































Graduate Student Journal of Psychology          Copyright 2012 by the Department of Counseling and Clinical Psychology 
2012, Vol. 14               Teachers College, Columbia University 

53 

  

Mediators and Mechanisms of Psychotherapy: 
Evaluating Criteria for Causality 

 
Joshua Wilt 

Northwestern University 
 
Although the efficacy of psychotherapy for treating many disorders has been demonstrated in 
randomized clinical trials (Ekers, Richards, & Gilbody, 2008; Hofman & Smits, 2008; Hoglend, 
1999), little is known about the mechanisms through which psychotherapy achieves its effects 
(Kazdin, 2005).  The purpose of this review is to examine the state of research on the mechanisms 
of psychotherapy.  First, a definition of mechanism is presented and the importance of studying 
mechanisms in psychotherapy is discussed.  Then, methods of demonstrating mediation in 
empirical studies are reviewed along with selected areas of empirical research on mechanisms of 
cognitive therapy.  Finally, suggestions for future directions in studying mechanisms are offered. 
 

 
A mechanism is defined as a step or series of steps in a 

process through which change is produced (Johannson & 
Hoglend, 2007).  Each intermediate step in a process of 
change can also be thought of as a mediator―a mediator is an 
intervening variable that is causally related to the mechanism 
of change (Doss & Atkins, 2006). This definition of 
mediation is crucial to research on how psychotherapy works, 
as research in the aim of elucidating psychotherapy 
mechanisms is primarily concerned with identifying 
mediators of change.  In psychotherapy research, a mediator 
is a construct that explains why therapy results in symptom 
improvement.  Mediation must therefore take place during the 
course of psychotherapy and before symptom change.   

It is important to distinguish mediation from causality.  
Logically, causality is a necessary condition for mediation; 
only when something has a causal effect can the intervening 
mediating pathway be established.  A common misconception 
in psychotherapy research until recently was that establishing 
causal effects gives information about possible mediators of 
those effects (Kazdin, 2007).  For example, if cognitive 
therapy were shown to be effective, it may have been 
assumed to work through cognitive change.  However, this 
line of reasoning is faulty.  Knowing that therapy works is not 
in any way informative of the way in which therapy 
works―based on efficacy research alone, it is likely to be 
difficult to draw any conclusions regarding the mechanism(s) 
of change without measurement of potential mediators.  In the 
example of cognitive therapy, there is a chance that affective 
change (or any variable, no matter how implausible) may 
actually explain why cognitive therapy was shown to be 
effective.  Mediators must be evaluated on their own terms, 
rather than being inferred from the modality of therapy. 

                                                 
Joshua Wilt, Department of Psychology, Northwestern 

University, Evanston, IL.  
Correspondence concerning this article should be addressed to 

Joshua Wilt, Northwestern University, Department Psychology, 
Swift Hall 109, Evanston, IL 6020. Email: 
joshuawilt2008@u.northwestern.edu. 

It is important to study the mechanisms of psychotherapy 
for at least four reasons.  The first reason is based on the 
extremely high proliferation of different types of 
psychotherapy.  It is highly unlikely that all of these types of 
therapy work in unique ways; therefore, identifying common 
mediators across psychotherapies may add parsimony to the 
varied theories of what happens when psychotherapy works 
(Johansson & Hoglend, 2007).  Second, identifying mediators 
may advance the understanding of differential responses to 
treatment.  Even in the most successful clinical trials, only 
60% of depressed patients show clinically significant 
symptom improvement (DeRubeis, Tang, & Beck, 2001).  
Third, identifying mediators may maximize treatment 
effectiveness (Laurenceau, Hayes, & Feldman, 2007).  
Psychotherapies contain multiple components, and yet each 
component may not be causally related to the mechanism by 
which therapy works.  For example, although Cognitive 
Behavioral Therapy (CBT) includes both cognitive and 
behavioral components, it is possible that only cognitive or 
only behavioral components are causally related to change 
depending on the disorder being treated.  Identifying 
mechanisms may allow therapists to focus on only those 
aspects that are actively contributing to the patient’s 
improvement, making therapy much more efficient and 
economical.  Knowledge about how therapy works also 
allows for generalization from clinical trials.  A common 
criticism of randomized clinical trials (RCTs), a clinical 
experiment in which participants are randomly allocated to 
receive different forms of treatment,  is that the various 
constraints and artificialities imposed on participants prevent 
the results from transferring to “real world” psychotherapy 
(Doss & Atkins, 2006).  If it was known why a specific 
psychotherapy worked, this knowledge would be potentially 
transferrable across a wide range of community settings.  
Fourth, there is the potential for benefits outside the realm of 
psychotherapy.  Specifically, knowing about the process by 
which maladaptive functioning changes into adaptive 
functioning may reflect upon the processes underlying 
different forms of psychopathology and healthy functioning 
in general. 



