Cognitive Mediators of Treatment for Social Anxiety Disorder:Comparing Acceptance and Commitment Therapy and

Cognitive-Behavioral Therapy

Andrea N. NilesLisa J. BurklundJoanna J. Arch

Matthew D. LiebermanDarby Saxbe

Michelle G. CraskeUniversity of California, Los Angeles

Objective: To assess the relationship between session-by-session mediators and treatment outcomes in traditionalcognitive-behavioral therapy (CBT) and acceptance andcommitment therapy (ACT) for social anxiety disorder.Method: Session-by-session changes in negative cognitions(a theorized mediator of CBT) and experiential avoidance(a theorized mediator of ACT) were assessed in 50 adultoutpatients randomized toCBT (n = 25) or ACT (n = 25) forDSM-IV social anxiety disorder. Results:Multilevel modelinganalyses revealed significant nonlinear decreases in theproposed mediators in both treatments, with ACT showingsteeper decline than CBT at the beginning of treatmentand CBT showing steeper decline than ACT at the end oftreatment. Curvature (or the nonlinear effect) of experientialavoidance during treatment significantly mediated posttreat-ment social anxiety symptoms and anhedonic depression inACT, but not in CBT, with steeper decline of the Acceptanceand Action Questionnaire at the beginning of treatmentpredicting fewer symptoms in ACT only. Curvature ofnegative cognitions during both treatments predicted out-come, with steeper decline of negative cognitions at the

beginning of treatment predicting lower posttreatment socialanxiety and depressive symptoms. Conclusions: Rate ofchange in negative cognitions at the beginning of treatment isan important predictor of change across both ACT and CBT,whereas rate of change in experiential avoidance at thebeginning of treatment is a mechanism specific to ACT.

Keywords: social anxiety disorder; treatment mediator; treatmentmechanism; cognitive-behavioral therapy; acceptance and commitmenttherapy

SOCIAL ANXIETY DISORDER IS AMONG THE MOST

common psychological disorders, affecting approxi-mately 13% of individuals at some point in their lives(Kessler, Petukhova, Sampson, Zaslavsky, &Wittchen, 2012). Cognitive-behavioral therapy(CBT) is an effective treatment for social anxietydisorder (Heimberg, 2002; Rodebaugh,Holaway,&Heimberg, 2004). However, a significant number ofindividuals do not benefit from CBT (Arch &Craske, 2009; Clark et al., 2006; Davidson et al.,2004). Recently, new behavioral treatments such asacceptance and commitment therapy (ACT; Hayes,Strosahl, & Wilson, 1999) have emerged that drawfrom Eastern mindfulness meditation practice, andpreliminary evidence supports their effectivenessfor anxiety disorders (Eifert & Forsyth, 2005).Understanding the mechanisms that drive treatmentresponse is essential for optimizing their delivery and

Available online at www.sciencedirect.com

ScienceDirectBehavior Therapy 45 (2014) 664–677

www.elsevier.com/locate/bt

This project was funded by the National Institutes of MentalHealth R21 MH081299 (PIs: Craske, Lieberman, and Taylor). Theclinical trial was registered onClinicalTrials.gov. under the identifierNCT00872820.

Address correspondence toMichelle G. Craske, Ph.D., Department ofPsychology, University of California, Los Angeles, 1285 Franz Hall, Box951563, Los Angeles, CA 90095-1563; e-mail: craske@psych.ucla.edu.0005-7894/45/664-677/$1.00/0© 2014 Association for Behavioral and Cognitive Therapies. Published byElsevier Ltd. All rights reserved.

improving outcomes (Kazdin, 2007). The goal of thecurrent study was to examine possible mediators oftreatment outcome in two treatments for socialanxiety disorder—CBT and ACT—to better under-stand why these treatments work.Testing mediators in randomized controlled trials

can tell us why and how treatments are effective(Kraemer, Wilson, Fairburn, & Agras, 2002), andcomparison of mechanisms across two activetreatments may ultimately help us tailor treatmentapproaches based on an individual’s presentation.For a rigorous test of treatment mediation, themediators must be tested during treatment andpreferably at multiple time points. Doing so ensuresthat the mediator temporally precedes the outcome(Kraemer et al., 2002), and assessing mediators atmultiple time points throughout treatment allowsassessment of change in mediators over time.Multilevel modeling is optimal for nested designswhere repeated measures are collected withinindividuals (Kenny, Kashy, & Bolger, 1998). Inaddition, multilevel modeling handles missing dataeffectively. As reviewed below, few studies haveexamined treatment mediators using this rigorousapproach.

Evidence for Treatment Mediators in CBTand ACT

The cognitive model for social anxiety disorder positsthat reductions in negative cognitions in relation tosocial situations explain subsequent symptom reduc-tion following CBT (Craske, 2010; Craske et al.,2008). In support of this purported mediator,Hofmann (2004) found that reduction in social costratings (patient ratings of “How bad would it be?” ifa feared social outcome occurred) from pre- to post-treatment predicted symptom reduction. However,since social cost ratings were not measured duringtreatment, rigorous testing of the mediator as atemporal precedent to the outcome was not conduct-ed. In another study, session-by-session ratings of theprobability of a negative social outcome predictedsubsequent fear reduction (Smits, Rosenfield,McDonald, & Telch, 2006). The mediator wasmeasured during treatment andprior to the outcome,and the authors used multilevel modeling to modelchange in the mediator over time. However, to fullyunderstand whether a mediator is specific to CBT(as opposed to common treatment processes), it isnecessary to compare CBT mediators with those ofanother active treatment (see Arch & Craske, 2008;Kraemer et al., 2002).ACT (Hayes et al., 1999) has been shown to be

effective for anxiety disorders (Arch, Eifert, et al.,2012), and in one randomized controlled trial, ACTwas effective for social anxiety disorder in particular

