Implementation intentions and repeated behaviour: augmenting the predictive validity of the theory...

Implementation intentions and repeatedbehaviour: augmenting the predictive

validity of the theory of plannedbehaviour

PASCHAL SHEERAN* and SHEINA ORBELL

University of She�eld, UK

Abstract

Two experiments based upon Gollwitzer's (1993) concept of implementation intentionsare described. In both experiments, attitudes, subjective norms, perceived behaviouralcontrol and intentions from Ajzen's (1991) theory of planned behaviour were used tomeasure participants' motivation prior to an intervention in which participants madeimplementation intentions specifying where and when they would take a vitamin C pilleach day. Behaviours were assessed by self-report and pill count at both 10 days and3 weeks in Experiment 1, and at 2 weeks and 5 weeks in Experiment 2. Resultssupported the view that participants who formed implementation intentions were lesslikely to miss taking a pill every day compared to controls. Evidence suggested thatimplementation intentions were e�ective because they allowed participants to passcontrol of behaviour to the environmental cues contained in the implementationintention. Implications of the study and some suggestions for future research areoutlined. Copyright # 1999 John Wiley & Sons, Ltd.

Understanding how people initiate and maintain behaviours such as condom use is animportant task for social psychologists. An approach that has had some success inthis area is the theory of reasoned action (Fishbein & Ajzen, 1975) and its successorthe theory of planned behaviour (Ajzen, 1985, 1991). According to these models, aperson's intention to perform a behaviour (e.g. `I intend going for cervical screening')is the key predictor of behavioural performance. Reviews of the literature are quitesupportive and suggest that measures of intention typically account for 20±30 per centof the variance in social and health behaviours in prospective studies (e.g. Randall &Wol�, 1994; Sheeran & Orbell, in press; Sheppard, Hartwick, & Warshaw, 1988).However, these ®ndings also mean that many people with positive intentions do notsucceed in establishing a new behaviour. For example, Orbell & Sheeran (in press)showed that only one-half of a sample of women who intended to undergo cervical

CCC 0046±2772/99/020349±21$17.50 Received 29 July 1997Copyright # 1999 John Wiley & Sons, Ltd. Accepted 15 April 1998

European Journal of Social PsychologyEur. J. Soc. Psychol. 29, 349±369 (1999)

*Correspondence to: Dr Paschal Sheeran, Department of Psychology, University of She�eld, She�eldS10 2TN, UK. e-mail: p.sheeran@she�eld.ac.uk

screening actually did so within a one-year period. The present study tries to addressthis problem using Gollwitzer's (1993; Gollwitzer & BrandstaÈ tter, 1997) concept of`implementation intentions'. In particular, we examine whether intentions to performa novel behaviour which have been supplemented by implementation intentionsspecifying where and when the behaviour will be enacted improves the likelihoodof the action being performed. We also investigate the processes that might beresponsible for the increased likelihood of action among people who have formedimplementation intentions.

THE THEORIES OF REASONED ACTION AND PLANNED BEHAVIOUR

According to the theory of reasoned action, intentions to perform a behaviourare determined by attitudes towards the behaviour and subjective norms. Attitudestowards the behaviour refer to people's positive or negative evaluation of theirperforming the behaviour (e.g. `Going for cervical screening would be wise/foolish').Subjective norms refer to people's perceptions of approval or disapproval fromsigni®cant others for performing the behaviour (e.g. `Most people who are importantto me think that I should go for cervical screening'). The theory of planned behaviourpostulates an additional determinant of intentions: perceived behavioural control.Perceived behavioural control refers to people's appraisals of their ability to performa behaviour (e.g. `Going for cervical screening would be easy/di�cult') and is closelyrelated to Bandura's (1977, 1986) concept of self-e�cacy. The more positive people'sattitudes and subjective norms are regarding the behaviour and the greater theirperceived behavioural control, the more likely it is that people will intend to performthe behaviour.

Intentions to perform a behaviour are conceptualized as the most immediate andimportant predictor of behavioural performance and mediate the e�ects of attitudes,subjective norms and, to a lesser extent, perceived behavioural control (Ajzen, 1991).Behavioural intentions are believed to summarize people's motivation to perform abehaviour. Ajzen (1991) states `Intentions are assumed to capture the motivationalfactors that in¯uence a behaviour, they are indicators of how hard people are willingto try, of how much e�ort they are planning to exert, in order to perform thebehaviour' (p. 181).

IMPLEMENTATION INTENTIONS

A distinction between a motivational phase during which a decision to perform abehaviour is made and a volitional phase during which speci®c plans are made toensure that one's decision is acted upon is central to Gollwitzer's (1993) concept ofimplementation intentions. According to Gollwitzer (1990) and Heckhausen (1991),the motivational or deliberative phase involves a subjective evaluation of the costs andbene®ts of performing the behaviour which culminates in the development of a `goalintention' or decision to perform the behaviour. This phase parallels the motivationalemphasis in the theory of planned behaviour where the decision to act (intention) is

350 P. Sheeran and S. Orbell

Copyright # 1999 John Wiley & Sons, Ltd. Eur. J. Soc. Psychol. 29, 349±369 (1999)

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based upon attitudes, subjective norms and perceived behavioural control. Unlike thetheory of planned behaviour, however, Gollwitzer and Heckhausen also posit avolitional or implemental phase which follows intention-formation. This phase ischaracterized by the development of plans which specify where and when one willenact one's intention (e.g. Ì will go for cervical screening at the University HealthCentre tomorrow morning'). These plans are termed ìmplementation intentions' byGollwitzer.

While only two experimental studies have investigated the utility of implementationintentions (Gollwitzer & BrandstaÈ tter, 1997; Orbell, Hodgkins, & Sheeran, 1997) bothhave demonstrated that planning has considerable volitional bene®ts. In the ®rst study,Gollwitzer and BrandstaÈ tter (1997) showed that students who supplemented their goalintention to write a report during the Christmas vacation with an implementationintention specifying where and when they would actually write the report were morethan twice as likely to complete their work on time compared to students who didnot form an implementation intention. In a ®eld study (Orbell et al., 1997), womencompleted measures of theory of planned behaviour constructs concerning theperformance of breast self-examination. Half of the sample were also asked to formimplementation intentions specifying where and when they would perform this breastself-examination. Despite equivalent intentions to perform the behaviour, all thewomenwho formed implementation intentions performed a breast self-examination inthe subsequent month compared with 53 per cent of the control group.