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54 
 

Criteria for Mediation 
 
Causality may be logically identified when an 

experiment utilizes random assignment of participants.  
Therefore, RCTs by definition are able to establish that 
psychotherapy causes symptom improvement.  However, 
there is no comparable method to show that a possible 
mediator is responsible for the effects of psychotherapy.  This 
is because the variables that may be responsible for change 
are left to vary freely rather than being randomly assigned.   

The lack of a definitive method for showing that 
mediators are part of a causal mechanism presents a difficult 
hurdle for achieving certainty regarding potential mediators. 
Numerous criteria have been proposed that supposedly 
strengthen the inferences one can make about whether a 
putative mediator is actually a part of the mechanism of 
psychotherapy (Doss & Atkins, 2006; Johansson & Hoglend, 
2007; Kazdin, 2007; Nock, 2007; Weersing, 2006).  
However, the myriad criteria have not been reviewed in a 
systematic way.  It is important to organize these criteria in 
order to establish a method for evaluating the degree of 
support for a given mediator as a link in the causal chain 
between psychotherapy and treatment outcome.   

There are two important clarifications to make before 
discussing the criteria for mediation.  First, because RCTs are 
the gold standard in psychotherapy research, much of the 
discussion of criteria below centers on research conducted in 
the context of an RCT; however, with the exception of 
Criterion 4 (that the mediator was discovered in the context of 
an RCT), each of the criteria may be applied to non-RCT 
designs.  Second, the distinction between full mediation (in 
which a mediator completely accounts for the association 
between treatment and outcome) and partial mediation (in 
which a mediator accounts for only part of the association 
between treatment and outcome) is an important issue but is 
outside the scope of this paper.  The discussion focuses on 
mediation in the general sense―that is, whether a particular 
mediator is part of the causal chain linking treatment to 
outcome. 

The first criterion, temporal precedence, is a necessary 
condition for mediation (i.e., the mediator must occur 
temporally before symptom change).  This criterion had not 
been explicitly recognized by psychotherapy research until 
recently, as it only just became common practice to measure 
mediator variables and symptom changes at multiple time 
points in psychotherapy studies (Hayes, Laurenceau, 
Feldman, Straus, & Cardaciatto, 2007; Laurenceau et al., 
2007).  It is a logical flaw to claim as a potential mediator any 
changes that occurred after symptoms have already changed.   

The most commonly cited evidence used to support 
mediation are statistical criteria (Baron & Kenny, 1986); see 
Figure 1 for a depiction of this model as applied to 
psychotherapy).  Each path in the figure signifies a criterion 
that must be met in order for mediation to be achieved.  Path 
“a” indicates that the relationship between treatment and 
outcome must be statistically significant.  Path “b” indicates 

that the relationship between the treatment and mediator must 
be significant, and path “c” indicates the relationship between 
the mediator and outcome must be significant.  Path “d” is 
shown as a dotted line, symbolizing that the relationship 
between treatment and outcome should be significantly 
reduced when taking the mediator into account.  More recent 
statistical mediation models (MacKinnon, Lockwood, 
Hoffman, West, & Sheets, 2002) have relaxed the Baron and 
Kenny model by excluding path “d” from the mediation 
model in order to maximize power to detect significant 
mediation effects.  There are various other statistics that 
assess mediation (Fritz & MacKinnon, 2007); however, 
distinguishing between those statistics is beyond the scope of 
this paper.  The point of establishing statistical mediation is 
that it lessens the possibility that a potential mediator went 
unidentified due to Type II error alone.  For example, 
consider a case in which multiple mediators (A, B, and C) are 
examined in the same study.  If mediator A meets statistical 
criteria and mediators B and C do not, then it is probable that 
mediators B and C failed to meet statistical significance 
because they are less causally implicated in the mechanism of 
psychotherapy (relative to mediator A), rather than for power 
reasons alone.  Establishing statistical mediation in the 
context of an RCT strengthens the case for the potential 
mediator being examined.  This is because, if random 
assignment was correctly implemented, pre-existing 
conditions cannot account for the effects of the mediator.  
However, it is still entirely possible that a different variable 
that changed as a result of therapy accounts for the effects of 
the mediator being examined.  In addition to statistical 
criteria, there are a number of other criteria that can be used 
to evaluate the likelihood that a potential mediator is a causal 
component of therapy.  These additional criteria are helpful in 
ruling out or at least reducing alternative possibilities, thereby 
strengthening inferences that can be made about potential 
mediators.  