(Dalrymple&Herbert, 2007). ACT aims to promotemindfulness, acceptance, and cognitive defusion(learning to detach from thoughts and observe themmore dispassionately) with the ultimate goal ofincreasing psychological flexibility and promotingbehavior change that aligns with one’s life values(Hayes et al., 1999). Decreased experiential avoid-ance, or becoming more willing to experienceuncomfortable physical sensations and emotions,has beenproposedas a possiblemechanismof change(Hayes et al., 2004).In a study of ACT for social anxiety disorder,

Dalrymple and Herbert (2007) found that greaterincreases in acceptance and cognitive defusion bymidtreatment predicted better outcomes posttreat-ment, whereas greater perceived control over anxiety(a more CBT consistent mediator measure) did not.However, the meaningfulness of these results waslimited by the fact that the mediator was assessedonly once midtreatment, and difference scores werecalculated to assess the effect of the mediator ontreatment outcomes. Repeated measurement of themediator at multiple time points throughout treat-ment and subsequent analysis using growth curvemodeling would allow for a more fine-grainedassessment of how the mediator changes throughouttreatment.To our knowledge, only one study has compared

treatment mediators in CBT and ACT. Arch,Wolitzky-Taylor, Eifert, andCraske (2012) examinedtreatment mediators in CBT and ACT for individualswith a variety of anxiety disorders. Participants withpanic disorder, social anxiety disorder, generalizedanxiety disorder, and specific phobia were random-ized to 12 sessions of either CBT orACT. Participantscompleted measures of purported treatment media-tors (negative beliefs in CBT and cognitive defusionin ACT) repeatedly throughout treatment. Usingmultilevel modeling, both purported mediators werefound to change significantly in both treatments, withexperiential avoidance and negative cognitionsdecreasing more in ACT than in CBT. Also, changein both purported mediators significantly predictedsymptom reduction and increased quality of life inboth treatments, suggesting similarity in the changemechanisms in ACT and CBT.

current studyThe current study included analysis of session-by-session data from a treatment study in which ACTand CBT were compared for the treatment of socialanxiety disorder. Patients in both treatment groupsdemonstrated significant symptom reduction follow-ing completion of treatment, and the two groupsdid not significantly differ posttreatment, or at 6- or12-month follow-ups (Craske et al., 2014).

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We assessed the following research questions:First, do CBT and ACT affect negative cognitionsand experiential avoidance, which have been positedasmediators of treatment outcome inCBT andACT,respectively? In accordance with the theoreticalmodels underlying each treatment approach, wehypothesized that negative cognitions woulddecrease more in CBT than in ACT, and thatexperiential avoidance would decrease more inACT than in CBT. Even though previous researchshowed that experiential avoidance and negativecognitions decreased to a greater extent in ACT thanin CBT, we linked our hypotheses to theory in theabsence of replicated empirical data to the contrary.Second, we addressed whether negative cognitions

and experiential avoidance mediated treatmentoutcomes. The Baron and Kenny (1986) approachto mediation requires that the independent variableX is related to the outcome Y. Although nodifferences were found between treatment groups(X) on treatment outcome (Y; Craske et al., 2014),more recent approaches to testing mediation nolonger require a significant relationship between Xand Y (MacKinnon, Lockwood, Hoffman, West, &Sheets, 2002). Fritz and MacKinnon (2007) suggestthat the original Baron and Kenny (1986) approachis underpowered and increases the likelihood of TypeII error. In addition, Arch and Craske (2008) arguethat even in the absence of significant differences intreatment outcome, the examination of mediatorscan address important questions about similaritiesand differences in how these two treatments producechange. Thus, we proceeded with testing mediationin this trial despite no group differences in treatmentoutcome.In line with the respective theoretical models,

change in negative cognitions should mediate out-comes in CBT, whereas change in experientialavoidance should mediate outcomes in ACT. There-fore,we hypothesized that negative cognitionswouldpredict greater improvement in CBT than in ACT,whereas experiential avoidance would predictgreater improvement in ACT than in CBT. Again,this hypothesis was not supported in previousresearch, in which reductions in negative cognitionsand experiential avoidance similarly predicted treat-ment outcome acrossACTandCBT (Arch,Wolitzky-Taylor, et al., 2012), but in the absence of replicatedempirical data, we deemed it more logical tohypothesize based on theorized mechanisms.

MethodparticipantsSeventy-one participants who met DSM-IV criteriafor a principal or co-principal diagnosis of socialanxiety disorder, generalized type, were randomized

to ACT (n = 34) or CBT (n = 37). Analyses includedonly participants who completed treatment (n = 27ACT, n = 25 CBT) because we were interested inexamining treatment mediators for participantscompleting a full course of treatment. Two partici-pants were excluded from analyses due to largeamounts of missing data (N50%) on the session-by-session treatment measures. The final sample ana-lyzed included 50 participants (n = 25 ACT, n = 25CBT). See Craske et al. (2014) for participant flow ofthe full sample. Participants were recruited from theLos Angeles area in response to local flyers, Internetand local newspaper advertisements, and referrals.The study took place at the Anxiety DisordersResearch Center at the University of California, LosAngeles (UCLA), Department of Psychology startingSeptember 2008 and ending March 2013 uponcompleting collection of the desired sample. Forty-three percent of the sample was female. In termsof ethnicity, 13% identified as Latino/HispanicAmerican, 15% as Asian American, 59% asCaucasian, and 13% did not respond or indicated“other.”Themean ageof participantswas 28.4 years(6.5 SD, range 18–42) with 15.5 years of education(1.9 SD, range 12–19 years), and 7% were married,83% were single, 4% were cohabitating, and 6%were separated or divorced. Twenty percent ofparticipants had comorbid anxiety disorders and20% met criteria for major depressive disorder ordysthymia.Participants were either medication free or stabi-

lized on psychotropic medications for a minimumlength of time (1month for benzodiazepines and betablockers, 3 months for SSRIs/SNRIs, heterocyclics,and MAO inhibitors). Also, participants werepsychotherapy free or stabilized on alternativepsychotherapies (other than cognitive or behavioraltherapies) that were not focused on their anxietydisorder for at least 6 months prior to study entry.Participants were encouraged not to change theirmedication or alternative psychotherapy during thecourse of the study. Exclusion criteria included activesuicidal ideation, severe depression (clinical severityrating N 6, see below), or a history of bipolardisorder, psychosis, mental retardation, or organicbrain damage. Participants with substance abuse ordependence within the last 6 months, or withrespiratory, cardiovascular, pulmonary, neurologi-cal, muscular-skeletal diseases, or pregnancy wereexcluded. Patients with asthma, high blood pressure,or thyroid diseases were included only if they werecurrently receiving treatment and were stabilized forthese conditions. Because our study included neuro-imaging (results to be reported elsewhere) additionalexclusion criteria were left-handedness, metalimplants, claustrophobia, and over 45 years of age.

