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Planned Behaviour Theory and Schools (EE)
This initial or rough draft of a summary (formatted in an Encyclopedia Entry format) has been started by the International School Health Network from a previous Canadian project. Visitors to and members of this wiki-based web site are welcome to edit the draft (using the Easy Edit" tool found at the top of the page or to comment on the draft by using the "thread" tool found at the bottom of the page. (All previous versions of this page are automatically saved by the system, so don't hesitate to edit this page). Eventually, when time and resources permit, this initial draft will be formatted in accordance with the outline for glossary terms, encyclopedia entries and handbook sections used in this knowledge exchange program for health,safety, educational and social development interventions.
This summary discusses one behavioural theory that underpins school health promotion, safety, social development and other similar programs. The summary is one of several that are being developed within this wiki-based web site. Please see our Call for Writers/Contributors/Sponsors on Behaviour Theories for a full listing of this series of summaries.
The Theory of Planned Behaviour (TPB) (Ajzen, 1991) is a theory of the factors that determine an individual‘s decision to adopt or maintain a particular behaviour. An extension of Ajzen and Fishbein‘s (1980) Theory of Reasoned Action, the TPB postulates that the most immediate determinant of a person‘s behaviour is behavioural intention—that is, what the person intends or plans to do.
According to the theory, intention is a function of three factors. The first of these factors is one‘s attitudes toward the behaviours, which reflects a personal evaluation of the action. The attitudinal component of the theory is a function of the person‘s salient beliefs about the likely outcomes of the behaviour. The second factor that predicts intention is the perceived social pressure (subjective norms) to perform the particular behaviour. Subjective norms are assumed to be determined by normative beliefs about what specific salient groups or individuals believe about the behaviour. While the TPB typically taps normative influences in terms of injunctive norms (i.e., pressure from others to engage in the specific behaviour), descriptive norms (i.e., perceived engagement in the given behaviour by others) have also been found to influence intentions (Conner, Martin, Silverdale & Grogan, 1996). The final motivational determinant that impacts behavioural intention is the perceived amount of control the individual has over the behaviour. Said another way, perceived behavioural control (PBC) refers to the relative ease or difficulty in performing the particular behaviour and is based on past experiences and the anticipation of obstacles that may prevent the behaviour from occurring. The model without PBC is the Theory of Reasoned Action. Perceived behavioural control was added to that theory to take account of non-volitional (without deliberate intention) behaviours.
In general, the more positive these three components (attitudes, subjective norms and perceived behavioural control) are, the stronger the intention is to perform the given behaviour (Ajzen, 1991). Thus, the behaviour is more likely to appear if it has a strong behavioural intention. It is important to note that Ajzen (1991; Ajzen & Fishbein, 1980) has indicated that the relative weight of the three components in the prediction of intention varies according to the behaviour that is being studied.
The Theory of Planned Behaviour is essentially an additive model—that is, it is assumed that the variables in the model are additive (or linear) in their effects on intentions and behaviour. Although linear models are generally preferred on grounds of parsimony, restrictions on the range of predictor variables can make this assumption problematic. The inclusion of interaction effects in a model removes this linearity assumption; several interaction effects have been proposed in the literature. In formulating his theory, Ajzen (1985) originally predicted an interaction between intentions and PBC such that intentions should only predict behaviour when a person correctly perceives high levels of control over the behaviour (high PBC).
The literature is mixed, however, as to whether such an interaction exists (Ajzen, 1991; Terry & O‘Leary, 1995). Another proposed interaction effect is the Contingency-Consistency Hypothesis, which suggests that attitudes are likely to be more predictive of the decision to act (intentions) when the social environment is supportive of the proposed action (Acock & DeFleur, 1972). The presence of such an interaction in the model would suggest that social influences operate to determine that impact of attitudes on behaviour (or intentions) rather than directly influencing behaviour.
While several studies have found support for this interaction effect, others have not (for a review, see Conner & McMillan, 1999). It has also been postulated that an interaction may exist between attitudes and PBC on intentions (Eagly & Chaiken, 1993). While one might expect increasing levels of PBC to be associated with increasing intentions to act for positively evaluated behaviour, this is not expected to be the case for negatively-evaluated behaviours. In the latter case, one might expect increasing PBC to be negatively related to intentions (i.e., greater control over a negatively-evaluated behaviour is associated with weaker intentions). In the case of drug use, one might expect PBC to be negatively related to intentions for those who evaluate drug use negatively; while for those who evaluate drug use positively, PBC might be expected to be positively related to intentions.