These studies strongly support the view that implementation intentions increase thelikelihood of a goal intention being enacted. Implementation intentions are likely tobe particularly e�ective in the context of behaviours or goals which are complex orwhere the timing and location for action are uncertain. For example, my intention totake up exercise might never be acted upon unless I specify what action I willundertake in order to promote my goal intention to exercise. An implementationintention such as Ì will join a ®tness club' would provide a valuable starting point inmoving towards my overall goal. Similarly, however, I might never join a ®tness clubunless I specify where and when I will do so. Again, an implementation intention suchas Ì will join the ®tness club on Pond Street tomorrow evening' would increase thelikelihood of goal achievement. Bargh (1990) points out that intentions which can beacted upon in a variety of di�erent ways or intentions which are poorly speci®ed areunlikely to result in action initiation. Implementation intentions can provide a simpleand e�ective solution to these problems.1

1We are grateful to our action editor, Guido Peeters, for pointing out that there is some ambiguityregarding whether implementations intentions involve specifying the timing and location for performing aparticular behaviour versus specifying the behaviour to be performed plus the timing and location for itsperformance. In our example of taking up exercise, we assume that specifying what action will beperformed (`join a ®tness club . . .') and where and when that action will be performed (`. . . on Pond Streettomorrow evening') could all be components of implementation intentions. An alternative view would beto suggest that the intention to exercise is a goal intention, (i.e. an outcome which can be achieved by avariety of di�erent actions), the intention to join a ®tness club is a behavioural intention (i.e. the speci®caction to be performed in the service of the goal intention) and that the term implementation intentionshould be used only to refer to when and where the speci®c action will be performed. Whetherimplementation intentions involve what, when and where or just when and where is open to debate.However, it should be noted that the studies presented here involve a speci®c action and, therefore, only thetiming and location of behavioural performance are speci®ed in participants' implementation intentions.We are aware of ongoing research where participants' implementation intentions have involved specifyingall three components with some success (Verplanken, personal communication).

Implementation intentions 351


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WHY ARE IMPLEMENTATION INTENTIONS EFFECTIVE INPROMOTING BEHAVIOUR?

Gollwitzer (1993) and Orbell et al. (1997) present evidence which suggests thatimplementation intentions are e�ective because they a�ect attentional and memorialprocesses relevant to successfully acting upon one's intentions. In particular, imple-mentation intentions appear to ensure that opportunities for initiating behavioursrelevant to the goal intention are not missed. For example, Gollwitzer (1993) reportsthat participants who formed implementation intentions regarding the timing, placeand method they would use to play a game were able to recall their chosen optionswith near-perfect accuracy. Similarly, Orbell et al. found that participants whoformed implementation intentions regarding breast self-examination reported thatthey had performed breast self-examination at the time and place they had speci®edone month earlier. These ®ndings suggest that planning a behaviour creates strongmemory traces that are easily accessed.

Implementation intentions also appear to a�ect speed of action initiation.Gollwitzer and BrandstaÈ tter (1997) found that participants who formed implementa-tion intentions to make particular counter-arguments at particular points during avideo presentation of a person making racist remarks were considerably faster inexpressing their counter-arguments than a control group who were asked only toidentify good opportunities. Making an implementation intention would, therefore,seem to ensure that when an opportunity for action presents itself, it will not only berapidly detected but a decisive answer as to whether one should act now or later, orwhat one should say or do, will be easily retrieved from memory. Behaviour followsrapidly after encountering the appropriate context for action.

Gollwitzer (1993) proposes that mentally forming a link between speci®cbehaviours and speci®c opportunities for enactment accounts for the speed of actioninitiation and the greater likelihood of action. Implementation intentions are notproposed to alter people's motivation (goal intentions). Rather, `[t]he underlyingtheory is that by forming implementation intentions people pass on control of goal-directed activities from the self to the environment. The intended behaviour is subjectto external control through the environmental cues speci®ed in one's implementationintention . . . when these cues . . . are encountered, they are expected to prompt theintended behaviour' (p. 153). Implementation intentions, therefore, a�ect thevolitional phase of performing a behaviour. They ensure that one's intention toperform a behaviour is speci®c with respect to the action to be performed and thetiming and location of its performance. They also ensure that one's intention toperform the behaviour is not forgotten. Implementation intentions stand in markedcontrast to the motivational emphasis of the theory of planned behaviour and othertraditional models of social and health behaviours.

The ®rst aim of this study is to test the hypothesis that intentions, as speci®ed by thetheory of planned behaviour, are more likely to be enacted if they have beensupplemented by implementation intentions which specify where and when thebehaviour will be performed. The second aim is to examine why implementationintentions are e�ective in increasing the likelihood of performing a behaviour. Inparticular, we test the hypothesis that there will be a strong correspondence betweenthe timing and location for performing the behaviour speci®ed in people's imple-mentation intentions and the timing and location of behavioural performance at







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follow-up. We also hypothesize that, because participants who form implementationintentions will remember where and when to perform the behaviour, they are lesslikely to report `forgetting' to perform the behaviour. Finally, we examine thehypothesis that memory for timing and location of behavioural performance accountsfor the e�ectiveness of implementation intentions. That is, we hypothesize thatreported `forgetting' to perform the behaviour will mediate the e�ects of formingimplementation intentions on subsequent behaviour.

THE PROBLEM OF REPEATED BEHAVIOURS

Sutton (1993) points out that almost all behaviours are capable of being repeated andthat many health behaviours must be repeated if they are to be e�ective (e.g. exercise,diet, brushing one's teeth). Repeated behaviours pose a signi®cant di�culty for thetheories of reasoned action and planned behaviour (Ronis, Yates, & Kirscht, 1989;Sutton, 1993). Several studies have found that past behaviour is a superior predictorof subsequent behaviour than intentions, attitudes, subjective norms and perceivedbehavioural control (e.g Bagozzi, 1981; Bentler & Speckart, 1979, Fredericks &Dossett, 1983; Triandis, 1980). It appears that the motivational variables speci®ed bythe theory of planned behaviour are unable to explain the relationship between pastand future behaviour.