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Participants received12weekly sessions of reduced-cost, sliding-scale treatment and were financiallycompensated for post and follow-up assessments.The study was fully approved by the UCLA HumanSubjects Protection Committee; full informed con-sent was obtained from all participants, including forvideo and audio recordings.

designParticipants were assessed at four time points:pretreatment (pre), posttreatment (post), and 6months (6MFU) and 12 months (12MFU) after pre(although only pre and post measures are used in thecurrent study). Assessments included a diagnosticinterview and self-report questionnaires. Participantswere randomly assigned to ACT, CBT, or a wait-listcondition using a random number generator.Because no treatment measures were collected fromwait-list participants, they were not included in themediation analysis. Participants were stratified byage and gender in CBT and ACT to ensure equaldistribution across groups; study personnel did notinform patients of their treatment condition using theterms CBT or ACT, but rather informed patientsthey were receiving behavioral treatment withcognitive strategies (CBT) or behavioral treatmentwith acceptance and mindfulness strategies (ACT).

diagnostic assessmentAnxiety Disorders Interview Schedule for DSM-IV(ADIS-IV)The ADIS-IV (Brown, Di Nardo, & Barlow, 1994) isa semistructured interview that assesses for anxietydisorders, mood disorders, psychotic disorders, andalcohol and substance abuse and dependence. Withthe exception of dysthymia, diagnosis of psycholog-ical disorders using the ADIS-IV evidenced good toexcellent interrater reliability (κs range from .67 to.86; Brown,DiNardo, Lehman,&Campbell, 2001).All interviews were audio recorded and a subset wasrandomly selected (n = 22) for blind rating by asecond interviewer.1 Interrater reliability on theprincipal diagnosis (n = 22) was 100%.After completing the ADIS-IV, interviewers rated

the severity of all diagnoses in the past month usinga 0 to 8 clinician severity rating (CSR) scale. Scores of1 and 2 indicate that at least some symptoms havebeen present in the past month but severity,impairment, and distress are subclinical. A score of3 indicates that symptoms may be clinically signifi-cant. A score of 4 or above indicates moderately

severe symptoms associatedwith clinically significantdistress or impairment. Participants were eligible forthe study if they received a CSR rating of 4 or higher.The CSR rating has demonstrated good to excellentinterrater reliability for anxiety disorders (Brownet al., 2001; Craske et al., 2007).

treatmentsParticipants in CBT or ACT received 12 weekly,1-hour, individual therapy sessions based on detailedtreatment manuals.2 ACT and CBT were matchedon number of sessions devoted to exposure butdiffered in framing of the intent of exposure. Asubsample of therapy sessions were reviewed forindependent assessment of therapist adherence andcompetency, and therapists adhered strongly to theirassigned treatment approach (for further details,see Craske et al., 2014). Following the 12 sessions,therapists conducted follow-up booster phone calls(20, 35 mins) once per month for 6 months toreinforce progress consistent with the assignedtherapy condition.

Cognitive-Behavioral TherapyCBT for social anxiety disorder was derived largelyfrom standardCBTprotocols (e.g.,Hope,Heimberg,Juster & Turk, 2000), but differed in its inclusion ofinteroceptive exposure. We have shown this partic-ular CBT to be effective for individuals with socialanxiety disorder (Craske et al., 2011). Session 1focused on assessment, self-monitoring, and psy-choeducation. Sessions 2–4 emphasized cognitiverestructuring errors of overestimation and catastro-phizing regarding negative evaluation, combinedwith hypothesis testing (i.e., conducting behavioralexperiments with the purpose of disconfirmingnegative thoughts), self-monitoring, and breathingretraining. Exposure to feared social cues (includingin vivo, imaginal, and interoceptive exposure com-bined with in vivo exposure) was introduced inSession 5, and emphasized strongly in Sessions 6–11.Session 12 focused on relapse prevention.

Acceptance and Commitment TherapyACT for anxiety disorders largely followed amanualauthored by Eifert and Forsyth (2005).3 Session 1focused on psychoeducation, experiential exercises,and discussion of acceptance and valued action.Sessions 2 and 3 explored creative hopelessness orexplored whether efforts to manage and controlanxiety had “worked” and how such efforts had ledto the reductionor elimination of valued life activities,

1 Given the mixed anxiety disorder sample and subsequently lown per disorder, intraclass correlation coefficients for individualdisorders should be interpreted cautiously.

2 See author for a copy of the CBT treatment manual; the ACTmanual is published (Eifert & Forsyth, 2005).

3 Creative hopelessness was moved from Session 1 to Session 2.

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and encouraged acceptance. Sessions 4 and 5emphasized mindfulness, acceptance, and cognitivedefusion, or the process of experiencing anxiety-related language (e.g., thoughts, self-talk) as part ofthe broader, ongoing stream of present experiencerather than getting stuck in responding to its literalmeaning. Sessions 6–11 continued to hone accep-tance, mindfulness, and defusion, and added valuesexploration and clarification with the goal ofincreasing willingness to pursue valued life activities.Behavioral exposures (e.g., interoceptive, in vivo,imaginal) were employed to provide opportunities topractice mindfully observing and accepting anxiety,and to practice engaging in valued activities whileexperiencing anxiety. Session 12 reviewed whatworked and how to continue moving forward.