The TPB (as well as its predecessor, the Theory of Reasoned Action) has been successfully applied to the prediction of a number of health behaviours, including physical exercise (Godin, Valois & Lepage, 1993), dieting (Conner et al., 1996), the use of condoms (Boldero, Moore & Rosenthal, 1992), drinking and driving (Beck, 1981; Marcil et al., 2001; Parker et al., 1992), cigarette smoking (McMillan & Conner, 2003b; McMillan, Higgins & Conner, 2005), and the use of marijuana and other illicit drugs (Conner & McMillan, 1999, 2003a).
Applications of the Theory
Several studies have successfully applied the TPB to understand the link between intentions to use and self-reported use of licit and illicit substances, primarily among young adults. Conner and McMillan (1999) employed the theory to investigate the factors underlying intentions and frequency of cannabis use over a three-month period in a sample of 249 undergraduate students from England aged 17–59 (but primarily in the 19–22 age range). This study also assessed whether youths‘ perceptions of others‘ behaviour (descriptive norms) would contribute to the prediction of intentions to use cannabis frequently, independent of perceived pressure from others to engage in frequent cannabis use (injunctive norms).
Findings revealed that attitudes, injunctive norms (pressure from others [best friend, other friends, family, health experts, partner, etc.] to engage in frequent cannabis use), descriptive norms (perceived pressure from best friend and partner to use cannabis frequently) and PBC were each significantly associated with intentions, with PBC having the greatest influence on intentions. More positive attitudes and perceived injunctive pressure to use cannabis were associated with intending to use cannabis more frequently, while higher levels of PBC were associated with intending to use cannabis less frequently. Thus, those youth who perceived more control over cannabis use intended to use it less frequently. Attitudes were found to explain approximately 10% unique variance in intentions, PBC accounted for about 14%, injunctive norms less than 1%, and descriptive norms about 2%. Youths‘ intentions to use cannabis more frequently were significantly positively related to their cannabis-using behaviour three months later (R2 = 0.71).
This work also found evidence of significant interaction effects. Specifically, a significant interaction was noted between intention and PBC such that increasing levels of PBC were associated with a weakening of the intention-behaviour relationship. This study also found support of an interaction between attitudes and PBC on intentions; PBC was negatively related to intentions only for those youth who evaluated cannabis use negatively or neutrally, whereas there was no relationship between PBC and intentions for those with strong positive evaluations/attitudes. As this is the first empirical demonstration of this interaction effect, this finding warrants replication. An important limitation of this study is that the cannabis use behaviour measure was assessed via self-report.
McMillan and Conner (2003a) also investigated whether the TPB could be used to explain the relationship between intentions and use of LSD, amphetamine, cannabis and ecstasy over six months in a sample of 461 undergraduate students from England aged 17–54 (but mainly in the 19–22 age range). Similar to their aforementioned work, this study assessed the predictive utility of students‘ attitudes, PBC, and injunctive and descriptive norms in explaining their intentions to use illicit drugs. The results from this work showed that attitudes, injunctive norms, descriptive norms and PBC were each significantly associated with intentions to use LSD, amphetamine, cannabis and ecstasy. Attitudes or PBC had the greatest influence on intentions compared to injunctive norms for all of the drugs. In each case, greater intention to use a drug was associated with holding a more positive attitude, perceiving greater pressure to use the drug, and perceiving factors to facilitate, rather than inhibit, drug use. Youths‘ intentions were significantly positively related to behaviour six months later for all of drugs (LSD R2 = 0.26; amphetamine R2 = 0.46; cannabis R2 = 0.70: ecstasy R2 = 0.41).
Consistent with their previous study focusing on youth cannabis use (Connor & McMillan, 1999), McMillan and Connor demonstrated two significant interaction effects. One interaction was observed between intention and PBC across all four drugs (although it was only marginally significant for amphetamine and ecstasy); as the level of PBC increased, the predictive power of intentions also increased. This finding is in contrast to their previous study (Connor & McMillan, 1999), which reported a negative PBC x intention interaction for cannabis use. In reconciling these differential findings, the authors noted differences in the wording of their PBC measures between the two studies. The other significant interaction effect that was noted consistently across all types of drugs was between attitudes and PBC. Specifically, PBC was more strongly associated with intention to use drugs when attitudes were more positive. The findings from this work are based on self-reported data, and need to be interpreted in the context of this limitation.
