expectations, and "exchange rules" of the relationship are firmly embedded. |
Equity theory was developed partly in response to a key weakness of SET: SET predicts that the most rewarding relationships are the most satisfying, yet research shows that people in relationships where they are over-rewarded are not maximally satisfied. Equity theory's central claim is that what matters is not profit but perceived fairness.
Key Definition: Equity is the condition in which each partner's ratio of perceived contributions (inputs/costs) to perceived benefits (outputs/rewards) is roughly equal. It concerns perceived fairness, not identical contributions.
A relationship is equitable when the ratio of one partner's benefits to contributions matches the ratio for the other. Formally, the equity principle can be expressed as:
Partner A’s contributionsPartner A’s benefits≈Partner B’s contributionsPartner B’s benefits
Equity does not require that both partners put in and get out the same amounts; a partner who contributes a great deal and receives a great deal in return can be in an equitable relationship with a partner who contributes and receives less. To illustrate: a partner who contributes "8 units" of effort and receives "8 units" of benefit (ratio 1:1) is in an equitable relationship with a partner who contributes "4 units" and receives "4 units" (also 1:1), even though their absolute contributions differ. Inequity arises only when the ratios diverge — for example, if one partner contributes "8" but receives only "4" (ratio 2:1, under-benefited) while the other contributes "4" and receives "8" (ratio 1:2, over-benefited). It is this perceived mismatch of ratios, not any difference in absolute amounts, that generates distress and the motivation to restore balance.
| Status | Experience | Typical emotional response |
|---|---|---|
| Equitable | Both partners perceive their ratios as fair | Satisfaction; relationship maintained |
| Over-benefited | A partner receives more relative to contribution | Guilt, discomfort, sometimes pity |
| Under-benefited | A partner contributes more relative to benefit | Anger, resentment, distress |
According to Walster et al., both over- and under-benefited partners experience distress, although the under-benefited partner typically experiences it most acutely. Partners are motivated to restore equity, by one of three routes:
A key prediction is that the greater the perceived inequity, the greater the dissatisfaction and the stronger the drive to restore balance — a prediction that lends itself to empirical testing.
Equity theory also makes a developmental claim: the importance of equity is theorised to increase as a relationship matures. In the earliest stage of a relationship, a partner may tolerate considerable imbalance (giving far more than they receive) because the relationship is new and exciting and they are still establishing it. As the relationship becomes established, however, partners increasingly monitor fairness, and persistent inequity becomes corrosive. This is one reason why equity theory is positioned as a theory of relationship maintenance rather than formation: the slow accumulation of perceived unfairness, rather than any single event, is what gradually erodes satisfaction and ultimately threatens the relationship's survival.
Caryl Rusbult argued that satisfaction and the quality of alternatives — the variables emphasised by SET — are not sufficient to predict whether a relationship persists. A third factor is decisive: investment size. This is why some people remain committed to relationships that are unsatisfying, and even abusive.
graph TD
A[Satisfaction level<br/>rewards minus costs vs CL] --> D[Commitment]
B[Quality of alternatives<br/>could needs be met elsewhere?] --> D
C[Investment size<br/>resources that would be lost] --> D
D --> E[Relationship maintenance<br/>+ accommodation, willingness to sacrifice]
Commitment, in turn, is theorised to produce relationship-maintenance behaviours such as accommodation (responding constructively rather than retaliating when a partner behaves badly), willingness to sacrifice, and positive illusions (idealising the partner and derogating tempting alternatives).
| Type | Description | Examples |
|---|---|---|
| Intrinsic investments | Resources put directly into the relationship | Time, emotional energy, self-disclosure, effort |
| Extrinsic investments | Resources that become tied to the relationship over time | Shared possessions, children, mutual friends, shared memories, financial entanglements |
The model's central insight is that high investment can sustain commitment even when satisfaction is low and alternatives are attractive, because ending the relationship would mean forfeiting everything that has been invested. This is the model's most distinctive and useful explanatory contribution.
Rusbult and Martz (1995) provided striking support in a study of "abused" women at a refuge. They found that the women who reported feeling most committed and most likely to return to an abusive partner were precisely those who had the greatest investment in the relationship and perceived the fewest desirable alternatives, even though their satisfaction was low. This directly corroborates the model's prediction that investment and alternatives — not satisfaction alone — drive commitment, and it illustrates why the model has become the dominant economic account of relationship maintenance. The investment model therefore reframes a difficult and sensitive social phenomenon — staying in a harmful relationship — not as irrationality but as the predictable output of the three commitment factors operating together.
| Feature | Social Exchange Theory | Equity Theory | Investment Model |
|---|---|---|---|
| Core variable | Profit (rewards − costs) vs CL and CL-alt | Perceived fairness of input/output ratios | Commitment via satisfaction, alternatives, and investment |
| Why people stay | Outcome exceeds CL and CL-alt | The relationship is perceived as equitable | High investment + low alternatives, even if satisfaction is modest |
| Why people leave | Costs exceed rewards, or a better alternative exists | Perceived inequity generates unresolved distress | Low investment, high-quality alternatives, low satisfaction |
| Signature limitation | Treats people as selfish "accountants" | Concept of equity is culturally specific | Largely correlational evidence |
Social exchange theory can explain why people remain in unsatisfying relationships, which is a genuine strength over a simple reward-maximisation account. By incorporating the CL-alt, SET predicts that a person will stay in a poor relationship if they perceive no superior alternative — for example, where they are financially dependent or socially isolated. There is supporting evidence: Simpson et al. (1990) found that people already in relationships rated opposite-sex others as less physically attractive than single participants did, consistent with the idea that committed individuals defensively lower their CL-alt to protect the relationship. The implication is that SET captures something real about the comparative, rather than absolute, nature of relationship judgements.
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