therapistsStudy therapists were advanced clinical psychologydoctoral students and recent Ph.D.s at UCLA, all ofwhom had at least 2 years of supervised training indelivering psychological treatments and at least 1year training in CBT or ACT. In addition, therapistscompleted intensive in-person 2-day workshops forCBT or ACT, prior to treating participants. Thera-pistswere assigned toACT,CBT, orboth (i.e., treatedin both CBT and ACT, though never at the sametime), depending on need.4 There were 28 therapists;13 therapists worked exclusively in CBT, 12 workedexclusively inACT, and 3 treated bothACTandCBTparticipants. There were no differences amongtherapists who provided CBT, ACT, or both interms of gender, age, or years since entering graduateschool (ps N .39). Generally, therapists treated one totwo patients at a time and two to five therapistsworked within each treatment condition at a time.The mean number of patients treated by CBT-onlytherapists was M = 2.38, SD = 1.56 (range 1–6,total = 31 participants), by ACT-only therapists wasM = 2.67, SD = 1.30 (range 1–5, total = 32 partic-ipants), and by therapists who treated both ACT andCBT was M = 5.67, SD = 2.52 (range 3–8, total =17 participants).Weekly, 90-minute group supervision meetings

were held separately for CBT and ACT. For CBT,the supervision was led by professors and postdoc-toral fellows at UCLA, and was held in person. ForACT, supervision was led by advanced therapistsfrom the University of Nevada, Reno, where ACTwas originally developed, via Skype.5

session-by-session measuresTo assess change over the course of treatment, twomeasures were administered at even numberedtreatment sessions (2, 4, 6, 8, and 10). The measureswere the 16-item Acceptance and Action Question-naire (AAQ; Hayes, Luoma, Bond, Masuda, &Lillis, 2006) and a modified version of the Self-Statements During Public Speaking Questionnaire(SSPS; Hofmann & DiBartolo, 2000). Clientscompleted the measures before each designatedtreatment session and returned them in a sealedenvelope to the clinic staff (not the therapist). TheAAQ was conceptualized as the ACT-specifictreatment mediator measure and the SSPS was theCBT-specific measure. Correlations between theAAQ and SSPS ranged from .47 to .71 (M = . 62)across the five treatment sessions at which thesemeasures were taken.

Acceptance and Action Questionnaire–16-ItemVersionThe AAQ (Hayes et al., 2004) was developedspecifically to measure the proposed mechanisms ofchange in ACT, namely experiential avoidance.Only psychometric properties for the 9-item versionhave been published, but Hayes and colleaguestested 7-, 9-, and 16-item versions, and state thatthe three scales perform “nearly identically” (Hayeset al., 2004). The authors also state that the 16-itemversion may be more useful to detect small changes(e.g., over the course of therapy) than the 9-itemversion because it contains more items (Hayes et al.,2004). The scalemeasures experiential avoidance andcontrol, negative evaluation of internal experience,psychological acceptance, and the tendency to actdespite emotional distress as opposed to avoiding dueto distress. Items are rated on a 7-point Likert scale(1 = never true, 7 = always true). Higher scoresindicate more experiential avoidance. Sample itemsinclude “I try to suppress thoughts and feelings that Ido not like by just not thinking about them” and “It’sokay to feel depressed or anxious” (reverse scored).Test–retest reliability for the 9-item version was .64over a 4-month period and internal consistency(Cronbach’s alpha) is .70 (Hayes et al., 2004). Forthe current sample, alpha was .86 across all treatmentsessions.

Self-StatementsDuringPublic SpeakingQuestionnaireThe SSPS (Hofmann&DiBartolo, 2000) is a 10-itemscale that measures negative and positive self-statements in the context of public speaking. Thescalewas developed to assess the cognitive componentof fear of public speaking. For purposes of the currentstudy, the scale was modified so that participantsrated their most anxiety-provoking social situa-tion rather than public speaking specifically. The

4 Therapists who provided CBT and ACT had experience inboth treatments.

5 UCLA supervisors observed but did not participate in ACTsupervision sessions.

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instructions read as follows: “Imagine your mostanxiety provoking social situation and consider thethoughts that might occur to you if you were to enterthis situation right now. Read each statement andrate the degree of agreement.” The scale consists oftwo subscales: one that assesses positive cognitions(SSPS-P), and one that assesses negative cognitions(SSPS-N). Although the SSPS-N subscale has shownthe greatest sensitivity to change (Hofmann &DiBartolo, 2000), to maximize the number of scaleitems, all 10 items were used and positive scale itemswere reverse coded. Cronbach’s alpha for the positivesubscale was .80 and for the negative subscale was.86, and test–retest reliability was acceptable for bothsubscales (Hofmann & DiBartolo, 2000). In thecurrent sample, alpha was .89 across all treatmentsessions for the full scale with positive items reversecoded.Alpha for thepositive subscalewas .86 and forthe negative subscale was .89.

outcome measuresWe assessed whether changes in SSPS and AAQduring treatment mediated outcomes at posttreat-ment.Given the emphasis on symptomreduction andsymptom mastery/control in CBT, and the emphasison acceptance, valued action, and living a meaning-ful life in ACT, we assessed social anxiety symptom-specific outcome measures as well as nonspecific orbroader outcomes across both treatments. Question-naires to assess social anxiety symptoms comprisedthe symptom-specific measures, and measures ofquality of life and depression comprised the nonspe-cific, broader outcomes. Each outcomequestionnaireis described briefly.

Social Anxiety Symptom ScalesWeselected threewidely used andwell-validated self-report measures of social anxiety symptoms. TheLiebowitz Social Anxiety Scale–Self-Report (LSAS-SR; Fresco et al., 2001) is a 24-item measure thatassesses fear and avoidance of social interactionaland performance situations. Each item is rated on ascale from 0 (no fear/never avoid) to 3 (severe fear/usually avoid). Scores were calculated as the sum offear and avoidance ratings across social and perfor-mance situations. In the current sample, Cronbach’sαs = .97 (pre) and .94 (post). The Social InteractionAnxiety Scale (SIAS; Mattick & Clarke, 1998) is a20-itemmeasure of cognitive, affective, or behavioralreactions to social interaction in dyads or groups.Participants respond on a Likert scale from 0 (notat all characteristic or true of me) to 4 (extremelycharacteristic or true of me). In the current sample,Cronbach’s αs = .96 (pre) and .95 (post). The SocialPhobia Scale (SPS; Mattick & Clarke, 1998) is a20-item measure describing situations or themes

related to being observed by others. Participants ratethe extent towhich each item is characteristic of themon a 0 to 4 scale. In the current sample,αs = .93 (pre)and .90 (post).