For example, RCTs can examine whether mediators meet 
a dose-response criterion; this means that, the more change in 
the mediator, the more change in the outcome.  Dose-
response increases confidence in the conclusion that 
mediators systematically relate to outcome across the range of 
possible outcomes; mediators that do not meet the dose-
response criterion may only be relevant or related to 
outcomes in a very narrow range.  For example, if a moderate 
amount of cognitive change is associated with moderate 
symptom improvement, and a high amount of cognitive 
change is associated with high symptom improvement, the 
range over which cognitive change is relevant to the outcome 
would be greater than if all levels of cognitive change were 
associated with moderate symptom change only.   

The specificity of mediators may also be examined.  
Specificity refers to the fidelity of a mediator to a particular 
form of psychotherapy.  For example, if cognitive change met 
criteria for statistical mediation in cognitive therapy but not in 
behavioral therapy, then cognitive change would have higher 
specificity than if it were shown to mediate the effects of both  



MEDIATORS AND MECHANISMS OF PSYCHOTHERAPY 
 

55 
 

 
Figure 1. The mediation model of Baron and Kenny (1986) as applied to psychotherapy.   
 
Note. Path “a” indicates that the relationship between treatment and outcome must be statistically significant.  Path “b” indicates that the 
relationship between the treatment and mediator must be significant, and path “c” indicates the relationship between the mediator and outcome 
must be significant.  Path “d” is shown as a dotted line, symbolizing that the path between treatment and outcome should be significantly 
reduced when taking the mediator into account. 
 
 
cognitive therapy and behavioral therapy.  Specificity says 
more about the psychotherapy in question than the mediator 
per se.  It is possible that all psychotherapies work through 
the same or similar mechanisms―in this extreme case, there 
would be no specificity for mediators even though the 
mediator would be completely valid.  However, if a particular 
brand of psychotherapy were not tied to a specific mediator, 
then the validity of the unique components of the 
psychotherapy could be called into question. 

If multiple RCTs for the same psychotherapy found 
evidence supporting a particular mediator, then the mediator 
would have high consistency.  High consistency reduces the 
possibility that the mediator in question met the statistical 
definition of mediation due to Type I error.  Similarly, 
converging evidence―evidence obtained across a variety of 
methodologies in a variety of settings―increases confidence 
in the conclusion that the mediator is not simply an artifact of 
the context of a particular RCT.  Converging evidence may be 
obtained through lab studies, field studies, and even studies 
examining different species.  For example, suppose that 
exposure therapy for spider phobia was thought to reduce 
phobic symptoms by reducing autonomic arousal.  Then, 
evidence that both individuals without clinically significant 
phobic symptoms as well as rats exhibit diminished 
autonomic responses to various potentially fear-inducing 
stimuli after undergoing exposure therapy would constitute 
converging evidence supporting reduced autonomic arousal 
as a mediator.   

Finally, plausibility (the degree to which a mediator 
makes intuitive sense) has been suggested as another criterion 
that may increase the confidence that can be placed in a 
mediator (Kazdin, 2007).  However, one difficulty in 
evaluating a mediator’s plausibility is that there may be 

disagreement in regard to whether a proposed mediator is a 
plausible explanation of change.  In general, potential 
mediators that are recognized as valid constructs have more 
plausibility compared to potential mediators that are not 
firmly established as valid scientific constructs.  For example, 
the empirical evidence for repressed memories (Davis & 
Loftus, 2009) is not as strong as the empirical evidence for 
cognitive biases (Hallion & Ruscio, 2011). Thus, the idea that 
reductions in cognitive bias mediated symptom improvement 
in a trial would be inherently more plausible than the idea that 
therapy worked by activating repressed memories.  
Examining mediators that have been well established within 
other empirical contexts reduces the likelihood that the 
potential mediator is not a valid scientific construct.  

