What Is the Theory of Planned Behavior?
The Theory of Planned Behavior (TPB) is a psychological model stating that behavioral intentions are the strongest predictor of actual behavior, and that intentions are determined by three factors: attitudes toward the behavior, subjective norms, and perceived behavioral control. Developed by Icek Ajzen at the University of Massachusetts Amherst, the TPB was published in 1991 in Organizational Behavior and Human Decision Processes as an extension of the earlier Theory of Reasoned Action (TRA), which Ajzen developed with Martin Fishbein in the late 1960s and 1970s.
The TPB is one of the most heavily cited theories in all of social science. Ajzen’s 1991 paper has accumulated over 40,000 Google Scholar citations. The model accounts for approximately 13% of all theory-based health behavior research (Painter et al., 2008), second only to the Transtheoretical Model (33%).
The TPB excels at predicting behavioral intentions. It is substantially weaker at predicting actual behavior. This distinction, the intention-behavior gap, is the central limitation of the model and one of the most important findings in behavioral science.
The Three Predictors of Intention
Attitudes Toward the Behavior
A person’s overall evaluation of performing the behavior. Attitudes are determined by behavioral beliefs: the person’s beliefs about the likely outcomes of the behavior, weighted by evaluations of those outcomes.
If a person believes that exercising will improve their energy, help them lose weight, and reduce disease risk, and if they value those outcomes, their attitude toward exercising will be positive. If they also believe exercising will cause joint pain and consume time, those negative beliefs diminish the attitude.
Attitudes are measured using semantic differential scales (good/bad, pleasant/unpleasant, beneficial/harmful). The TPB treats attitudes as a weighted composite, not a simple thumbs-up or thumbs-down.
Subjective Norms
The perceived social pressure to perform or not perform the behavior. Subjective norms are determined by normative beliefs: beliefs about whether important others approve of the behavior, weighted by the person’s motivation to comply with those others.
If a person believes their spouse, doctor, and close friends all think they should exercise, and if they care about those people’s opinions, subjective norms favor exercise. If their work culture glorifies overwork and treats exercise as frivolous, subjective norms work against it.
Subjective norms consistently emerge as the weakest of the three TPB predictors. Armitage and Conner (2001) found that subjective norms accounted for significantly less variance in intentions than attitudes or perceived behavioral control. This weakness has generated ongoing debate about whether the TPB adequately captures social influence. Critics argue that the model measures perceived social pressure (injunctive norms) but not perceived social behavior (descriptive norms: what people actually do), and that descriptive norms often matter more.
Perceived Behavioral Control (PBC)
The person’s perception of how easy or difficult it is to perform the behavior. PBC is determined by control beliefs: beliefs about factors that facilitate or impede the behavior, weighted by the perceived power of those factors.
PBC was the critical addition that transformed the TRA into the TPB. Ajzen added it because many behaviors are not under full volitional control. A person may intend to quit smoking but perceive the behavior as extremely difficult. A person may intend to save money but perceive their financial situation as leaving no room for savings.
PBC has two components: self-efficacy (confidence in one’s ability) and perceived controllability (belief about external factors). These map closely onto Albert Bandura’s self-efficacy construct and onto COM-B’s distinction between capability and opportunity.
PBC is the strongest single predictor of behavior in many TPB studies, stronger than intention itself. This is the TPB’s most important empirical contribution: the finding that perceived control over a behavior predicts actual behavior above and beyond intention.
How Well Does the TPB Predict?
Predicting Intentions
The TPB explains a substantial proportion of variance in behavioral intentions.
Armitage and Conner (2001) published the most comprehensive meta-analysis of the TPB in the British Journal of Social Psychology, covering 185 studies. The TPB accounted for 39% of the variance in behavioral intentions on average (multiple R = .63). Attitudes and perceived behavioral control were the strongest predictors. Subjective norms were consistently the weakest.
McEachan, Conner, Taylor, and Lawton (2011) published a larger meta-analysis in Health Psychology Review covering 237 prospective studies of health behavior. The TPB accounted for 44.3% of variance in intentions.
Predicting Behavior
The TPB is substantially less successful at predicting actual behavior than at predicting intentions.
Armitage and Conner (2001) found the TPB accounted for 27% of variance in self-reported behavior and 20% in objectively measured behavior. The gap between intention prediction (39%) and behavior prediction (20-27%) is the intention-behavior gap in statistical form.