Umeh and Patel (2004) examined the components of the TPB in relation to ecstasy use in a cross-sectional study of 200 young adults (mean age 21 years) from England. The results indicated that students‘ attitudes and subjective norms were significantly associated with their intentions to use ecstasy in the next two months. Stronger intent to use ecstasy was significantly associated with greater previous use of ecstasy. A significant interaction effect was documented between PBC and attitude; that is, perceived controllability of ecstasy use predicted stronger intentions to use the drug in the future only to the extent that respondents endorsed ecstasy use. Those youth who felt confident that they could procure and use the drug may nevertheless have failed to do so if they disapproved of such behaviour. The pattern of this interaction seems to contradict the moderator effect that was reported by Conner and McMillan (1999), who noted that PBC better predicts intentions to use cannabis given more negative evaluations of cannabis use. Conner and McMillan argued that the failure of PBC to predict intentions in persons who endorse cannabis use is attributable to studying a non-socially desirable behaviour.
McMillan and colleagues (2005) employed an extended version of the TPB to investigate the factors underlying cigarette smoking intentions and subsequent smoking behaviour three months later in a sample of 741 schoolchildren, aged 12–13, from northern England. Findings revealed that attitudes, subjective norms and PBC were significant predictors of intentions to smoke (R2 = .52). Attitudes and PBC each explained approximately 5% unique variance in intentions, whereas subjective norms only accounted for around 1%. Youths‘ intentions and PBC were found to be significant predictors of future cigarette smoking, correctly classifying 85% of study participants into smokers and non-smokers. Several other additional variables were also found to significantly increase the predictive utility of the model, including perceived friends‘ smoking and perceived family members‘ smoking (descriptive norms).
The TPB has also been used to understand alcohol and tobacco use among young adults. In their study of 494 undergraduate students in England aged 17–54 years (but mainly in the 19–22 age range), McMillan and Conner (2003b) investigated the links between attitudes, PBC, injunctive and descriptive norms and intentions to use alcohol and tobacco six months later. Attitudes, PBC, and injunctive and descriptive norms were found to be significant predictors of alcohol intentions, with PBC being the only significant predictor of intentions to use tobacco. For both drugs, PBC had the greatest influence on intentions. Students‘ intentions to use alcohol and tobacco were associated with their subsequent self-reported quantity of substance use, with stronger intentions and greater PBC being associated with higher levels of self-reported alcohol and tobacco use.
More recently, Judson and Langdon (2009) tested the TPB to explain the use of illicit prescription stimulant use among a sample of 333 New England undergraduate students (mean age 20 years). Of all participants, 6.3% (n = 21) were stimulant prescription holders and 93.6% (n = 312) were not. Students were provided with a self-report questionnaire that contained questions pertaining to attitudes (i.e., beliefs about adverse health effects, ethics of use), subjective norms (i.e., extent to which it is socially acceptable to use) and perceived control (i.e., perceived need to use stimulants) toward the illicit use of stimulant medications. Results showed that about 20% of the sampled youth reported illicit use of prescription stimulants. Independent of prescription status, illicit users thought that illicit use was significantly more ethical than non-users. Of non-prescription holders, illicit users were significantly less concerned with health risks associated with illicit use and felt more dependent on stimulant medications compared to non-users. Both holders and non-holders of prescriptions who were illicit users reported higher social norms for illicit use compared with non-users.
The predictors of illicit prescription stimulant use were tested in a model and the results were consistent with the TPB. Non-holders of a prescription who used illicitly had specific attitudes, normative beliefs and perceived control that differed from non-illicit users. Lower scores in concerns about adverse health effects and ethics of use, higher perceived positive subjective norms and lower perceived control were related to illicit use among non-prescription holders. Although intention to use was not directly measured in this study, the authors suggest that the link between these attitudes and beliefs were directly related to illicit use. The limitations of this study included the reliance on self-reported measures. There were also a small number of prescription holders, which limited statistical power of tests and reduced comparisons that could be made.