It is important to note that the criteria intended to bolster 
the case for mediators only lessens the likelihood of 
alternative explanations (or rules out alternative explanations) 
instead of providing evidence in favor of mediation effects.  
However, evidence that some potential mediators meet the 
aforementioned criteria to a greater extent than other potential 
mediators does carry some very important implications. 
Specifically, practical decisions can be informed by taking 
into account the cumulative evidence for or against any 
possible mediator.  When deciding which psychotherapies 
should be widely disseminated or which components of 
psychotherapies should be emphasized, decisions that are 
normally best made by examining efficacy research may be 
well-served by also taking into account mechanism research. 
Based on the rationale that potential mediators meeting 
criteria are less likely to be the result of extraneous 
explanations compared to potential mediators not meeting 
criteria, interventions based on mediators meeting criteria 
may have a greater likelihood of success. However, this idea   

d 

 
Treatment 

 
Outcome 

 
Mediator 

b 

a 

c 



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56 
 

is currently only speculation.  Examination of interventions 
based on potential mediators meeting higher numbers of 
criteria versus interventions that are not based on this 
evidence could clarify whether using the additional criteria to 
evaluate mediation is indeed practically useful.   

In addition to the possibility that a potential mediator is 
actually part of the causal mechanism, evidence to support 
mediation may be due to a variety of alternative explanations 
(e.g., statistical artifacts or methodological artifacts).  If a 
mediator meets many of the criteria above, the likelihood that 
the mediator can be explained by such alternative 
explanations is decreased.  Thus, more confidence can be 
placed in mediators that meet additional criteria than potential 
mediators that do not meet said criteria (see Figure 2 for a 
schematic summary).   

 
Evaluating Mediation in RCTs: 

Common Statistical and Methodological Pitfalls 
 
As discussed above, various methods may be used to 

obtain evidence for mediation.  However the primary 
evidence for mediation should be garnered within the context 
of RCTs because RCTs can establish causality, a necessary 
condition for mediation.  The most common way to evaluate 
mediation in RCTs is to use the statistical criteria for 
mediation proposed by Baron and Kenny (1986).  Although 
these criteria are rather straightforward, there are some 
subtleties involved in applying them to RCTs that have led to 
confusion in some research studies.  First, when examining 
the relationship between treatment and outcome (path “a”), 

evidence that multiple treatments are equally effective does 
not prohibit testing for mediation.  If any treatment is shown 
to be significantly related to the outcome, then path “a” is 
said to be established for that treatment.  Second, if a 
significant path exists between a treatment and mediator, then 
path “b” is said to be established.  Third, a non-significant 
relationship between mediator and outcome (path “c”) across 
all participants does not necessarily preclude the possibility 
that mediation occurred.  This is because path “c” may only 
be significant within the treatment group that improved 
significantly on the outcome variable.  Tests for treatment-by-
mediator interactions should be used to detect this possibility.  
These considerations apply regardless of the particular 
statistical modeling technique being used to evaluate 
mediation (e.g., multiple regression, multilevel-modeling, or 
structural equation modeling) as they each deal with the same 
fundamental assumptions of mediation.   

Even if all statistical criteria for mediation are met, many 
existing studies purporting to have found mediation effects 
suffer from methodological limitations related to the timing 
of mechanism and outcome assessments.  As a result, these 
authors often conclude erroneously that mediation occurred 
without having determined that the mediator changed before 
outcome variable changed.  Examples of flawed designs are 
enumerated in Table 1, along with an empirical example of 
each design variation. One example of flawed methodology 
includes studies that assess mediators and outcomes at two 
time points only, prior to the experimental manipulation and 
after the experimental manipulation.  One study used this type 
of design to find evidence suggesting that CBT helped to 

Criterion 1 (necessary condition): Change in mediator precedes change in outcome 

Criterion 2: Mediator meets statistical criteria proposed by Baron and Kenny (1986) –
rules out Type II error 

Criterion 3: Mediator is found consistently across studies – reduces chances that
mediator was due to Type I error 

Criterion 4: Mediator was discovered in the context of an RCT – reduces the likelihood
that pre-existing conditions account for mediation effects. 