McEachan and colleagues (2011) found the TPB accounted for 19.3% of variance in behavior. They also found significant variation by behavior type: the TPB predicted physical activity and dietary behavior moderately well but predicted risk behaviors (drug use, risky sexual behavior), screening/detection, and abstinence behaviors poorly.
Sheeran (2002) quantified the intention-behavior gap directly: across studies, 47% of people who form positive intentions fail to act on them. Webb and Sheeran (2006) later showed that a medium-to-large change in intention (d = 0.66) produces only a small-to-medium change in behavior (d = 0.36). This is the TPB’s fundamental limitation. Intention is necessary but not sufficient.
What Explains the Gap?
Webb and Sheeran (2006) published a meta-analysis of 47 experimental studies that successfully changed intentions. They found that interventions producing medium-to-large changes in intention produced only small-to-medium changes in behavior. The intention-behavior gap is not an artifact of measurement. It is a real phenomenon.
Several factors explain why intentions fail to translate into behavior:
Forgetting. People form intentions and then forget them at the moment of action. Implementation intentions (specific if-then plans) address this directly.
Competing goals. The intended behavior competes with other goals and priorities at the moment of action.
Habits. Past behavior often predicts future behavior better than intentions. Automatic behaviors bypass the deliberative process the TPB models.
Environmental barriers. The TPB captures perceived control but not actual environmental constraints. A person may feel confident they can exercise but face genuine structural barriers (no sidewalks, no childcare, a 12-hour workday).
Temporal discounting. The costs of behavior change are immediate (effort, discomfort), while the benefits are delayed (health, fitness). People systematically overweight the immediate costs.
Real-World Applications
Physical Activity
The TPB has been applied to exercise research more than any other behavioral domain. Hagger, Chatzisarantis, and Biddle (2002, Journal of Sport and Exercise Psychology) published a meta-analysis of 72 studies applying the TPB to physical activity. Perceived behavioral control was the strongest predictor of exercise intentions and behavior. Attitudes were second. Subjective norms were weak.
Condom Use
Albarracin, Johnson, Fishbein, and Muellerleile (2001, Psychological Bulletin) published a meta-analysis of 96 studies applying the TRA/TPB to condom use. Attitudes predicted condom use intentions, and past behavior was the strongest predictor of future condom use, stronger than intention itself. This finding reinforced the importance of habit and past behavior as factors the TPB underweights.
Environmental Behavior
Bamberg and Moser (2007, Journal of Environmental Psychology) published a meta-analysis applying the TPB to pro-environmental behavior (recycling, energy conservation, sustainable transport). The model worked moderately well: attitudes and perceived behavioral control predicted intentions, and intentions predicted behavior. Moral norms (a sense of personal obligation) added predictive value beyond the standard TPB variables.
Technology Adoption
The Technology Acceptance Model (TAM), developed by Fred Davis in 1989, is essentially a domain-specific adaptation of the TPB. TAM’s “perceived usefulness” maps to attitudes, and “perceived ease of use” maps to perceived behavioral control. TAM became the dominant model in information systems research, accumulating tens of thousands of citations.
Limitations and Criticisms
The intention-behavior gap. The TPB’s central practical limitation. Knowing someone’s intention only partially predicts their behavior. The model identifies the problem (intention matters) without solving it (how to convert intentions into action). Implementation intentions, developed by Peter Gollwitzer as a separate research program, specifically addresses this gap.
Static snapshot. The TPB measures beliefs, attitudes, norms, and control at a single point in time. Behavior unfolds over time in dynamic contexts. The model does not address how these factors change, interact with environmental shifts, or are influenced by past behavior.
Rationalistic assumptions. The TPB assumes that behavior results from a deliberative process: evaluate outcomes, assess norms, judge control, form intention, act. Dual-process models suggest that much behavior is automatic, habitual, and driven by environmental cues rather than conscious intention. The TPB’s reflective bias makes it better at predicting deliberate, one-time decisions than habitual, repeated behaviors.
Weak social influence. Subjective norms consistently underperform as a predictor. Rivis and Sheeran (2003, British Journal of Social Psychology) showed that adding descriptive norms (what people actually do) to the TPB significantly improved prediction. The model’s original formulation captures only injunctive norms (what people approve of), missing the powerful influence of observed behavior.