One explanation of substantive associations between past and future behaviour isthe use of equivalent scales to measure these constructs. Ajzen (1991) has noted thisproblem but acknowledges that while shared method variance in measures of past andfuture behaviour contributes to large correlations between these measures, the size ofthe e�ect is such that it cannot be entirely attributed to measurement error (p. 204). Asecond explanation for the signi®cant e�ects of past behaviour over and above thee�ects of intentions concerns the possibility that intentions may be unstable. Sinceintentions can change over time because of new information, the association betweenintention and behaviour might become attenuated as the time interval between thetwo measures increases (Ajzen, 1985). There is some empirical support for this hypo-thesis. Doll and Ajzen (1992) found that stable intentions were more predictive ofbehaviour than unstable intentions. Relatedly, Sheeran and Orbell (in press) foundthat intentions were better predictors of behaviour over `short' compared to `long'time intervals.

A third explanation of the strength of the relationship between past and futurebehaviour is that past behaviour is related to subsequent behaviour directlyÐvia anon-conscious process usually termed habit. According to this view, behaviourswhich have been performed many times in the past are encoded in memory such thatenvironmental cues serve to automatically elicit the behaviour. One's intentions toperform a new behaviour in a familiar context may simply be forgotten. As Roniset al. (1989) point out: `Memory is often a critical problem for repeated health-relatedbehaviours. It may be particularly di�cult to remember to perform novel behavioursin familiar settings because behaviour in such settings is largely habitual andautomatic' (p. 228).

Gollwitzer (1993) and Orbell et al. (1997) point out that the process by which pastbehaviour controls future behaviour and the process by which implementation





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intentions direct future behaviour might be the same. In both cases there is anassociation in memory between the behaviour and certain environmental cues. Orbellet al. also point to a crucial distinction, however. Habits are generally believed to beformed by a repeated pairing of a behaviour with its context of performance. Roniset al., for example, de®ne habit as àn action that has been done many times and hasbecome automatic. That is, it is done without conscious thought'. Implementationintentions, on the other hand, involve a cognitive rehearsal of the link between beha-viour and the context of performance (the time and place speci®ed in the implementa-tion intention). Whether the pairing of behaviour and context is behavioural (habit)or cognitive (implementation intentions), the outcome should be the sameÐin bothcases there is an association in memory between a behaviour and certain contextualcues.

Based upon this line of reasoning, Orbell et al. (1997) proceeded to demonstratethat implementation intentions could break the link between past and futurebehaviour: for participants who formed implementation intentions, past behaviourexerted no signi®cant e�ect upon their subsequent performance of a breast self-examination. The subsequent behaviour of participants who had not formed imple-mentation intentions, on the other hand, was highly predictable from their past breastself-examination behaviour. The study provided support for the contention thatimplementation intentions can mimic the e�ects of habit.

EXPERIMENT 1

The present study seeks to extend Orbell et al.'s (1997) ®ndings in a number of ways.Whereas Orbell et al. were concerned with a discrete òne-o�' behaviour (a singlebreast self-examination over 1 month), the present study concerns a behaviour thatmust be repeated every dayÐtaking vitamin C tablets. In addition, we utilized twodependent measures in the present study. Self-reports of pill consumption weresupplemented by a pill count conducted by a researcher who was blind to partici-pants' experimental condition. The present study also adopts a more explicitlyìnterventionist' approach. A di�culty experienced in the Orbell et al. study was thatrelatively few people intended to perform a breast self-examination in either theintervention or control group. We wanted to ensure (1) that participants in the presentstudy intended to take the pills, (2) that the behaviour was relatively novel forparticipants and hence their past behaviour would not in¯uence their intentions, and(3) that material factors (e.g. the price of vitamin C pills, access to pharmacist) did nota�ect our intervention. We therefore provided participants with a free bottle ofvitamin C tablets, reasoning that participants were unlikely to be taking vitamin Cpills every day before the intervention but that they would be likely to intend to do soif the pills were provided gratis.

Another important feature of our design was that we collected data concerning goalintentions both before and after the implementation intention intervention. It ispossible that the formation of an implementation intention might change partici-pants' goal intentions. By collecting data at three time points we are able to investig-ate the possibility that the implementation intention intervention simply changesparticipants' goal intentions. Previous studies did not examine this possibility. Hence,





the e�ects obtained in those studies could have been due to motivational rather thanvolitional factors.

There were three reasons why taking a vitamin C tablet each day was the behaviourwe chose to study. First, vitamin C intake is an important health behaviour in its ownright. Adequate vitamin C is important for the prevention of scurvy (NationalResearch Council, 1989). Vitamin C has also received attention in recent medicalresearch as an antioxidant nutrient associated with a lower risk of heart disease andsome cancers (Block, 1992; Block, Henson, & Levine, 1991; Enstrom, Kanim, &Klein, 1992). Vitamin C intake has been reliably associated with demographicvariables including socioeconomic status (Bureau of the Census, 1989; Hargreaves,Baquet, & Gamshadzahi, 1989; Shapiro, Samuals, Breslow, & Camacho, 1983),ethnicity (Bowering & Clancy, 1986; Koplan, Annest, Layde, & Rubin, 1986), age(Schneider & Nardlund, 1983; Sobal & Daly, 1990) and sex (Koplan et al., 1986; Read& Graney, 1982).

The second reason for choosing taking a vitamin C tablet each day was that thisbehaviour presents no di�culties, in principle, for the concept of implementationintentions. Population surveys (e.g. Moss, Levy, Kim, & Parki, 1989) seem to indicatethat taking a vitamin C pill every day can become part of people's routine whichsuggests that an approach based on implementation intentions could be used here.

Finally, and most importantly, taking a vitamin C tablet each day presents nodi�culties, in principle, for the theory of planned behaviour since our design ensuresthat taking a vitamin C pill every day does not require resources or cooperationÐfactors which might diminish the predictive validity of the TPB (Eagly & Chaiken,1993; Liska, 1984). Madden, Ellen and Ajzen (1992) found that TPB variablesexplained 59 per cent of the variance in taking vitamin supplements over two weekswhile Warshaw and Davis (1985) obtained a correlation of 0.73 between intentionsand vitamin intake over one week. If anything, vitamin intake appears to be morepredictable from TPB variables than many other behaviours (see Godin & Kok, 1996;Sheppard et al., 1988). Thus, taking vitamin C pills every day could be seen torepresent a stringent test of the ability of implementation intentions to improve thelikelihood of action over and above the e�ects of the TPB.