Limitations
In interpreting the evidence supporting the TPB in explaining youth substance use, several important limitations must be considered. Even though the theory accounts for considerable proportions of variance in youth drug use behaviour, some variance is still left unexplained, suggesting that other factors may be influencing youths‘ intentions to use drugs; additional research is required to investigate further constructs that can account for additional variance in drug use behaviour. Another important limitation is the reliance on self-reported measures in the studies substantiating this theory. Finally, with the exception of one study (McMillan et al., 2005), all of the reviewed studies were conducted using samples of undergraduate students. As such, it is unclear whether these findings would generalize to younger age groups.
This summary discusses one behavioural theory that underpins school health promotion, safety, social development and other similar programs. The summary is one of several that are being developed within this wiki-based web site. Please see our Call for Writers/Contributors/Sponsors on Behaviour Theories for a full listing of this series of summaries.
Theory of Planned Behaviour
The Theory of Planned Behaviour (TPB) (Ajzen, 1991) is a theory of the factors that determine an individual‘s decision to adopt or maintain a particular behaviour. An extension of Ajzen and Fishbein‘s (1980) Theory of Reasoned Action, the TPB postulates that the most immediate determinant of a person‘s behaviour is behavioural intention—that is, what the person intends or plans to do.
According to the theory, intention is a function of three factors. The first of these factors is one‘s attitudes toward the behaviours, which reflects a personal evaluation of the action. The attitudinal component of the theory is a function of the person‘s salient beliefs about the likely outcomes of the behaviour. The second factor that predicts intention is the perceived social pressure (subjective norms) to perform the particular behaviour. Subjective norms are assumed to be determined by normative beliefs about what specific salient groups or individuals believe about the behaviour. While the TPB typically taps normative influences in terms of injunctive norms (i.e., pressure from others to engage in the specific behaviour), descriptive norms (i.e., perceived engagement in the given behaviour by others) have also been found to influence intentions (Conner, Martin, Silverdale & Grogan, 1996). The final motivational determinant that impacts behavioural intention is the perceived amount of control the individual has over the behaviour. Said another way, perceived behavioural control (PBC) refers to the relative ease or difficulty in performing the particular behaviour and is based on past experiences and the anticipation of obstacles that may prevent the behaviour from occurring. The model without PBC is the Theory of Reasoned Action. Perceived behavioural control was added to that theory to take account of non-volitional (without deliberate intention) behaviours.
In general, the more positive these three components (attitudes, subjective norms and perceived behavioural control) are, the stronger the intention is to perform the given behaviour (Ajzen, 1991). Thus, the behaviour is more likely to appear if it has a strong behavioural intention. It is important to note that Ajzen (1991; Ajzen & Fishbein, 1980) has indicated that the relative weight of the three components in the prediction of intention varies according to the behaviour that is being studied.
The Theory of Planned Behaviour is essentially an additive model—that is, it is assumed that the variables in the model are additive (or linear) in their effects on intentions and behaviour. Although linear models are generally preferred on grounds of parsimony, restrictions on the range of predictor variables can make this assumption problematic. The inclusion of interaction effects in a model removes this linearity assumption; several interaction effects have been proposed in the literature. In formulating his theory, Ajzen (1985) originally predicted an interaction between intentions and PBC such that intentions should only predict behaviour when a person correctly perceives high levels of control over the behaviour (high PBC).
The literature is mixed, however, as to whether such an interaction exists (Ajzen, 1991; Terry & O‘Leary, 1995). Another proposed interaction effect is the Contingency-Consistency Hypothesis, which suggests that attitudes are likely to be more predictive of the decision to act (intentions) when the social environment is supportive of the proposed action (Acock & DeFleur, 1972). The presence of such an interaction in the model would suggest that social influences operate to determine that impact of attitudes on behaviour (or intentions) rather than directly influencing behaviour.
While several studies have found support for this interaction effect, others have not (for a review, see Conner & McMillan, 1999). It has also been postulated that an interaction may exist between attitudes and PBC on intentions (Eagly & Chaiken, 1993). While one might expect increasing levels of PBC to be associated with increasing intentions to act for positively evaluated behaviour, this is not expected to be the case for negatively-evaluated behaviours. In the latter case, one might expect increasing PBC to be negatively related to intentions (i.e., greater control over a negatively-evaluated behaviour is associated with weaker intentions). In the case of drug use, one might expect PBC to be negatively related to intentions for those who evaluate drug use negatively; while for those who evaluate drug use positively, PBC might be expected to be positively related to intentions.