Poor at guiding interventions. The TPB identifies predictors (attitudes, norms, control) but does not specify how to change them. The model is diagnostic, not prescriptive. COM-B and the Behaviour Change Wheel were designed to fill this gap by mapping behavioral barriers to specific intervention strategies.
Past behavior. When past behavior is added to the TPB, it typically becomes the strongest predictor of future behavior, often reducing the predictive contribution of intentions. This suggests that habitual behavior operates through pathways the TPB does not model.
TPB vs. Other Models
| Feature | TPB | COM-B | Social Cognitive Theory | Health Belief Model |
|---|---|---|---|---|
| Core mechanism | Attitudes + Norms + Control = Intention | C + O + M = B | Self-efficacy drives behavior | Perceived threat drives behavior |
| Primary strength | Predicting intentions (R = .63) | Diagnosing barriers | Predicting confidence | Historical importance |
| Primary weakness | Intention-behavior gap | Does not predict without intervention data | Limited environmental scope | Small effect sizes across all constructs |
| Addresses habits | No (but past behavior is powerful) | Yes (automatic motivation) | Partially | No |
| Intervention guidance | Low (identifies predictors, not strategies) | High (BCW maps to interventions) | Moderate (mastery experiences, modeling) | Low |
| Best for | Predicting who will intend to act | Designing interventions | Building self-efficacy | Screening/vaccination contexts |
Frequently Asked Questions
What is the Theory of Planned Behavior? The Theory of Planned Behavior (TPB) is a psychological model stating that behavior is predicted by behavioral intentions, which in turn are determined by three factors: attitudes toward the behavior (positive or negative evaluations), subjective norms (perceived social pressure), and perceived behavioral control (perceived ease or difficulty). Developed by Icek Ajzen and published in 1991, it is one of the most cited theories in social science with over 40,000 citations.
What is the difference between the Theory of Reasoned Action and the Theory of Planned Behavior? The Theory of Reasoned Action (TRA) was the predecessor, developed by Ajzen and Martin Fishbein. The TRA included only attitudes and subjective norms as predictors of intention. The TPB added perceived behavioral control as a third predictor. This addition was critical because many behaviors are not under full volitional control. Perceived behavioral control became the strongest predictor in many studies, making it the most important theoretical contribution of the TPB.
What is the intention-behavior gap? The intention-behavior gap is the finding that forming a positive intention to act does not guarantee action. Sheeran (2002) found that 47% of people who intend to perform a behavior fail to do so. Webb and Sheeran (2006) showed that a medium-to-large change in intention produces only a small-to-medium change in behavior. This gap is the central limitation of the TPB and has driven research into implementation intentions (if-then plans), habits, and environmental design as ways to bridge it.
Does the Theory of Planned Behavior predict actual behavior? The TPB predicts intentions well (explaining 39-44% of variance) and behavior moderately (explaining 19-27% of variance). It performs best for deliberate, one-time decisions and worst for habitual, repeated behaviors. Perceived behavioral control often predicts behavior better than intention itself, suggesting that perceived ability matters more than motivation for many health behaviors.
How does the TPB compare to COM-B? The TPB is primarily a prediction model: it identifies what predicts intentions and behavior. COM-B is primarily a diagnostic and intervention design model: it identifies why behavior is not occurring and maps those barriers to intervention strategies. The TPB’s “perceived behavioral control” overlaps with COM-B’s Capability and Opportunity. The TPB’s “attitudes” map partly to COM-B’s Reflective Motivation. COM-B’s distinction between reflective and automatic motivation addresses habitual behavior that the TPB does not capture.
Sources and Further Reading
- 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.
- McEachan, R. R. C., Conner, M., Taylor, N. J., & Lawton, R. J. (2011). Prospective prediction of health-related behaviours with the theory of planned behaviour: A meta-analysis. Health Psychology Review, 5(2), 97-144.
- Sheeran, P. (2002). Intention-behavior relations: A conceptual and empirical review. European Review of Social Psychology, 12(1), 1-36.
- Webb, T. L., & Sheeran, P. (2006). Does changing behavioral intentions engender behavior change? A meta-analysis. Psychological Bulletin, 132(2), 249-268.