Hypotheses

This study has the following hypotheses:

(1) Participants who form implementation intentions concerning where and whenthey will take a vitamin C pill each day will be less likely to miss taking their pillsthan participants who do not form implementation intentions.

(2) Participants who form implementation intentions will show strong correspond-ence between the timing and location speci®ed in their implementation intentionand the timing and location of actual behaviour.

(3) Participants who form implementation intentions will be less likely to report`forgetting' to take pills compared to controls.

(4) Memory for timing and location of behavioural performance will mediate thee�ects of implementation intentions on behaviour.



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Method

Participants and Procedure

Participants were undergraduates selected at random from two university halls ofresidence. They were approached in their rooms during the evening and asked toparticipate in a study concerned with taking vitamin C tablets. Participants weregiven a sealed bottle containing 50 vitamin C tablets and asked to complete the Time 1questionnaire. They were informed that they would be asked to complete two furtherquestionnaires, one 10 days later (Time 2) and one 3 weeks later (Time 3). Theintervention took the form of a non-obtrusive manipulation contained in the Time 1questionnaire. The researcher was unaware whether participants were in the controlor experimental conditions and participants were not informed that they were takingpart in an experimental study.

One hundred and thirty-six participants completed the questionnaire at Time 1,N� 111 completed the questionnaire at Time 2, and N� 103 completed the Time 3questionnaire. Six participants who had consumed all of the tablets at Time 2 wereexcluded from the study. This meant that there were complete data for all three time-points from N� 78 participants. Fifty-six per cent of this sample were women(N� 44) and 44 per cent were men (N� 34). Participants' mean age was 18.83 years(SD� 1.53).

Questionnaires

Time 1 questionnaire Items assessing attitude, subjective norm, perceived beha-vioural control, intentions and past behaviour were presented in random order(cf. Sheeran & Orbell, 1996). Attitude towards taking vitamin C was measured using 7bipolar scales (`good±bad', `pleasant±unpleasant', `healthy±unhealthy', `nice±nasty',`sensible±foolish', `harmful±bene®cial', ùseful±useless') and the item Ì think thattaking one vitamin C tablet per day would increase my resistance to commoninfections' (àgree±disagree') which had satisfactory internal reliability (alpha� 0.78).Subjective norm was measured by a single item: `Most people whose opinion mattersto me think I should take a vitamin C tablet every day' (7-point àgree±disagree'scale). Four items were used to assess perceived behavioural control. All the items weremeasured using 7-point bipolar scales: `For me, taking a vitamin C tablet every dayfor the next three weeks would be . . .' (èasy±di�cult'); `How many obstacles are therethat would prevent you taking a vitamin C tablet every day for the next 3 weeks?'(`numerous obstacles would prevent me±few obstacles would prevent me'); `Howmuch control do you think you have over whether or not you take a vitamin C tabletevery day for the next 3 weeks?' (`complete control±no control'); and `How con®dentare you that you will be able to take a vitamin C tablet every day for the next 3 weeks?'(`con®dent±uncon®dent'). These four items formed a reliable scale (alpha� 0.72).Intentionwas measured by two items: Ì intend to take a vitamin C tablet every day forthe next 3 weeks' and Ì will take a vitamin C tablet every day for the next 3 weeks'(both 7-point àgree±disagree' scales). Reliability was satisfactory (alpha� 0.71),Past behaviour was measured by one item `How often in the past have you takenvitamin C tablets?' (5-point scale `never±always').



Time 2 questionnaire Participants were asked to indicate how many vitamin Ctablets they had taken in the last 10 days and `How many days since the study began(i.e. 10 days ago) have you been away (e.g. weekends away)?' Participants alsocompleted the two intention items above.

Time 3 questionnaire Participants were asked to indicate how many vitamin Ctablets they had taken in the last 11 days and `How many days in the last 11 (i.e. sinceyou completed the second questionnaire) have you been away (e.g. weekends away)?'Participants were also asked to write down where and when they had taken thevitamin C tablets and `If you did not take a vitamin C tablet every day, why not?'

Pill Count

The number of pills remaining in each bottle was counted by the researcher at bothTime 2 and Time 3 while participants completed the questionnaires.

Implementation Intention Intervention

Following the procedure described by Gollwitzer (1993), participants in theintervention group were asked to make implementation intentions specifying whereand when they would take a vitamin C tablet every day for the next 3 weeks. Thefollowing paragraph was added to the Time 1 questionnaire for the interventiongroup, and was presented after the items assessing theory of planned behaviourvariables and previous use of vitamins:

`You are more likely to carry out your intention to take a vitamin C tablet everyday for the next 3 weeks if you make a decision about where and when you willdo so. Decide now where and when you will take the vitamin C tablets in the next3 weeks. You may ®nd it useful to take a tablet just before or just after somethingelse that you do regularly, such as brushing your teeth. Please write in belowwhere and when you will take a vitamin C tablet every day for the next 3 weeks(e.g. in my room at 7 pm just after my evening meal).'

The Time 1 questionnaire for the control group was identical in all respects to that ofthe intervention group except for the omission of this item.

Results

Representativeness and Randomization Checks

Fifty-seven per cent of the original sample could be contacted to complete theirquestionnaires at all three time-points. To ensure that the ®nal sample was repre-sentative we compared the Time 1 questionnaire responses of participants whocompleted all questionnaires (N� 78) with those who did not (N� 52).2 AMANOVA

2N� 130 because six participants who had consumed all the vitamin tablets at Time 2 were excluded fromthe study.



conducted on the continuous measures (i.e. attitude, subjective norm, perceivedbehavioural control, intention, past behaviour, and age) showed no di�erencebetween the two groups (F 6,123)� 1.79, ns) which was con®rmed by univariatetests. Chi-square tests showed that men and women were equally likely to remain inthe study (Chi-square� 0.87, ns) and that intervention and control participants werealso equally likely to complete all three time-points (Chi-square� 0.62, ns). Thus,there is good reason to believe that participants who remained in the study arerepresentative of the original sample.

