The interdependence principle is the idea that the outcomes any one person experiences are shaped not just by their own choices, but by the choices and actions of others they’re connected to. It shows up across psychology, economics, and biology, but its most developed form comes from social psychology, where John Thibaut and Harold Kelley built it into a full framework for understanding how relationships work. At its core, the principle says that whenever two or more people (or groups, or organisms) rely on each other for outcomes they care about, their behavior, satisfaction, and power all become linked.
The Principle in Social Psychology
Interdependence theory, as Thibaut and Kelley originally developed it in the late 1950s, seeks to capture the essence of social life by providing a framework for understanding social interaction. The theory starts from a simple premise: in any relationship, each person’s outcomes (happiness, resources, security) depend partly on what the other person does. That mutual influence is the interdependence.
From this foundation, the theory addresses some of the biggest themes in social psychology: dependence and power, rules and norms, coordination and cooperation. It treats every interaction as a kind of situation where two people’s interests can align, conflict, or fall somewhere in between. How much your outcomes depend on someone else, and how much theirs depend on you, determines the structure of the relationship.
How People Evaluate Their Relationships
Thibaut and Kelley introduced two benchmarks people use, often unconsciously, to judge how a relationship is going. The first is the comparison level: a personal standard based on what you’ve come to expect from relationships. If your current relationship delivers outcomes above that standard, you feel satisfied. If it falls below, you don’t, even if the relationship looks fine from the outside.
The second benchmark is the comparison level for alternatives. This isn’t about satisfaction. It’s about dependence. You compare what you’re getting now to what you believe you could get from your best available alternative, whether that’s a different relationship, being alone, or some other arrangement. When your current relationship exceeds what alternatives could offer, you’re more dependent on it and more likely to stay. When alternatives look better, your dependence drops. These two benchmarks are related but not identical: you can be dissatisfied with a relationship yet still stay because your alternatives look worse.
The Investment Model
Psychologist Caryl Rusbult expanded on Thibaut and Kelley’s framework by adding a third factor: investment size. In her investment model, commitment to a relationship is influenced by three independent forces. Satisfaction level captures how rewarding the relationship feels. Quality of alternatives captures what else is available. And investment size captures everything you’ve put into the relationship that you’d lose if it ended: time, shared memories, mutual friends, financial entanglements, emotional energy.
This third factor explains why people sometimes stay in relationships that are neither satisfying nor lacking in alternatives. The sheer weight of what they’ve invested creates its own form of dependence. Rusbult’s model treats all three factors as working together to produce commitment, which in turn predicts whether someone stays or leaves.
How Partners Shape Each Other’s Motivation
One of the more nuanced parts of interdependence involves how partners influence each other’s internal motivations, not just their external outcomes. Research on motivational interdependence in couples identifies two phases where this happens.
In the first phase, called motive expression, one partner’s behavior can activate the other person’s own desires and goals. For example, watching your partner pursue a creative project might awaken your own creative ambitions. In the second phase, motive satisfaction, a partner’s behavior either creates opportunities or obstacles for fulfilling those goals. Your partner’s enthusiasm for traveling might open up chances for you to satisfy your own curiosity about new places, or their resistance might block it.
Crucially, this process runs through subjective perception. It’s not what your partner objectively does that matters most, but how you perceive what they’re doing. Two people can observe the same behavior from a partner and have entirely different motivational responses. This means that in interdependent relationships, people are constantly interpreting each other’s actions and adjusting their own motivations in response, often without realizing it.
The Principle in Economics and Game Theory
The interdependence principle also sits at the heart of game theory in economics. In any strategic situation, whether it’s two companies setting prices, countries negotiating trade deals, or roommates deciding who does the dishes, the outcome for each participant depends on the choices of all participants. This shared feature of interdependence is what defines a “game” in the formal sense.
The challenge interdependence creates is circular reasoning: your best move depends on what the other person does, but their best move depends on what you do. The mathematician John Nash addressed this with his concept of equilibrium, where each player’s strategy is the best response to the strategies of everyone else. At a Nash equilibrium, no one has a reason to change their behavior given what everyone else is doing. It doesn’t mean everyone is happy with the outcome. It means the system has reached a stable point where all the interdependent choices lock into place.
Nash equilibrium remains an incomplete solution to the problem, particularly in situations where multiple equilibria exist or where players act simultaneously without knowing each other’s choices. But it gave economists a practical way to analyze situations defined by mutual dependence.
Interdependence in Biological Systems
In biology, the interdependence principle describes how the parts of a complex system rely on each other to function. Ensuring synchronization between all the processes in a living system requires functional interdependence, where multiple mechanisms cooperate to achieve a specific outcome. These mutual dependencies take two main forms: cooperation, where parts actively support each other, and coordination, where parts align their timing and activity.
At the cellular level, this plays out through rapid effects like the activation and inhibition of enzymes, and slower effects related to gene expression. Cells depend on the organism for signals and resources, while the organism depends on cells performing their specialized roles. Neither level can function without the other. This mirrors the social psychology version of the principle: outcomes at every level are contingent on what happens at the other levels.
Why the Principle Matters Across Fields
What makes the interdependence principle so widely applicable is that it captures a structural reality. Whenever outcomes are shared or linked, the behavior of any one party ripples outward. In a romantic relationship, that ripple affects satisfaction, commitment, and power dynamics. In an economy, it shapes pricing, trade, and strategic competition. In a living organism, it determines whether cells cooperate smoothly or fall out of sync.
The practical takeaway is that interdependence isn’t just a description of connectedness. It’s a tool for understanding why people stay in relationships, why negotiations stall, why ecosystems collapse when one species disappears, and why your choices rarely affect only you. Recognizing the structure of interdependence in any situation helps clarify who holds power, what each party stands to lose, and where cooperation or conflict is likely to emerge.