Criterion 5: Mediator exhibits dose-responsive relationship to outcome – reduces the
likelihood that the mediator works only in a narrow range  

Criterion 6: Mediator is specific to certain types of therapy – reduces the likelihood that
mediator is due to general effects of psychotherapy 

Criterion 7: Evidence supporting mediation converges across methods – reduces the
likelihood that the mediator is due to methodological artifact 

Criterion 8: Mediator is well-grounded in empirical research – reduces the likelihood
that the mediator is not a valid scientific construct 

Figure 2. This figure shows how achieving criteria for mediation reduces the likelihood that mediation effects are due to extraneous 
explanations.  

 



MEDIATORS AND MECHANISMS OF PSYCHOTHERAPY 
 

57 
 

 
Table 1 
 
Examples of flawed designs commonly used for studying mediation effects  
 
Study Disorder 

Studied 
Mediator 

Assessment 
Outcome 

Assessment 
Limitation 

  Pre During Post Pre During Post  
Smits, Powers, Cho, 
and Telch (2004) 

Panic 
Disorder 

Y N Y Y N Y Cannot determine whether 
the mediator changed prior 
to the outcome due to lack 
of assessments during the 
study 

Hofmann (2004) Social 
Phobia 

N Y N Y N Y Cannot determine whether 
mediator changed during 
the study due to only 
assessing mediator one 
time 

Wilson, Fairburn, 
Agras, Walsh, and 
Kraemer, H. (2002) 

Bulimia 
Nervosa 

Y Y Y Y N Y Although it is possible to 
determine that change in 
the mediator preceded the 
final outcome assessment, 
it is not possible to 
establish that the mediator 
changed before the 
outcome because the 
outcome was not assessed 
during the study 

 
Note. “Y” indicates that the mediator or outcome is measured at the time specified by the column label: “Pre” = prior to treatment, “During” = 
during treatment, and “Post” = following treatment. 

decrease panic symptoms through the mechanism of reducing 
“fear of fear,” or feelings of anxiety related to physical 
symptoms (Smits, Powers, Cho, & Telch, 2004).  This study 
consisted of 130 participants who were referred to the study 
by mental health professionals for treatment of panic disorder.  
Ninety participants were randomized to the treatment 
condition and 40 to a waitlist condition.  “Fear of fear,” 
assessed by the Anxiety Sensitivity Index (Peterson & Reiss, 
1987) met all of Baron and Kenny’s (1986) criteria for 
mediation; however, as “fear of fear” was only measured 
concurrently with panic symptoms, there is no evidence to 
make the claim that “fear of fear” changed before panic 
symptoms.   

Failing to include pre-treatment measures of mediators 
also limits the conclusions that can be drawn from a study.  
For example, one study presented evidence suggesting that 
cognitive behavioral group therapy reduced symptoms of 
social phobia through the putative mediator of patients’ 
appraisals of hypothetical events associated with social 
phobia (Hofmann, 2004).  This study consisted of 90 
participants who were randomly selected from among 
individuals seeking outpatient treatment at a university 
anxiety clinic.  Participants were randomly assigned to 
receive either cognitive behavioral group therapy or exposure 
therapy, or were assigned to a waitlist control condition. 

Although judgments of events, as assessed by the Social Cost 
Questionnaire (Foa, Franklin, Perry, & Herbert, 1996) were 
reported during the study and thus before the post-treatment 
assessment of social phobia, the lack of a baseline assessment 
of the mediator prohibits conclusions about whether the 
mediator changed during treatment.   

Designs that do not measure outcome variables during 
the study are also flawed.  One study showed that the effects 
of CBT and interpersonal psychotherapy (IPT) on symptoms 
of bulimia nervosa were statistically mediated by changes in 
dietary restraint (Wilson, Fairburn, Agras, Walsh, & 
Kraemer, 2002).  This study consisted of 220 participants 
diagnosed with bulimia nervosa and was conducted on an 
outpatient basis at two university treatment centers.  
Participants were randomly assigned to either the CBT or IPT 
conditions  The study’s design was able to establish that the 
mediator of dietary restraint (as assessed by the Eating 
Disorder Examination Questionnaire; Fairburn & Beglin, 
1994) changed in response to treatment, and that the mediator 
changed during the treatment and thus before the post-
treatment outcome assessment.  However, as no assessments 
of bulimia symptoms were made during the study, the 
possibility still exists that those symptoms changed prior to 
the mediator of dietary restraint.  It is worth noting that a 
substantial body of research ostensibly supporting the 