Composite Symptom ScaleA composite was created from the LSAS, SIAS, andSPS to enhance the assessment of each constructand reduce the number of analyses. Z scores werecalculated for each measure at pre and standardi-zation was based on pre means and standarddeviations for the follow-up assessment. Thecomposite score represented averages of the threemeasures and was used as our assessment of socialanxiety symptoms.

Quality of life. The Quality of Life Inventory(QOLI; Frisch, 1994) assesses values and lifesatisfaction across 16 broad life domains and hasgood test–retest reliability and internal validity(Frisch et al., 2005). In the current study, wecalculated a weighted score that accounts for bothsatisfaction and importance ratings for each lifedomain. In the current sample, Cronbach’s αs = .85(pre) and .84 (post).

Depression. The Anhedonic Depression Scale ofthe Mood and Anxiety Symptom Questionnaire(Watson & Clark, 1991) is a 22-item measure ofdepressive symptoms. Participants indicate theextent to which they have experienced each of the22 symptoms on a 1 to 5 Likert scale where 1 = notat all and 5 = extremely. The scale demonstratesgood convergent and discriminant validity in com-munity samples (Watson,Clark, et al., 1995;Watson,Weber, et al., 1995). Current αs were .93 (pre) and.94 (post).

statistical analysesTreatment of Missing DataFor the session-by-session measures (AAQ andSSPS), therapists administered the questionnaires ateven-numbered treatment sessions. Thirty-eightparticipants completed measures at all five sessions,11 completed four measures, 1 completed threemeasures, and 2 completed two measures. The 2participants who completed less than 50% of thesession measures were excluded from analyses. Forthe outcomemeasures (symptoms, quality of life, andanhedonic depression), 4 participants were missingdata at baseline with 2 participants missing only thequality-of-life measure and 2 missing all threemeasures. At posttreatment, 7 participants weremissing data: 3 missing data only on the quality-of-life measure and 3 missing data on all threemeasures.

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To account for missing data in analyses, multilevelmodeling was used to analyze change in session-by-session measures over time (see details below), andmultiple imputation using a multivariate normalmethod with 20 imputations was used to estimatemissing data on outcome measures for mediationanalyses.Consequently, 50participantswere includedin data analyses.

Change in experiential avoidance (AAQ) and cogni-tions (SSPS) during treatment. To test whether therewas a significant decline in AAQ and SSPS duringtreatment and whether the rate of change in thesemeasures differed by treatment group, we usedmultilevel modeling in Stata 12. The ACT groupwas coded as 1 and the CBT group as 0. To estimatechange in AAQ and SSPS over time, we modeled timeat Level 1, and individual at Level 2. The Level 1predictor was session, and the Level 2 predictor wasgroup. To test whether change in each mediatordiffered by group, we included the Session × Groupinteraction. We modeled session as a continuousvariable and examined the quadratic effect. Therefore,the model included the following predictors: Session,group , Ses s ion × Group , Ses s ion 2 , andGroup × Session2. Random effects of intercept, linearslope, and the covariance among them were included(i.e., unstructured Level 2 variance/covariancestructure). The random effect of the session2 termwas not includedbecause its inclusion did not improvethe model fit. For Level 1 residuals, within-grouperrorsweremodeledusing anautoregressive structure.

Experiential avoidance (AAQ) and cognitions (SSPS)as mediators of treatment outcome. Mediation wastested using the MacArthur guidelines as outlined byKraemer et al. (2002). Using this method, mediatorsof treatment outcome must (a) occur during treat-ment, (b) correlate with treatment condition, and(c) either relate directly to treatment outcome orinteract with treatment group in relation to theoutcome. For all analyses using AAQ and SSPS aspossible mediators, criterion (a) was met given thatthese variables were assessed during treatment.Criteria (b) and (c) were tested using seeminglyunrelated regression (SUR) in Stata 12. SUR allowsfor simultaneous estimation of multiple equations.This method calculates parameter estimates andstandard errors accounting for the correlationamong residual variances among models. Thismethod produces more efficient parameter esti-mates than ordinary least squares (OLS) regres-sion, and allows for calculation of the indirecteffect for multiply, imputed data. For details onhow to implement this method, see the UCLA Stata

Frequently Asked Questions page (UCLA: StatisticalConsulting Group, 2013).Outcome variables included symptoms, quality of

life, and anhedonic depression. Because quadratictrends were found in the AAQ and SSPS (see theResults section and Fig. 2), using Stata 12, estimates ofthe quadratic effect for each participantwere obtainedby extracting random effects of session2 for eachparticipant and adding those random effects to thefixed effect of session2. These estimates represent thedirection and steepness of curvature for each partic-ipant. Positive curvature indicates that the curve isconvex, or that change occurs more quickly at thebeginning of treatment and levels off toward the end.Negative curvature indicates that the curve is concave,or that change occurs more slowly at the beginningof treatment and then speeds up toward the end.The estimates of curvature were examined as media-tors using SUR. The independent variable wastreatment condition with ACT coded as 1 and CBTcoded as 0.For each mediator and outcome, we first tested

moderated mediation, or whether treatment groupinteracted with the mediator to affect outcome(see Fig. 1A). For moderated mediation, the tworegression equations included in each SUR were asfollows: The first equation included the mediatorvariable regressed on the outcome variable at pre(covariate) and treatment condition (independentvariable). The second equation included the out-come variable at post regressed on the outcomevariable at pre (covariate), the mediator, treatmentcondition (independent variable), and mediator ×treatment condition. If moderated mediation wasnot found, mediation was tested (see Fig. 1B). Formediation, the two regression equations included ineach SUR were as follows: The first equationincluded the mediator variable regressed on theoutcome variable at baseline (covariate)and treatment condition (independent variable).The second equation included the outcome variableat post regressed on the outcome variable at pre(covariate), the mediator, and treatment condition(independent variable). Finally, if mediationwas not found, we tested the mediators aspredictors of treatment outcome using OLS regres-sion (see Fig. 1C): the equation included theoutcome variable at post regressed on the outcomevariable at pre (covariate), the predictor, andtreatment condition (as a covariate).

