Theory of Planned Behavior (TPB)
Overview
The Theory of Planned Behavior (Ajzen, 1991) extends the Theory of Reasoned Action by adding perceived behavioral control as a predictor of both intention and behavior. Behavioral intention is determined by three factors: attitude toward the behavior, subjective norms, and perceived behavioral control. Intention is the proximal predictor of behavior, moderated by actual control.
When to Use
- Predicting whether a target population will adopt a new behavior (health, technology, policy)
- Diagnosing the intention-behavior gap — why people intend to act but do not follow through
- Designing persuasion or behavior change interventions by targeting the weakest predictor
- Evaluating campaign effectiveness on attitude, norm, and control dimensions
When NOT to Use
- For habitual or automatic behaviors where intention plays a minimal role
- When behavior is primarily driven by unconscious or emotional processes (use dual-process models)
- For behaviors under complete external control (no volitional component)
Assumptions
IRON LAW: Intention predicts behavior ONLY when perceived
behavioral control is high — without actual control, intention
alone is insufficient. The intention-behavior gap widens as
volitional control decreases.Key assumptions:
- People make reasoned (though not necessarily rational) decisions based on available information
- Behavioral beliefs, normative beliefs, and control beliefs are the informational foundations
- The relative weight of attitude, norms, and PBC varies across behaviors and populations
Methodology
Step 1 — Specify the Target Behavior
Define the behavior precisely using the TACT framework:
- Target: at what object or person
- Action: what specific action
- Context: in what situation
- Time: in what time frame
Step 2 — Measure the Three Predictors
| Predictor | Definition | Underlying Beliefs |
|---|---|---|
| Attitude | Favorable/unfavorable evaluation of performing the behavior | Behavioral beliefs (outcomes x evaluations) |
| Subjective norms | Perceived social pressure to perform or not perform | Normative beliefs (referents x motivation to comply) |
| Perceived behavioral control (PBC) | Perceived ease or difficulty of performing the behavior | Control beliefs (facilitators/barriers x power) |
Step 3 — Assess Intention and the Intention-Behavior Gap
- Measure behavioral intention as a function of the three predictors
- Identify which predictor is the weakest link (highest leverage for intervention)
- Evaluate actual behavioral control to assess gap risk
Step 4 — Design Targeted Intervention
- Attitude-focused: provide new outcome information, reframe consequences
- Norm-focused: make social norms visible, use social proof, engage opinion leaders
- PBC-focused: reduce barriers, build self-efficacy, provide skills training
- Implementation intentions: bridge the intention-behavior gap with "if-then" planning
Output Format
## TPB Analysis: [Target Behavior]
### Behavior Specification (TACT)
- Target: [object/person]
- Action: [specific behavior]
- Context: [situation]
- Time: [time frame]
### Predictor Assessment
| Predictor | Score | Key Beliefs | Intervention Potential |
|-----------|-------|-------------|----------------------|
| Attitude | [+/-] | [salient beliefs] | [High/Medium/Low] |
| Subjective norms | [+/-] | [key referents] | [High/Medium/Low] |
| PBC | [+/-] | [barriers/facilitators] | [High/Medium/Low] |
### Intention Strength: [Strong/Moderate/Weak]
### Intention-Behavior Gap Risk: [High/Medium/Low]
### Recommended Intervention
1. [Primary lever: weakest predictor]
2. [Implementation intention strategy]
3. [Barrier removal or facilitator enhancement]Gotchas
- TPB assumes rational information processing; it underestimates the role of emotions, habits, and unconscious drivers
- Subjective norms are consistently the weakest predictor in meta-analyses — but this may reflect measurement issues rather than true irrelevance
- Past behavior often explains more variance than TPB constructs, suggesting habit and automaticity are under-represented
- PBC is not the same as actual control — people systematically overestimate or underestimate their control
- The theory works best for deliberate, one-time decisions; for recurring behaviors, habit formation models should supplement TPB
- Cross-cultural application requires recalibrating normative beliefs — collectivist cultures weight subjective norms more heavily
References
- Ajzen, I. (1991). The theory of planned behavior. *Organizational Behavior and Human Decision Processes*, 50(2), 179-211.
- Armitage, C. J. & Conner, M. (2001). Efficacy of the theory of planned behaviour: a meta-analytic review. *British Journal of Social Psychology*, 40(4), 471-499.
- Fishbein, M. & Ajzen, I. (2010). *Predicting and changing behavior: the reasoned action approach*. Psychology Press.