The TPB (as well as its predecessor, the Theory of Reasoned Action) has been successfully applied to the prediction of a number of health behaviours, including physical exercise (Godin, Valois & Lepage, 1993), dieting (Conner et al., 1996), the use of condoms (Boldero, Moore & Rosenthal, 1992), drinking and driving (Beck, 1981; Marcil et al., 2001; Parker et al., 1992), cigarette smoking (McMillan & Conner, 2003b; McMillan, Higgins & Conner, 2005), and the use of marijuana and other illicit drugs (Conner & McMillan, 1999, 2003a).
Applications of the Theory
Several studies have successfully applied the TPB to understand the link between intentions to use and self-reported use of licit and illicit substances, primarily among young adults. Conner and McMillan (1999) employed the theory to investigate the factors underlying intentions and frequency of cannabis use over a three-month period in a sample of 249 undergraduate students from England aged 17–59 (but primarily in the 19–22 age range). This study also assessed whether youths‘ perceptions of others‘ behaviour (descriptive norms) would contribute to the prediction of intentions to use cannabis frequently, independent of perceived pressure from others to engage in frequent cannabis use (injunctive norms).
Findings revealed that attitudes, injunctive norms (pressure from others [best friend, other friends, family, health experts, partner, etc.] to engage in frequent cannabis use), descriptive norms (perceived pressure from best friend and partner to use cannabis frequently) and PBC were each significantly associated with intentions, with PBC having the greatest influence on intentions. More positive attitudes and perceived injunctive pressure to use cannabis were associated with intending to use cannabis more frequently, while higher levels of PBC were associated with intending to use cannabis less frequently. Thus, those youth who perceived more control over cannabis use intended to use it less frequently. Attitudes were found to explain approximately 10% unique variance in intentions, PBC accounted for about 14%, injunctive norms less than 1%, and descriptive norms about 2%. Youths‘ intentions to use cannabis more frequently were significantly positively related to their cannabis-using behaviour three months later (R2 = 0.71).
This work also found evidence of significant interaction effects. Specifically, a significant interaction was noted between intention and PBC such that increasing levels of PBC were associated with a weakening of the intention-behaviour relationship. This study also found support of an interaction between attitudes and PBC on intentions; PBC was negatively related to intentions only for those youth who evaluated cannabis use negatively or neutrally, whereas there was no relationship between PBC and intentions for those with strong positive evaluations/attitudes. As this is the first empirical demonstration of this interaction effect, this finding warrants replication. An important limitation of this study is that the cannabis use behaviour measure was assessed via self-report.
McMillan and Conner (2003a) also investigated whether the TPB could be used to explain the relationship between intentions and use of LSD, amphetamine, cannabis and ecstasy over six months in a sample of 461 undergraduate students from England aged 17–54 (but mainly in the 19–22 age range). Similar to their aforementioned work, this study assessed the predictive utility of students‘ attitudes, PBC, and injunctive and descriptive norms in explaining their intentions to use illicit drugs. The results from this work showed that attitudes, injunctive norms, descriptive norms and PBC were each significantly associated with intentions to use LSD, amphetamine, cannabis and ecstasy. Attitudes or PBC had the greatest influence on intentions compared to injunctive norms for all of the drugs. In each case, greater intention to use a drug was associated with holding a more positive attitude, perceiving greater pressure to use the drug, and perceiving factors to facilitate, rather than inhibit, drug use. Youths‘ intentions were significantly positively related to behaviour six months later for all of drugs (LSD R2 = 0.26; amphetamine R2 = 0.46; cannabis R2 = 0.70: ecstasy R2 = 0.41).
Consistent with their previous study focusing on youth cannabis use (Connor & McMillan, 1999), McMillan and Connor demonstrated two significant interaction effects. One interaction was observed between intention and PBC across all four drugs (although it was only marginally significant for amphetamine and ecstasy); as the level of PBC increased, the predictive power of intentions also increased. This finding is in contrast to their previous study (Connor & McMillan, 1999), which reported a negative PBC x intention interaction for cannabis use. In reconciling these differential findings, the authors noted differences in the wording of their PBC measures between the two studies. The other significant interaction effect that was noted consistently across all types of drugs was between attitudes and PBC. Specifically, PBC was more strongly associated with intention to use drugs when attitudes were more positive. The findings from this work are based on self-reported data, and need to be interpreted in the context of this limitation.