Similar analyses were conducted in order to ensure that our randomization ofparticipants to the experimental (N� 38) and control conditions (N� 40) wassuccessful. Table 1 presents the means and standard deviations for the theory ofplanned behaviour variables and past behaviour for control and intervention part-icipants. A MANOVA comparing the two groups was non-signi®cant (F 5,72)� 0.20,ns) and univariate F-tests con®rmed that there were no di�erences between the groupson pre-intervention measures of attitude, subjective norm, perceived behaviouralcontrol, intention or past behaviour. Similarly, there were equivalent proportions ofmen and women in the control and intervention groups (Chi-square� 0.51, ns) and nosigni®cant age di�erences between the groups (t� 1.14, ns).

We also inspected the correlations between past behaviour and intention for thetwo groups. Consistent with the requirements of our design, past behaviour was notsigni®cantly associated with intentions for the experimental (r� 0.19, ns) or controlgroups (r� 0.10, ns). Table 1 shows that both groups had positive attitudes,subjective norms, perceived behavioural control and intentions. Behaviouralintentions were strong in both conditions, with mean scores greater than 6.0 on a1±7 scale. We can conclude that both groups did not base their intentions to take avitamin C tablet every day upon their previous behaviour and that both groups werehighly motivated to take the tablets. Di�erences between the groups on the number ofpills taken must, therefore, be attributed to volitional rather than motivationalfactors.

E�ects of the Implementation Intention Intervention

Table 2 shows the number of tablets missed by participants in the control andintervention groups. At Time 2, ten days into the intervention, both the pill count andself-report showed no signi®cant di�erences between the groups on the numbers of

Table 1. Comparison of exerimental and control group on theory of planned behaviourvariables and past behaviour (Experiment 1)

Experimental group Control group F

Attitude 5.34 (0.70) 5.28 (0.87) 0.10Subjective norm 4.34 (1.48) 4.05 (1.66) 0.67Perceived behavioural control 5.68 (1.27) 5.53 (1.03) 0.37Intention 6.23 (1.21) 6.05 (0.99) 0.51Past behaviour 2.29 (1.16) 2.23 (1.14) 0.06

Note: Standard deviations are presented in parentheses. *p5 0.05.



missed tablets (t� 0.87 and 1.03, respectively, ns). By Time 3, however, theintervention group had missed signi®cantly fewer pills than the control accordingto both the subjective (t� 2.05, p5 0.05) and pill count indices (t� 1.86, p5 0.05).On average, the control group had missed two vitamin C tablets more than theexperimental group over the 3-week period.

In order to ensure that these ®ndings were robust we conducted two reliabilitychecks. First, we compared the correlations between the self-report and pill countmeasures at Time 3 for the control and intervention groups. The correlations weresimilar (r� 0.90 and 0.92 for the experimental and control groups, respectively)indicating that measurement error is unlikely to be responsible for di�erences betweenthe two groups. A second reliability check was conducted by examining the pill countsand self-reports from the N� 103 participants who completed both the Time 1 andTime 3 questionnaires.3 Con®rming the ®ndings obtained above, the interventiongroup (N� 52) missed fewer tablets (M� 2.21, SD� 4.25) than the control group(N� 51, M� 3.80, SD� 5.44) according to the pill count measure (t� 1.65,p5 0.05). Similarly, the intervention group reported missing fewer tablets than thecontrol group (t� 1.95, p5 0.05). We can conclude that the implementationintention intervention was successful in ensuring that vitamin C tablets were takenby participants.

Implementation Intentions and Action Memory

Gollwitzer and BrandstaÈ tter (1997) and Orbell et al. (1993) have demonstrated thatimplementation intentions create strong memory traces for the timing and location ofaction which means that opportunities to act are unlikely to be missed. In order to testthe hypothesis that intervention participants would perform the behaviour at the timeand in the location speci®ed by their implementation intentions, we cross-tabulatedwhere and when participants said they would take a vitamin C tablet each day withwhere and when participants reported having taken tablets in the Time 3 question-naire. Data were available for 37 intervention participants at both time-points. Allparticipants reported that they would take the tablet in their room and all reportedhaving taken them in that location at Time 3. Content analysis of the speci®ed timesrevealed three meaningful categories; morning/breakfast, evening/teatime, and other.

Table 2. Comparison of experimental and control group on the number of pills missed atTime 2 and Time 3 (Experiment 1)

Experimental group Control group t

Pill count (Time 3) 1.57 (3.41) 3.53 (5.62) 1.86*

Self-report (Time 3) 2.68 (4.03) 4.85 (5.20) 2.05*

Pill count (Time 2) 0.61 (1.75) 1.00 (2.24) 0.87Self-report (Time 2) 0.92 (2.05) 1.43 (2.28) 1.03


3There were no di�erences between participants who completed both the Time 1 and Time 3 questionnaires(N� 103) and participants who did not complete these measures (N� 27) on the theory of plannedbehaviour variables, past behaviour or age, or in the proportions of men/women or experimental/controlparticipants in the sample.



Of the 27 participants who speci®ed morning/breakfast, 77 per cent reported takingthe tablet there. Eight participants speci®ed evening/teatime, of whom 63 per centreported taking the tablet there while one of the two participants who speci®ed `other'time reported taking the tablet at that time. AChi-square test showed that there was ahighly dependent relationship between the timing and location for action speci®ed inparticipants' implementation intention and the timing and location of behaviouralenactment reported at Time 3 (Chi-square (4)� 16.00, p5 0.005). Since (1) a numberof possible times and locations for taking the vitamin tablets could have been chosen,(2) three weeks had elapsed between the two questionnaires, and (3) since the item onthe Time 3 questionnaire came as a surprise to participants, these ®ndings supportGollwitzer's view that implementation intentions produce strong memory traces.

In order to examine the e�ects of the intervention on missing opportunities foraction we examined the reasons for non-enactment given by participants in theintervention and control groups. Following Orbell et al., the Time 3 questionnaireswere examined for reports of `forgetting' to take the vitamin C pills by someone whowas blind to the experimental condition that participants were in. Forty-three per centof the control group (N� 17) reported forgetting to take a pill compared to just16 per cent (N� 6) of the implementation intention group (Chi-square (1)� 6.69,p5 0.01). This ®nding con®rms our third hypothesis, that participants who formedimplementation intentions would be less likely to report `forgetting' to take vitamin Cpills than controls.