ResultsReferences to AAQ and SSPS refer to scores onthe mediation measures taken every other sessionfrom Session 2 through Session 10 (5 total datapoints).

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change in experiential avoidance (aaq) andcognitions (ssps) during treatmentAcceptance and Action QuestionnaireResults for the AAQ are displayed in Fig. 2A.The Session2 × Group interaction was significant(b = .35, 95% confidence interval (CI) = .17 to.54, p = .001). AAQ scores declined in bothgroups. The quadratic term was significant withinACT (b = .20, z = 2.76, p = .006) and CBT (b =–.16, z = –2.60, p = .009). Because the beta coef-ficient associated with the quadratic term waspositive in ACT, the curve was convex, meaningthat the decline in ACT was steeper at the beginningof treatment and leveled off toward the end oftreatment. In CBT, because the beta coefficientassociated with the quadratic term was negative,the curvewas concave,meaning that declinewas levelat the beginning of treatment and became steeper

toward the end of treatment. AAQ scores weresignificantly lower in ACT than CBT at Sessions 4,6, 8, and 10 (ps b .012), but did not significantlydiffer at Session 2 (p = .051).

Self-StatementsDuringPublic SpeakingQuestionnaireResults for the SSPS are displayed in Fig. 2B. TheSession2 × Group interactionwas significant (b = .21,CI = .03 to .40, p = .025). SSPS scores declined inboth groups. The quadratic term was significant inACT (b = .19, z = 2.33, p = .020), and the linearterm, but not the quadratic term, was significant inCBT (b = −1.56, z = −8.60, p b .001). The curve inACT was convex, indicating that the decline in ACTwas steeper at the beginning of treatment and leveledoff toward the end of treatment. The rate of declinein CBT remained constant throughout treatment.SSPS scores were significantly lower in ACT than in

FIGURE 1 Moderated mediation (A), mediation (B), and prediction (C) models tested.

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CBT at Sessions 4, 6, and 8 (ps b .039), but did notsignificantly differ at Sessions 2 or 10 (ps N .327).

experiential avoidance (aaq) andcognitions(ssps) as mediators of treatment outcomeFor all analyses using AAQ and SSPS as possiblemediators, criterion (a) was met given that thesevariables were assessed during treatment. In thissection, we tested simultaneously whether criteria(b) and (c) were met using SUR. For these analyses,the mediators were the curvature (coefficientassociated with Session2) of AAQ and SSPS foreach individual participant. For each mediator, wefirst tested whether treatment group interacted withthe mediator to affect outcome (moderated medi-ation). In the absence of a significant interaction, wetested the mediated effect regardless of treatmentgroup. Finally, if no mediation was found, we tested

the mediators as predictors of treatment outcome(see Fig. 1 for models).

Social Anxiety SymptomsResults are displayed in Table 1. For AAQ curvature,the moderated mediation model was significant.Group was a significant predictor of AAQ curvature(b = .23,p b .001) such that participants inACThadsignificantly more positive curvature (more convexcurve) than participants in CBT. Group significantlyinteracted with AAQ curvature to predict socialanxiety symptoms (b = −2.72, p = .024, η2 = .16).Tests of simple effects revealed a significant indirecteffect (a × b) for ACT (b = − .57, p = .006), but notCBT (b = .05, p = .813), such that more positiveAAQ curvature (more convex curve) was associatedwith greater symptom reduction in ACT, but not inCBT. Therefore, criterion (c) was met and AAQ

FIGURE 2 Change in (A) Acceptance and Action Questionnaire and(B) Negative Self-Statements over sessions 2 through 10.

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curvature mediated symptom reduction in ACT, butnot in CBT.For SSPS curvature, neither the moderated medi-

ation model nor the mediation model was significant(ps N .451). For the prediction model, SSPS curva-ture significantly predicted social anxiety symptoms(b = −1.97, p = .002, η2 = .14) such that morepositive SSPS curvature (more convex curve) wasassociated with fewer social anxiety symptoms atpost across both groups. Criterion (c) was not metand SSPS curvature did not significantly mediatesymptom outcome. However, SSPS curvature didsignificantly predict symptom outcome.

Quality of LifeFor AAQ and SSPS curvature, neither the moderatedmediation model, nor the mediation model, nor thepredictor model was significant (ps N .106). There-fore, criterion (c) was not met and neither AAQ norSSPS curvature mediated quality of life.

Anhedonic DepressionResults are displayed in Table 1. For AAQ curvature,the moderated mediation model was significant.Group was a significant predictor of AAQ (b = .23,p b .001) such that participants in ACT hadsignificantly higher AAQ curvature than participantsin CBT. Group significantly interacted with AAQcurvature to predict anhedonic depression (b =−74.76, p = .001, η2 = .21). Tests of simple effectsrevealed a significant indirect effect (a × b) for ACT(b = −8.19, p = .030), but not CBT (b = 8.85, p =.071) such that more positive AAQ curvature (moreconvex curve) was associatedwith greater anhedonic

depression reduction in ACT, but not in CBT.Therefore, criterion (c) was met and AAQ curvaturemediated anhedonic depression in ACT, but not inCBT.For SSPS curvature, neither the moderated medi-

ationmodel nor the mediationmodel was significant(ps N .453). For the prediction model, SSPS curva-ture significantly predicted anhedonic depression(b = −29.74, p = .017, η2 = .15) such that morepositive curvature of SSPS (more convex curve) wasassociated with fewer social anxiety symptoms atpost across both groups. Criterion (c) was not metand SSPS curvature did not significantly mediateanhedonic depression. However, SSPS curvature didsignificantly predict anhedonic depression.