Umeh and Patel (2004) examined the components of the TPB in relation to ecstasy use in a cross-sectional study of 200 young adults (mean age 21 years) from England. The results indicated that students‘ attitudes and subjective norms were significantly associated with their intentions to use ecstasy in the next two months. Stronger intent to use ecstasy was significantly associated with greater previous use of ecstasy. A significant interaction effect was documented between PBC and attitude; that is, perceived controllability of ecstasy use predicted stronger intentions to use the drug in the future only to the extent that respondents endorsed ecstasy use. Those youth who felt confident that they could procure and use the drug may nevertheless have failed to do so if they disapproved of such behaviour. The pattern of this interaction seems to contradict the moderator effect that was reported by Conner and McMillan (1999), who noted that PBC better predicts intentions to use cannabis given more negative evaluations of cannabis use. Conner and McMillan argued that the failure of PBC to predict intentions in persons who endorse cannabis use is attributable to studying a non-socially desirable behaviour.
McMillan and colleagues (2005) employed an extended version of the TPB to investigate the factors underlying cigarette smoking intentions and subsequent smoking behaviour three months later in a sample of 741 schoolchildren, aged 12–13, from northern England. Findings revealed that attitudes, subjective norms and PBC were significant predictors of intentions to smoke (R2 = .52). Attitudes and PBC each explained approximately 5% unique variance in intentions, whereas subjective norms only accounted for around 1%. Youths‘ intentions and PBC were found to be significant predictors of future cigarette smoking, correctly classifying 85% of study participants into smokers and non-smokers. Several other additional variables were also found to significantly increase the predictive utility of the model, including perceived friends‘ smoking and perceived family members‘ smoking (descriptive norms).
The TPB has also been used to understand alcohol and tobacco use among young adults. In their study of 494 undergraduate students in England aged 17–54 years (but mainly in the 19–22 age range), McMillan and Conner (2003b) investigated the links between attitudes, PBC, injunctive and descriptive norms and intentions to use alcohol and tobacco six months later. Attitudes, PBC, and injunctive and descriptive norms were found to be significant predictors of alcohol intentions, with PBC being the only significant predictor of intentions to use tobacco. For both drugs, PBC had the greatest influence on intentions. Students‘ intentions to use alcohol and tobacco were associated with their subsequent self-reported quantity of substance use, with stronger intentions and greater PBC being associated with higher levels of self-reported alcohol and tobacco use.
More recently, Judson and Langdon (2009) tested the TPB to explain the use of illicit prescription stimulant use among a sample of 333 New England undergraduate students (mean age 20 years). Of all participants, 6.3% (n = 21) were stimulant prescription holders and 93.6% (n = 312) were not. Students were provided with a self-report questionnaire that contained questions pertaining to attitudes (i.e., beliefs about adverse health effects, ethics of use), subjective norms (i.e., extent to which it is socially acceptable to use) and perceived control (i.e., perceived need to use stimulants) toward the illicit use of stimulant medications. Results showed that about 20% of the sampled youth reported illicit use of prescription stimulants. Independent of prescription status, illicit users thought that illicit use was significantly more ethical than non-users. Of non-prescription holders, illicit users were significantly less concerned with health risks associated with illicit use and felt more dependent on stimulant medications compared to non-users. Both holders and non-holders of prescriptions who were illicit users reported higher social norms for illicit use compared with non-users.
The predictors of illicit prescription stimulant use were tested in a model and the results were consistent with the TPB. Non-holders of a prescription who used illicitly had specific attitudes, normative beliefs and perceived control that differed from non-illicit users. Lower scores in concerns about adverse health effects and ethics of use, higher perceived positive subjective norms and lower perceived control were related to illicit use among non-prescription holders. Although intention to use was not directly measured in this study, the authors suggest that the link between these attitudes and beliefs were directly related to illicit use. The limitations of this study included the reliance on self-reported measures. There were also a small number of prescription holders, which limited statistical power of tests and reduced comparisons that could be made.
Limitations
In interpreting the evidence supporting the TPB in explaining youth substance use, several important limitations must be considered. Even though the theory accounts for considerable proportions of variance in youth drug use behaviour, some variance is still left unexplained, suggesting that other factors may be influencing youths‘ intentions to use drugs; additional research is required to investigate further constructs that can account for additional variance in drug use behaviour. Another important limitation is the reliance on self-reported measures in the studies substantiating this theory. Finally, with the exception of one study (McMillan et al., 2005), all of the reviewed studies were conducted using samples of undergraduate students. As such, it is unclear whether these findings would generalize to younger age groups.
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