Our fourth hypothesis was that remembering to perform the behaviour couldexplain the e�ects of the implementation intention manipulation. To test thisproposition, a forced entry regression procedure to test for mediation was conductedin the manner speci®ed by Baron and Kenny (1986). Mediation was estimated, ®rst,by regressing the Time 3 pill count on experimental condition, and, second, bysimultaneously regressing the pill count on experimental condition and the mediator.Experimental condition should have a signi®cant e�ect in the ®rst equation and themediator should have a signi®cant e�ect in the second. In addition, the e�ect ofexperimental condition should be reduced to non-signi®cance in the second equation.The mediator, memory for intention, was a dichotomous item derived from reported`forgetting' (cf. Orbell et al., 1997). The results of the regression analyses are shown inTable 3. In the ®rst equation, experimental condition signi®cantly predicted thenumber of tablets missed (Beta� 0.206, t� 1.84, p5 0.05). When memory forintention was added to the equation, the overall model remained signi®cant(F 2,75)� 30.15, p5 0.001) and the mediator signi®cantly predicted behaviour(Beta� 0.653, t� 7.39, p5 0.001) accounting for 45 per cent of the variance.However, the e�ect of experimental condition was reduced to non-signi®cance(Beta�ÿ0.052, t� 0.58, ns). This analysis supports the view that remembering to

Table 3. Regession coe�cients for test of memory for intention as the mediator of therelationship between experimental condition and Time 3 pill count (Experiment 1)

Without mediator Memory for intention as mediator

Experimental condition 0.206* ÿ0.052Mediator Ð 0.653***

*p5 0.05, **p5 0.01, ***p5 0.001.



perform the behaviour mediated the e�ect of implementation intentions on vitamin Cintake.

We also wanted to ensure that the e�ects of the intervention could not be attributedto changes in the goal intentions of intervention participants compared to controls. Ifthe implementation intention intervention altered participants' intentions, then thesuccess of the intervention could simply be attributed to changes in motivation to takethe vitamin C pills. To rule out these hypotheses, a 2� 2 mixed-model ANOVA wasconducted comparing the goal intentions of experimental and control participants atTime 1 and Time 2. Consistent with our proposal that these ®ndings cannot beexplained by motivational factors, the experimental and control groups did not di�erin their intentions (F 1,76)� 2.93, ns), there were no signi®cant changes in goalintentions across time (F 1,76)� 1.39, ns), and no signi®cant interaction betweenexperimental condition and time (F 1,76)� 0.38, ns).

Discussion

Experiment 1 used the theory of planned behaviour to measure participants' motiva-tion prior to an intervention which required half of the sample to form implementa-tion intentions specifying where and when they would take a vitamin C tablet everyday. Participants in the implementation intention and control conditions weresimilarly highly motivated. While there were no di�erences in the number of pillsmissed by the two groups after 10 days, by 3 weeks the intervention group had missedsigni®cantly fewer pills than the control group according to both self-reports and apill count. These ®ndings would seem to support the view that implementationintentions increase the likelihood of action among people motivated to perform thebehaviour.

Nonetheless, there are a number of points worth noting about ®ndings fromExperiment 1. First, the e�ect of the implementation intention manipulation emergedat the 3-week follow up but not at 10 days. Second, although one-tailed signi®cancetests are appropriate when predictions are made about the direction of the e�ect forexperimental versus control conditions (Howell, 1987), it should be acknowledgedthat the pill count data would show no statistically discernible di�erences if two-tailedtests had been employed. Finally, it is possible that idiosyncratic aspects of the design,manipulation or measures could have been responsible for our ®ndings. For thesereasons, we conducted a second experiment to validate our ®ndings.

EXPERIMENT 2

Experiment 2 was intended to provide a conceptual replication of our ®rst experi-ment. Taking vitamin C pills every day for three weeks was again the behaviourstudied. The experimental design we employed was adapted from Howard, Curtinand Johnson's (1991) research on volition and involved two phases. In the baseline ormotivational phase, participants were given a bottle of 50 vitamin C pills and weretold that it was entirely up to them whether or not they took the pills. After twoweeks, the researcher counted how many pills had been consumed and the



intervention phase began. Here, participants were asked if they would try to take avitamin C pill every day for three weeks. Motivation to take the pills was determinedusing di�erent measures of TPB variables to those employed in Experiment 1. Half ofthe sample received a manipulation designed to make these respondents form imple-mentation intentions. Again, the manipulation varied the procedure employed inExperiment 1.

The reason for employing this experimental design is that it provides a cleardi�erentiation between the motivational and volitional action phases (cf. Gollwitzer,1990; Heckhausen, 1991). In the initial phase pill consumption is a function ofparticipants' motivation to take the pills. This phase also provides direct experiencewith the behaviour and enables participants to base their attitudes, subjective norms,perceived behavioural control and intentions upon that experience (cf. Fazio, 1990).The intervention or volitional phase, on the other hand, requires that respondents arede®nitely motivated to perform the behaviour and the issue is whether participantsenact their intentions (i.e. demonstrate volition). According to Howard et al. (1992), itis only possible to conclude that the volitional action phase is addressed if (1)participants have experience with the focal behaviour and, therefore, their intentionsare realistic, and (2) participants have clear intentions and are de®nitely trying toenact those intentions.

The hypothesis in the present experiment is that the formation of an implementa-tion intention specifying where and when one plans to take a vitamin C pill will leadto fewer missed pills among participants who are trying to take a pill every daycompared to participants who are similarly motivated but have not formed animplementation intention.

Method

Participants and Procedure

Participants were undergraduate students contacted by means of a `snowballing'technique whereby initial contacts of the researchers provided the names of furtherpotential participants. Participants were approached in their own homes. They wereinformed that the study concerned vitamin C intake and were provided with a sealedbottle of 50 vitamin C tablets. Participants were told that it was entirely up to themwhether or not they took the tablets and that the researcher would return in twoweeks to see how many pills they had taken. Two weeks later the researcher returnedto participants' homes, counted the number of pills remaining, and asked participantsif they would try to take a pill every day for the next 3 weeks. Participants thencompleted a questionnaire containing the TPB measures and were told that theresearcher would return to count the pills. The implementation intention interventiontook the form of a non-obtrusive questionnaire manipulation. The researcher wasunaware whether participants were in the control or experimental conditions andparticipants were not informed that they were taking part in an experimental study.

