Theory of planned behavior
The Theory of Planned Behavior (TPB) is a model in social psychology developed by Icek Ajzen to predict deliberate behavior through the proximal determinant of behavioral intention, which is a function of three core constructs: attitude toward the behavior (evaluation of the behavior's outcomes), subjective norm (perceived social pressure to perform or not perform the behavior), and perceived behavioral control (perceived ease or difficulty of performing the behavior).[1]90020-T) The model further posits that actual behavior is determined jointly by intention and perceived behavioral control, particularly when the latter reflects actual control.[2] Introduced in a 1985 book chapter and elaborated in subsequent publications, TPB extends the earlier Theory of Reasoned Action by incorporating perceived control to address behaviors where individuals lack complete volition.[3]90020-T) Empirical tests of TPB have demonstrated its utility across diverse domains, including health-related actions, environmental conservation, and consumer choices, with meta-analyses showing that the three predictors account for approximately 39% of variance in intentions and intentions explaining 27% of variance in behavior.[4] Despite strong predictive validity, the theory acknowledges limitations such as the intention-behavior gap, where intentions do not always translate to actions due to unforeseen barriers or habit influences, prompting refinements like integrating past behavior or moral norms in extended versions.[5] TPB's emphasis on modifiable beliefs underlying attitudes, norms, and control facilitates targeted interventions to change behavior, though causal claims rely on correlational evidence rather than experimental manipulation of intentions.[6] As of 2020, over 4,200 studies have applied or tested the framework, underscoring its robustness while highlighting needs for addressing contextual and automatic processes.[4]Historical Development
Origins in the Theory of Reasoned Action
The Theory of Reasoned Action (TRA) posits that an individual's intention to perform a specific behavior is the primary predictor of whether that behavior will occur, with intentions formed as a function of two key constructs: the attitude toward the behavior, reflecting the individual's evaluation of performing it, and the subjective norm, capturing perceived social pressure from relevant others to engage or not engage in the behavior.[7] This framework assumes that behaviors are largely under volitional control, such that strong intentions reliably lead to corresponding actions when external barriers are minimal.[5] Martin Fishbein and Icek Ajzen formalized TRA in their 1975 book Belief, Attitude, Intention and Behavior: An Introduction to Theory and Research, building on earlier work in attitude theory and social psychology to provide an empirically grounded model for predicting volitional behaviors across contexts such as health decisions and consumer choices.[7] Attitudes in TRA derive from behavioral beliefs—expectations about the likely outcomes of the action—weighted by evaluations of those outcomes, while subjective norms arise from normative beliefs about referents' expectations, multiplied by motivation to comply with them.[7] The model emphasizes specificity: predictions are most accurate when attitudes, norms, intentions, and behaviors target the same action, target, context, and time frame.[8] TRA's predictive validity was demonstrated in numerous studies, with meta-analyses showing that attitudes and subjective norms together accounting for approximately 40% of variance in intentions for volitional behaviors.[8] However, the theory's reliance on the assumption of complete volitional control limited its applicability to behaviors influenced by non-volitional factors, such as resource availability, skills, or environmental constraints, where intentions often failed to translate into actions.[5] This shortfall highlighted the need for an expanded model to incorporate perceptions of control, laying the groundwork for subsequent theoretical developments.[5]Introduction of Perceived Behavioral Control
The perceived behavioral control (PBC) construct was introduced by Icek Ajzen in 1985 to extend the Theory of Reasoned Action (TRA), which presupposed complete volitional control over behavior but failed to account for situations involving external constraints or limited personal resources.[9] Ajzen proposed PBC as a direct determinant of both behavioral intention and action, serving to regulate the intention-behavior relationship by reflecting individuals' perceptions of their ability to perform the behavior despite potential impediments.[10] This concept drew partial influence from Albert Bandura's (1977) self-efficacy, defined as confidence in one's capabilities to organize and execute actions required to manage prospective situations, yet Ajzen differentiated PBC by emphasizing its dual components: self-efficacy regarding internal capabilities and perceived controllability over external barriers such as time, opportunities, or dependencies on others.[10] Initial empirical validation occurred through studies in the mid-1980s, notably Schifter and Ajzen's (1985) investigation of weight loss among college students, where PBC significantly improved prediction of dieting intentions and actual weight reduction beyond TRA variables, explaining an additional 5-10% of variance in behavior.[11] Applications to exercise behaviors similarly revealed PBC's utility in forecasting adherence under conditions of incomplete control, such as scheduling conflicts or environmental factors.[12] The integration of PBC culminated in the formal delineation of the Theory of Planned Behavior in Ajzen's 1991 publication in Organizational Behavior and Human Decision Processes, which synthesized prior work and positioned TPB as a comprehensive model for behaviors with varying degrees of volitional influence.[13]Post-1991 Refinements and Theoretical Evolution