DiscussionThe first goal of the current study was to examinehow theorized mediators in ACT and CBT changedover the course of treatment. In particular, wewere interested in whether avoidance of uncom-fortable internal experiences (experiential avoidance)decreased to a greater extent in ACT than in CBT,and whether negative cognitions in social situationsdecreased to a greater extent in CBT than in ACT.Experiential avoidance showed a nonlinear decline inboth treatments, with ACT showing a convex curve(steeper at the beginning of treatment) and CBTshowing a concave curve (steeper at the end oftreatment). As hypothesized, experiential avoidancedecreased to a greater extent in ACT than inCBT. This finding is not surprising given that ACTdirectly targets experiential avoidance in the first

Table 1Significant moderated mediation and prediction models (see Fig. 1 for models)

Moderated Mediation (mediator interacts with treatment group)

Dependent Variable Mediator X → M(a path)

X*M → Y(b2 path)

M → Y(b1 path)

indirect effectACT (a*(b2 + b1))

indirect effectCBT (a*b1)

Social Anxiety Symptoms AAQ curvature .23⁎⁎⁎

(.12 to .34)−2.7⁎(-5.1 - .36)

.23(-1.7 to 2.2)

-.57⁎⁎

(-.97 to -.16).05(-.39 to .50)

Depressive Symptoms AAQ curvature .23⁎⁎⁎

(.12 to .33)−75⁎⁎(-118 to -32)

39(.89 to 77)

−8.2⁎(-16 to -.80)

8.9(-.76 to 18)

Prediction (non-significant mediators)

Dependent Variable Predictor P → Y(b path)

Social Anxiety Symptoms SSPS curvature −2.0⁎⁎(-3.2 to -.76)

Depressive Symptoms SSPS curvature −30⁎(-54 to -5.8)

Note. * p b .05, **p b .01, *** p b .001; X = treatment condition (0 = CBT and 1 = ACT); M = mediating variable; Y = outcome at Post;P = predictor variable; AAQ curvature = non-linear effect of Acceptance and Action Questionnaire; SSPS curvature = non-linear effect ofSelf Statements Questionnaire.

673med iators of treatment for soc ial anx i e ty

five sessions. However, during the second half oftreatment in which exposure to reduce fearfulness isemphasized in CBT and exposure to increase living avalued life is emphasized in ACT, the rate of declinein experiential avoidance increased in CBT andslowed in ACT. This suggests that the cognitivedefusion approach used in ACT and the behavioralexposure approach used in CBT may have themost pronounced effect on willingness to experienceuncomfortable internal experiences, although thiscannot be directly tested in the current study. Thisfinding replicates previous research, which showedgreater decreases in experiential avoidance in ACTthan in CBT (Arch, Wolitzky-Taylor, et al., 2012).Negative cognitions also decreased significantly in

both treatments. The decrease was nonlinear andconvex (steeper at the beginning of treatment) inACT, andwas linear inCBT.Contrary to hypotheses,at the beginning of treatment, negative cognitionsdecreased more quickly in ACT than in CBT.However, by the end of treatment, both groups hadshown the same reduction in negative cognitions asthe decline slowed in ACT but remained constant inCBT. This finding replicates previous research, whichshowed greater decreases in negative cognitions inACT than in CBT (Arch, Wolitzky-Taylor, et al.,2012). Again, although not directly tested, thecognitive defusion approach used in ACT and thebehavioral exposure approach in CBT seem to havecaused the greatest reduction in negative cognitions.This finding is surprising given that ACT does notdirectly aim to change thinking patterns while CBTdoes. However, cognitive restructuring has beencompared with thought suppression (Hayes et al.,1999) because it labels certain cognitions as “faulty,”which may encourage suppression of those thoughts.Suppression is thought to be counterproductive,according to the ACT framework, as it can result inincreased intrusion of suppressed thoughts (Wegner,1994). In addition, Eifert and Forsyth (2005) haveargued that focusing attention on the content ofnegative thoughts maintains the ruminative cycle.These findings lend support to the idea that mindful-ness and acceptance may be a more effective way ofreducing negative thoughts than cognitive restructur-ing, while behavioral exposure to feared situations inCBT may prove to be more effective at reducingnegative thoughts than the values-driven approachemphasized in ACT. In other words, despite similar-ities between the behavioral approaches in ACT andCBT, the differences in framing and structure appearto yield differential effects on cognitive change.The second goal of the study was to examine

experiential avoidance and negative cognitions asmediators of treatment outcome.We tested whetherthe curvature (or nonlinear effect) of experiential

avoidance andnegative cognitionsmediatedoutcomedifferentially in ACT and CBT (moderated media-tion), and if there were not group differences, whetherthe mediators were shared between the two treat-ments. If neither mediation model was significant, wetested a prediction model. Using the MacArthurguidelines as outlined by Kraemer et al. (2002), thecurvature in experiential avoidance emerged as asignificant mediator of social anxiety symptomreduction and depressive symptom reduction inACT and not in CBT such that participants withgreater reduction in experiential avoidance at thebeginning of treatment had better outcomes onlywithinACT.Consistentwith hypotheses and contraryto previous results examining treatment mediation(Arch, Wolitzky-Taylor, et al., 2012), experientialavoidance was a mechanism of treatment outcomeonly within ACT. Although curvature in negativecognitions did not differ between the two treatmentsand therefore was not a significant mediator, it was apredictor of improvement regardless of group suchthat greater reduction in negative cognitions at thebeginning of treatment predicted better outcome.These findings indicate that how quickly negativecognitions change in treatment is an important factorin improvement across both treatments.It is important to note that no significant differ-

ences in treatment outcome emerged between ACTand CBT, which makes interpretation of mediationresults more complex. For experiential avoidance,greater reductions were found in ACT than in CBT,and experiential avoidance was related to bettertreatment outcome more so in ACT than in CBT.These findings imply better outcomes in ACT thanCBT, but we did not find such differences. There areat least two factors that may explain this contradic-tion. The first is an additionalmediator that decreasesto a greater extent in CBT than in ACTmay not havebeen measured in the current study. Possibilitiesinclude control over negative thoughts, self-efficacy,behavioral avoidance, or adherence to behavioralexposures. The second possibility is that ACT did, infact, work better than CBT, but that we did not havesufficient power to detect this difference. As indicatedin Craske et al. (2014), we had enough power todetect only a large effect size difference between ACTand CBT. That being said, differences between ACTand CBT did not even approach significance in thecomplete sample, and for some outcome measures,participants in CBT showed marginally greatersymptom reduction than did those in ACT (Niles,Mesri, Burklund, Lieberman, & Craske, 2013). Inanother study comparing ACT and CBT acrossmultiple anxiety disorders with a larger sample size,no group differences were found (Arch, Eifert, et al.,2012). Therefore, it is unlikely that ACT would have