Forty participants were initially recruited. Two participants were unwilling to try totake a vitamin C pill every day for three weeks and were excluded from the study. Oneparticipant who did not complete the implementation intention manipulationcorrectly was also excluded. This meant that there were 37 participants in total,



19 in the experimental condition and 18 in the control condition. There were 20women and 18 men in the sample.

Questionnaire

Responses to all items were on 7-point scales. Responses to the item `For me, takingone-a-day vitamin pills every day for the next three weeks would be . . .' on ®ve bipolarscales (`good±bad', `pleasant±unpleasant', `harmful±bene®cial', ùseful±useless',ènjoyable±unenjoyable') constituted the measure of attitude. Reliability was high(alpha� 0.86). Subjective norm was measured by the item `Most people who areimportant to me think I should take one-a-day vitamin tablets for the next threeweeks' (`likely±unlikely'). Perceived behavioural control comprised four items: `For meto take one-a-day vitamin tablets every day for the next three weeks would be . . .'(`very easy±very di�cult'), Ìf I want to I will easily be able to take one-a-day vitamintablets every day for the next three weeks' (`strongly agree±strongly disagree'), `Thenumber of external in¯uences that may prevent me from taking one-a-day vitamintablets for the next three weeks are . . .' (`numerous-none at all'), and `How muchcontrol do you think you have over your ability to take one-a-day vitamin tabletsevery day for the next three weeks?' (`complete control-absolutely no control').Reliability was high (alpha� 0.90). Finally, three items were used to measureintentions; Ì will try to take one-a-day vitamin tablets every day for the next threeweeks' (`de®nitely will±de®nitely will not'), Ì will make an e�ort to take one-a-dayvitamin tablets every day for the next three weeks `de®nitely true±de®nitely false'),and Ì intend to take one-a-day vitamin tablets every day for the next three weeks'(`de®nitely do-de®nitely do not'). Reliability was satisfactory (alpha� 0.73).

Pill Count

The number of pills remaining in each bottle was counted by the researcher whileparticipants completed questionnaires at the end of the 2-week baseline period andagain 3 weeks later.

Implementation Intention Intervention

Participants in the experimental group were asked to form implementation intentionsspecifying where and when they would take a vitamin C tablet every day for the next3 weeks. The following paragraph was added to the end of the questionnaire for theexperimental group:

`You are more likely to be successful in your intentions to take a one-a-dayvitamin C tablet for the next 3 weeks if you make a decision about where you willtake these tablets and at what time. Decide now where you will take the tabletsand at what time.'

The questionnaire for the control group was identical in all respects to that of theintervention group save for the omission of this item.



Results

There were no di�erences between the experimental and control conditions on thenumber of men and women in the sample or on participants' age. There were also nodi�erences on the number of pills missed by the experimental (M� 1.21, SD� 2.07)and control participants (M� 1.28, SD� 3.53) during the 2-week baseline period(t� 0.07, ns). Thus both groups had equivalent direct experience with the behaviourprior to the volitional phase of the experiment.

In order to ensure that both groups were similarly highly motivated to take thevitamin C pills every day for three weeks, MANOVA was conducted on attitude,subjective norm, perceived behavioural control and intentions. The multivariate testwas non-signi®cant (F 4,32)� 0.67, ns) and so were the univariate Fs. As Table 4shows, participants had positive attitudes, subjective norms, perceived control andintentions. Intentions were extremely strong in both groups with means greater than6.70 on a 1±7 scale.

Analysis of the pill count data showed that, as predicted, participants who formedimplementation intentions missed signi®cantly fewer pills (M� 0.90, SD� 1.60) thancontrol participants (M� 2.50, SD� 3.42)(t� 1.81, p5 0.05, one-tailed). In order toguard against the possibility that these mean di�erences might be due to outliers, wereanalysed the data in terms of the number of participants who missed one or morepills. There were no di�erences between the experimental and control conditionsduring the baseline period (Chi-square (1)� 0.22, ns). However, during the 3-weekvolitional phase, only 26 per cent (N� 5) of participants who formed implementationintentions missed taking a pill compared to 61 per cent (N� 11) of controlparticipants (Chi-square (1)� 4.56, p5 0.05). Moreover, when we analysed the datafor those participants who missed a pill during the baseline period only, fewer peoplein the experimental condition missed taking a pill compared to controls (45 per centversus 85 per cent, respectively, Chi-square (1)� 4.11, p5 0.05). We conclude thatthese ®ndings provided strong evidence that the formation of an implementationintention improves the likelihood of vitamin C intake over a 3-week period.

Discussion

Experiment 2 employed a di�erent design, di�erent measures of TPB variables and amodi®ed implementation intention manipulation in order to validate the key ®ndingfrom Experiment 1 that implementation intentions improve the likelihood of action

Table 4. Comparison of experimental and control group on theory of planned behaviourvariables (Experiment 2)

Experimental group Control group F

Attitude 5.20 (1.12) 5.46 (0.98) 0.54Subjective norm 4.79 (1.75) 4.55 (1.79) 0.16Perceived behavioural control 5.67 (1.43) 6.04 (1.00) 0.83Intention 6.75 (0.35) 6.70 (0.41) 0.17




among participants who have strong behavioural intentions according to the TPB.Convergent evidence demonstrated that implementation intentions enhanced thepredictive validity of the TPB. Participants who formed implementation intentionsmissed fewer pills on average over a 3-week period; fewer participants who formedimplementation intentions missed taking a pill compared to controls; and this e�ectremained even when only those participants who missed pills during the baselineperiod were considered. It should be acknowledged that the mean di�erence on thenumber of missed pills by implementation intention versus control participants wasagain detectable using a one-tailed test only. However, a meta-analysis indicated thatalthough the e�ect size associated with implementation intention manipulations isrelatively modest (r� 0.24, N� 116, k� 2), this e�ect is homogenous (Chi-square(1)� 0.19, ns) and highly signi®cant (95 per cent CI� 0.05 to 0.40, p� 0.005). Takentogether, ®ndings from the two experiments speak of the utility of implementationintentions in closing the gap between intentions and action.