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58 
 

meditational effects of therapeutic alliance (i.e., the degree to 
which a patient and therapist collaborate, agree on goals, and 
form a personal bond; Henry, Strupp, Schacth, & Gaston, 
1994) has been largely discredited due to failure to measure 
symptom change during studies (DeRubeis et al., 2001).  
When symptom change has been included in assessments 
during the course of treatment, therapeutic alliance has not 
emerged as a significant predictor of post-treatment symptom 
change (DeRubeis & Feeley, 1990; Tang & DeRubeis, 1999).   

To avoid the pitfalls of these studies, the best possible 
design would include assessments of potential mediators and 
outcome variables before and after the treatment is 
implemented, and also at multiple time points while the 
treatment is being implemented.   

 
Cognitive Therapy for Depression Works  

Through Cognitive Change 
 

The methodological limitations described above have the 
consequence of preventing the accumulation of knowledge 
about how psychotherapy works.  However, one exception is 
the study of cognitive therapy for depression.  This section 
describes how various studies are converging on the 
possibility that cognitive therapy works through producing 
cognitive change, and that cognitive change mediates not only 
response to treatment but also reduction of relapse rates.  
Examples of studies providing evidence in support of the 
criterion for mediation are presented below (see Figure 2 for a 
list of each criterion identified by number).   
 The first direct evidence for mediation by cognitive 
change was provided by Tang and DeRubeis (1999). In this 
study, cognitive therapy was found to be effective for treating 
depression in the context of an RCT (thus establishing 
causality and meeting Criterion 4, that the research was 
conducted in the context of an RCT). Importantly, symptoms 
of depression as well as cognitive change were monitored at 
multiple time points throughout the course of treatment.  
Cognitive change was assessed with a well-validated measure 
of cognition in depression, the Patient Cognitive Change 
Scale (Tang, DeRubeis, Beberman, & Pham, 2005), 
suggesting that the proposed mediator is a valid scientific 
construct (Criterion 8).  When considered together, two 
noteworthy findings of this study make a strong case that 
cognitive change mediated the effects of therapy.  First, 
individuals who experienced “sudden gains,” or large 
reductions in depressive symptoms from one session to the 
next, were much more likely to improve than individuals who 
did not experience sudden gains (meeting the statistical 
criterion from Criterion 2).  Second, the session in which 
sudden gains occurred was likely to have been preceded by 
cognitive change in the previous session (Criterion 1), but not 
during a control session (i.e., two sessions prior to sudden 
gains).  These findings were subsequently replicated in an 
independent sample (Tang et al., 2005), providing some 
evidence that cognitive change meets the consistency criteria 
for mediation (Criterion 3).  Additionally, there is some 
evidence that cognitive change meets specificity criteria 

(Criterion 6), as cognitive change has not been found to 
mediate the effects of Supportive-Expressive therapy 
(Andrusyna, Luborsky, Pham, & Tang, 2006) on depression.  
Encouraging findings are also occurring in the domain of 
relapse prevention.  It was recently found that lower rates of 
relapse in depressed patients receiving cognitive therapy were 
predicted by sudden gains (Tang, DeRubeis, Hollon, 
Amsterdan, & Shelton, 2007) as well as by competence in 
cognitive techniques emphasized during cognitive therapy 
(Strunk, DeRubeis, Chiu, & Alvarez, 2007).  These findings 
provide more evidence for consistency, and they also suggest 
that cognitive change is durable and robust rather than a 
transient effect of cognitive therapy.  Thus, cognitive change 
does reasonably well (meeting six of eight criteria) when 
evaluated on the criteria of mediation reviewed in this paper.  
Future research should test for a dose-response relationship 
(Criterion 5) and use additional measures of cognitive change 
(Criterion 7). 

 
Conclusions and Future Directions 

 
Research on the mechanisms of psychotherapy is still in 

its earliest stages.  Although much has been written on the 
ways in which psychotherapy might work, little empirical 
research has investigated the process of psychotherapy, and a 
high percentage of this research was plagued by serious 
methodological flaws.  Therefore, there is an urgent need for 
mechanism research that is conducted with appropriate 
methodological rigor and that establishes treatment efficacy 
before evaluating mechanisms.  While this area holds great 
promise for future researchers, moving forward with blind 
enthusiasm (by not establishing treatment efficacy first) 
would be just as detrimental to progress as the 
methodological pitfalls described in this paper. 