674 n i le s et al .

outperformed CBT had our sample been larger. It ismore likely that the key mechanism of change forCBT was not measured in the current study.These findings provide evidence that the rate of

change in experiential avoidance and negativecognitions, particularly at the beginning of treatment,does, in fact, explain a significant portion of thevariance in treatment outcome (approximately 15–20%), albeit differentially in ACT and CBT. This isthe first study to show that change mechanisms maydiffer in ACT and CBT. Given that experientialavoidance is the primary target of change in ACT, itis not surprising that experiential avoidance was asignificant mediator only for ACT. However, theresults conflict with an earlier study with a mixedanxiety disorder sample (Arch, Wolitzky-Taylor,et al., 2012), where differential mediation in ACTand CBT was not established. One possible explana-tion of the contradictory findings is that treatmentmechanisms differ for a social anxiety sample and amixed anxiety sample. The studies also differed in theway the mediators were measured and treated inanalyses. For the ACT-specific mediator, the currentstudy used the AAQ, which is more widely validatedthan the Believability of Anxious Feelings andThoughts Questionnaire used in the previous study.TheCBT-specificmediator for the current studywas acognitive measure specific to social situations, where-as the measure in the previous study, the AnxietySensitivity Index, was intended to capture cognitivemechanisms across anxiety disorders. Finally, thecurrent study examined nonlinear change of themediators and used simultaneous regression modelsto test mediation, while the previous study assessedlinear change for the mediators and tested mediationusing two separate models. Therefore, although bothapproaches can be used to assess mediation, thedifferent methods used in the two studies couldexplain the difference in findings.Several study limitations should be noted. First, we

were only able to test one hypothesized mediator ofeach treatment model given that measurements weretaken repeatedly throughout treatment and concernsabout participant burden limited the number ofmediators that could be assessed. Future studies mayaim to assess additional mediators, including con-trollability of negative thinking, self-efficacy, or thecontent of anxiety-related thoughts for CBT, andacceptance, cognitive defusion, behavioral commit-ments, or values clarification for ACT. Second, ACTand CBT were matched on the number of sessionsspent on exposure exercises, which likely minimizedgroupdifferences inmediationpathways.Comparingcognitive therapy to a mindfulness-based approachto anxiety treatment would likely result in greatergroup differences in mediation. Third, our assessed

mediators and outcomes were based on self-reportquestionnaires. Future studies would benefit fromintegrating behavioral and brain-based measure-ments of mediators and outcomes. Fourth, the SSPSwas developed for use in a public speaking contextand reflects cognitions that individuals with socialphobia report following public speaking. We there-fore cannot be certain that the negative cognitionscaptured with this measure are the same as thoseexperienced in other social situations. Fifth, the test–retest reliability of the AAQ-9 was only .64 , whichindicates that the measure may not be sufficientlystable over time. The 16-item version, however,demonstrated a correlation of .86 between Sessions 8and 10 of treatment, indicating that within oursample, themeasurewas quite stable. Sixth, the AAQand SSPSwere significantly correlated (r ranged from.47 to .71), indicating that these measures assesssimilar constructs. Despite significant overlap, how-ever, it is notable that mediation results differedbetween the two measures. Finally, our sample wasrelatively young,well educated, andmore likely to besingle compared with the population. Therefore, it ispossible that our results would not generalize to amore diverse sample.In conclusion, these findings shed light on how

cognitive processes change in ACT andCBT for socialanxiety disorder and how these processes relate totreatment outcome. For both treatment approaches,negative thoughts decreased and willingness tohave uncomfortable internal experiences increased,indicating that these treatment approaches overlapsignificantly in terms of their effect on cognitivechange. Arch and Craske (2008) reviewed thesimilarities between these two treatment approaches,and the current project adds to existing research onthese similarities (Arch, Wolitzky-Taylor, et al.,2012). Patients whose negative thinking decreasedmore quickly at the beginning of treatment demon-strated the greatest symptom reduction across bothtreatment approaches, and those with the fastestincrease in willingness to experience uncomfortablesensations had greater symptom reduction in ACTbut not in CBT. In other words, faster reduction innegative cognitions at the beginning of treatmentpredicted overall improvement, and faster reductionin experiential avoidance predicted improvementonly for ACT.These findings have a number of important

implications for treatment. First, change in the coremechanisms of psychopathology early in treatmentmay enable subsequent symptom change to occur inthe time remaining before the end of treatment. Byextension, the role of early change may differ ifsymptom change was examined over a lengthierinterval. Second, these findings could indicate that

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change in negative beliefs is central to social anxietypsychopathology independent of treatment ap-proach, whereas change in acceptance is less relevantto psychopathology in general, but more relevant tothe effects of a specific treatment strategy. Third,early change in core mechanisms may allow othercritical mechanisms, such as engagement in exposureand values-driven activities, to take place, whichmaycontribute to better symptom outcome. Finally, thesefindings highlight the importance of strategiesspecifically designed to reduce negative cognitionsand experiential avoidance early in treatment, suchas emphasizing cognitive defusion and exposurewithin the first few sessions of treatment. Overall,these findings add to a growing body of literature ontreatment mediation for anxiety disorders and beginto shed light on differences in mechanisms betweenACT and CBT for social anxiety disorder.

Conflict of Interest StatementThe authors declare that there are no conflict of interest.

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