GENERAL DISCUSSION

Gollwitzer (1990) and Heckhausen (1991) have developed a model of behaviouralperformance which distinguishes between a motivational phase which culminates inthe intention to perform a behaviour and a subsequent volitional phase which ischaracterized by plans to initiate the behaviour at a speci®c time and place. In twoexperiments we conducted an intervention on the volitional phase by requiring half ofour sample to form implementation intentions specifying where and when they wouldtake a vitamin C tablet every day for 3 weeks. Variables speci®ed by the theory ofplanned behaviour were used to characterize participants' motivation to perform thebehaviour. Despite equivalent strong intentions (and attitudes, subjective norms andperceived behavioural control), both experiments showed that participants whoformed implementation intentions missed fewer pills at 3-week follow-ups comparedto controls. These ®ndings provide strong support for the view that implementationintentions increase the likelihood of successful performance of a repeated behaviouramong people who are motivated to perform that behaviour.

The fact that di�erences between the implementation intention and control groupsemerged at three weeks but not 10 days in Experiment 1 deserves comment. Webelieve that this ®nding is supportive of the idea that motivation or intention is likelyto be highly predictive of behaviour in the short term (see Sheeran & Orbell, in press).Consistent with this view, Warshaw and Davis (1985) and Madden et al. (1992) haveshown that theory of reasoned action/planned behaviour variables o�er very goodprediction of supplement intake over one week and two weeks, respectively. In thelonger term, however, motivation to perform a behaviour may not, on its own, besu�cient to ensure adherence and speci®c plans which allow the individual to `pass oncontrol of goal-directed activities from the self to the environment' (Gollwitzer, 1993,p. 153) will be especially helpful. Further indirect support for this proposition comesfrom McCaul, Glasgow, and O'Neill (1992). In three experimental studies, theseresearchers tried to create healthy dental habits usingmotivational interventions basedupon Bandura's (1986) social cognitive theory. Despite some initial success, thee�ectiveness of these interventions declined rapidly over time. This contrasts with



®ndings from the present volitional intervention which actually shows increasede�ectiveness over time among participants who were highly motivated (cf. Atkinson& Birch, 1970, 1978). Thus, implementation intentions could be seen as a valuablesupplement to motivational interventions which would contribute to increasedlikelihood of action in the longer term.

Experiment 1 also provides insight into the processes by which implementationintentions increase the likelihood of behavioural performance. Two lines of evidencesuggested that implementation intentions were e�ective because they improvedmemory for initiating the behaviour. First, there was a strong correspondencebetween the time and place of enactment speci®ed in participants' implementationintentions and the time and place of actual behavioural performance reported atfollow-up. This ®nding suggests that forming an implementation intention creates amental link between a behaviour and certain contextual cues which produces strongmemory traces such that encountering the relevant contextual cues serves to cueperformance of the behaviour. The second line of evidence showed that participantswho formed implementation intentions were less likely to report `forgetting' toperform the behaviour than control participants. Regression analyses showed thatthese reports of `forgetting' to take the pills entirely mediated the e�ects of forming animplementation intention on subsequent behaviour. It is important to acknowledgethat reported `forgetting' does not constitute a direct test of memory for behaviouralperformance among the intervention group. It should be noted, however, thatreported `forgetting' constitutes as clear an analogue of not missing opportunities foraction as a ®eld study such as this permits. Laboratory studies by Gollwitzer (1993)have shown that participants who form implementation intentions have excellentmemory for the timings and locations speci®ed in their intentions. Our ®ndings forreported `forgetting' are certainly consistent with this laboratory research and lendsupport to the view that the automatic initiation of behaviour is responsible for thee�ectiveness of implementation intention interventions.

Experiment 1 was also able to rule out the hypothesis that implementation inten-tions were e�ective simply because they altered participants' motivation to take avitamin C pill every day. Our ®ndings showed no evidence that the goal intentions ofthe experimental group changed as a result of the interventionÐthe intentions ofexperimental and control participants were equivalent both before and after theintervention.

Findings obtained here support the utility of a volitional approach to promotingvitamin C intake. Previous research indicates that TPB variables are very goodpredictors of vitamin supplement consumption (Madden et al., 1992; Warshaw &Davis, 1985). Nonetheless, our experiments demonstrate that it is possible to enhancethe predictive validity of the TPB even when intentions and perceived behaviouralcontrol are strong. These ®ndings suggest that psychological interventions can bee�ective in this area, especially if participants' intentions to take pills are supple-mented by implementation intentions specifying when and where they will do so.

A possible criticism of this study should be addressed. The present experimentswere conducted over a relatively short time periodÐ3 weeks. It must be acknowl-edged that a longer-term study would have been desirable. For example, the McCaulet al. (1992) study described earlier included 2-month and 6-month follow-ups. It isworth noting, however, that both experiments involved an objective pill count whichis extremely labour-intensive and di�cult to conduct over a longer time period. We



would also point out that the results obtained here show that while a 10-day follow-upshowed no di�erences, three-week follow-ups were su�cient to demonstrate thee�ectiveness of implementation intention interventions. We do not believe that thereare strong grounds to suppose that the e�ectiveness of our intervention would haveseriously diminished over a longer time period since taking the pills is likely to havebecome `habitual' after 3 weeks. Clearly, however, further research is required to testthis proposition.

Notwithstanding this limitation, the present research has both theoretical andpractical importance. While meta-analytic reviews show that the behaviouralintention construct from the theories of reasoned action and planned behaviourhas good predictive validity (Randall & Wol�, 1994; Sheeran & Orbell, in press;Sheppard et al., 1988), it is also apparent that many people who intend to perform abehaviour are not successful in their e�orts to act upon their positive intentions(Orbell & Sheeran, in press). Many of the behaviours of interest to social psychol-ogists are actually complex goals which require the enactment of speci®c behavioursat speci®c times in speci®c locations. Intentions to start exercising, to protect oneselfagainst sunburn, or to take medication recommended by one's doctor might never beenacted unless an implementation intention is formed concerning where and when toinitiate relevant behaviours. Our ®ndings indicate that implementation intentions aree�ective in promoting vitamin C pill intake over a 3-week period. One obviousextension of this study would be to examine medication adherence among a clinicalpopulation over a longer period. More generally, the concept of implementationintentions may provide a valuable strategy for attempting to change people's beha-viour. The e�ectiveness of this type of volitional intervention with other behavioursshould be investigated in future research.

ACKNOWLEDGEMENTS

The authors would like to thank David Silverman and James Mackie for their help inconducting the experiments.

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