One way that mechanism research could have a 
significant impact is by finding some common ground across 
the vast number of psychotherapies in existence.  New 
psychotherapies are developed frequently, each laying claim 
to a somewhat unique theoretical basis.  Thus, the theoretical 
landscape of psychotherapy is likely much more complex 
than it needs to be, which has the important practical 
implication of making treatment choices seem overwhelming 
and impossible to navigate for clients.  Mechanism research 
has the potential to reveal the processes that are shared 
between different therapies.  This development would 
increase parsimony and would hopefully lead to the 
incorporation of techniques relevant to the ways in which 
psychotherapy works across its disparate forms. Mechanism 
research incorporating multiple efficacious psychotherapies 
and multiple putative mediators in the same RCT may be 
especially helpful for identifying mediators that are common 
across psychotherapies.  The identification of mediators also 
has the potential to distill treatments down to their essential 
components, which might result in reductions to the time and 
cost to train therapists as well as to disseminate 
psychotherapy.  Mechanism research may thus increase the 
efficiency of psychotherapy.  Perhaps the most important 



MEDIATORS AND MECHANISMS OF PSYCHOTHERAPY 
 

59 
 

benefit of increased efficiency is that more economical 
therapies increase the availability of psychotherapy, 
especially to underserved populations (Kazdin & Blase, 
2011).  

Another suggestion for future research is made 
specifically for investigating cognitive change.  Cognitive 
change has a special status among potential mediators due to 
the increasing evidence which suggests that cognitive change 
mediates the effects of cognitive therapy on depressive 
symptom reduction.  Future efforts to further validate this 
mediator may focus on meeting the various criteria described 
earlier (e.g., dose-response and consistency).  Meeting these 
criteria may strengthen confidence in the conclusion that 
cognitive change truly is involved in the causal pathway from 
cognitive therapy to symptom reduction.  Perhaps most 
important would be research that examines the “convergence 
of evidence” criterion by investigating whether cognitive 
therapy works through cognitive change across various 
disorders.  If this were the case, the validity of applying 
cognitive change techniques across disorders would be 
strengthened.  The studies summarized in Table 1  (Hofmann, 
2004; Smits et al., 2004; Wilson et al., 2002) each 
investigated potential mediators of the effects of cognitive 
therapy on different disorders (panic disorder, social phobia, 
and bulimia nervosa); however, these studies included 
methodological flaws.  Thus, new research taking multiple 
assessments of potential mediators and outcomes is needed 
across the realm of psychopathologies for which cognitive 
therapy has proven effective.   

Any mediator may be evaluated based on the criteria 
proposed in this paper, and the degree to which a mediator 
meets those criteria could serve as a useful basis for making 
judgments about whether the mediator should be considered a 
causal mechanism.  In summary, it is necessary for changes in 
mediators to precede changes in outcomes (Criterion 1).  
Establishing that a mediator meets the statistical criteria 
proposed by Baron and Kenny (1986) rules out the possibility 
of Type II error (Criterion 2).  Finding evidence for a 
mediator across studies reduces the chances that the any 
individual study committed a Type I error (Criterion 3).  If 
the mediator was discovered in the context of an RCT, it is 
less likely that pre-existing conditions could have accounted 
for mediation effects (Criterion 4).  Evidence for a dose-
response relationship between a mediator and outcome 
reduces the likelihood that the mediator works only in a 
narrow set of circumstances (Criterion 5).  Showing that a 
mediator is specific to certain type of therapy reduces the 
likelihood that the mediator is due to general effects of 
psychotherapy (Criterion 6).  Finding that mediation 
converges across methodologies reduces the likelihood that 
the mediator is due to methodological artifact (Criterion 7).  
Finally, examining potential mediators that have that have 
been well established within other empirical contexts reduces 
the likelihood that the mediator is not a valid scientific 
construct (Criterion 8).  These criteria for mediation have the 
potential to clarify the scientific status of potential mediators 

by serving as a framework in which research on mediation in 
psychotherapy may be systematically organized.   

 
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Baron, R. M., & Kenny, D. A. (1986). The moderator-
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