Following the establishment of the Theory of Planned Behavior (TPB) in its 1991 formulation, Icek Ajzen addressed conceptual ambiguities through subsequent publications, particularly clarifying the multifaceted nature of perceived behavioral control (PBC). In a 2002 analysis, Ajzen delineated PBC as a superordinate construct comprising two distinct components: self-efficacy, which pertains to an individual's confidence in their ability to perform the behavior despite internal challenges such as skills or effort, and controllability, which assesses the perceived influence of external factors like obstacles or resources.[10] This refinement responded to early empirical observations where PBC measures often conflated internal capabilities with external barriers, emphasizing that comprehensive assessment requires items tapping both dimensions to enhance predictive validity without altering the theory's core causal pathways.[14] Ajzen further reinforced these distinctions in responses to critiques during the 2010s, underscoring PBC's perceptual basis while noting its approximation of actual behavioral control under conditions of accurate self-appraisal. For instance, in addressing misconceptions about PBC equating solely to self-efficacy, Ajzen argued that the former's inclusion of external controllability provides a broader predictor of intention and behavior, particularly in volitional contexts where real impediments vary.[5] This evolution drew from accumulated empirical feedback, such as studies revealing that PBC's dual role moderates the intention-behavior relationship more effectively when external factors are salient, though PBC remains a subjective perception rather than objective control.[10] Theoretical refinements also incorporated considerations of intention stability, informed by longitudinal data showing that intentions predict behavior more reliably when they remain consistent over time, as fluctuations often stem from changing beliefs or unforeseen controls. Ajzen's later works highlighted that past behavior, while correlated with future actions, primarily reflects intention stability or habit-like automaticity rather than a direct causal antecedent, preserving TPB's focus on reasoned deliberation.[5] Habit integration was acknowledged peripherally as a potential moderator in scenarios of repeated behaviors, where automatic processes may weaken intention's proximal role, yet Ajzen maintained that TPB's structure accommodates such extensions without necessitating core revisions, as evidenced in defenses against claims of theoretical obsolescence.[15] These adjustments, grounded in meta-analytic evidence up to the mid-2010s, refined measurement and boundary conditions while upholding the model's parsimony.[5]Core Constructs and Mechanisms
Attitudes Toward the Behavior
Attitudes toward the behavior in the theory of planned behavior represent an individual's overall evaluation of performing a specific behavior, ranging from favorable to unfavorable.[9] This construct is derived from expectancy-value theory, positing that attitudes arise from beliefs about the likely outcomes of the behavior and the personal evaluation of those outcomes.[5] Formally, attitude (A) is operationalized as the weighted sum of behavioral beliefs (b_i), reflecting the subjective probability that the behavior will produce outcome i, multiplied by the evaluation of that outcome (e_i):This formulation emphasizes target-specific assessments rather than diffuse or general attitudes toward objects or abstract concepts, ensuring relevance to the precise behavioral intention under study.[9] The causal mechanism links these attitudes directly to behavioral intentions, where more positive evaluations strengthen the motivation to intend the behavior, independent of normative or control factors.[9] Empirical tests, rooted in the precursor theory of reasoned action and extended in TPB, demonstrate that attitudes reliably predict intention formation, particularly for behaviors under varying degrees of volitional control.[16] Meta-analytic reviews of TPB applications across health, environmental, and consumer behaviors confirm that attitudes toward the behavior explain 20-30% of the variance in intentions, with a weighted average correlation of approximately 0.50 after correcting for measurement error.[16] In contexts involving partial behavioral control, such as exercise adherence or dietary choices, this predictive power holds, though it diminishes slightly when actual control is low, underscoring attitudes' role in proximal motivation rather than overriding external barriers.[16] These findings derive from aggregated data in over 200 studies, highlighting consistent but moderated efficacy without implying universality across all behavioral domains.[16]