The relationship between dissociation and voices: A systematic literature review and meta-analysis
Introduction
Auditory verbal hallucinations (AVHs) or “hearing voices” have been defined as the experience of hearing a voice in the absence of an appropriate external stimulus (Stanghellini & Cutting, 2003). Despite this deceivingly simple definition, the exact characterisation of the experience of hearing voices, and the extent to which voices are phenomenologically independent from other intrusive, unwanted and/or unintended cognitions, has been a matter of enduring academic and clinical debate (e.g., Aleman and Larøi, 2008, Slade and Bentall, 1988). Many cognitive researchers have argued that hearing voices can be conceptualised as common internally generated cognitive events (for example, auditory imagery, intrusive thoughts or acts of inner speech) that are erroneously attributed to a source alien or external to self (e.g., Bentall, 1990, Frith and Done, 1988, Morrison et al., 1995, Waters et al., 2006). Although they have been traditionally regarded as a core symptom of psychotic illness (e.g., Schneider, 1959), AVHs are not intrinsically pathological. Recent evidence from case–control and epidemiological studies suggests that AVHs are experienced by a sizable number of individuals in the general population (a recent literature review found a median prevalence rate of 13.2%; Beavan, Read, & Cartwright, 2011), and can occur in the absence of psychological distress and/or diagnosable mental health difficulties (e.g., McCarthy-Jones and Davidson, 2013, Sommer et al., 2010). A plethora of cross-sectional studies, often employing measures of “hallucination proneness” (e.g., Bentall & Slade, 1985), have confirmed that the predisposition to experience hallucinatory phenomena is common in non-clinical samples (for a review, Aleman & Larøi, 2008). This supports the notion that hallucinatory experiences cannot be uniquely ascribed to the realm of psychopathology; rather, they can be better understood as lying on a continuum with “normal” functioning (e.g., van Os et al., 2000, van Os et al., 2009) or, in more prosaic terms, as relatively common experiences that become a cause of distress and disability in a minority of individuals.
Despite the above findings, AVHs are often, but not always, associated with severe distress and social and occupational impairment in clinical samples (McCarthy-Jones, 2012a). Voices are almost routinely present within schizophrenia-spectrum disorders (with a prevalence estimate of approximately 70%; McCarthy-Jones, 2012a) and dissociative identity diagnoses (DID; Dell, 2006), with further studies reporting that 31% of individuals with a bipolar affective diagnosis (Hammersley et al., 2003), 46% of individuals with a borderline personality diagnosis, 66% of individuals with a schizophrenia diagnosis and 90% of individuals with both BPD and schizophrenia diagnoses (Kingdon et al., 2010) hear voices. As voices are frequently experienced as distressing within patient samples (e.g., Morrison, Nothard, Bowe, & Wells, 2004), ongoing research is being carried out in an attempt to identify potential underlying mechanisms of voice-hearing.
Although much research into the aetiology of hearing voices has focused on the role of possible cognitive anomalies or deficits (e.g., source monitoring biases, self-monitoring deficits; Waters et al., 2012a, Waters et al., 2012b, Brookwell et al., 2013, Ditman and Kuperberg, 2005), in recent years a growing number of studies has also considered the potential influence of dissociative processes (e.g., Anketell et al., 2010, Moskowitz, 2011, Moskowitz et al., 2009, Varese et al., 2012). Dissociation has been described as the failure to integrate information regarding psychological functioning, such as memory, consciousness and perception; it is often experienced as a sense of detachment from the self and/or environment (International Society for Study of Dissociation, 2011). A number of varied psychological symptoms and processes have been associated with dissociation, such as divided attention, hypnotic suggestion, flashbacks and identity confusion (see Brown, 2006 for a more comprehensive list).
Dissociative experiences have been found to be a common feature within the general population (Ross, Joshi, & Currie, 1990) and within mental health samples (e.g., Anketell et al., 2010, Haugen and Costillo, 1999, Koopman et al., 2003). The aetiological origins of dissociation have been a matter of enduring controversy. Several “post-traumatic” explanatory accounts of dissociation have been frequently considered by researchers and clinicians. For example, some researchers have conceptualised dissociation as a defensive or adaptive psychological mechanism aimed at reducing the emotional and physical pain of the traumatic experience (e.g., Bierre, 1996). Conversely, others have proposed that dissociation may represent a direct consequence of trauma, resulting from the unintegrated contextual representation of the trauma with other experiences in the cognitive structures of the affected individuals, largely as a consequence of the overwhelming effect experienced during traumatic experiences (Dorahy and Van der Hart, 2007, Van der Hart et al., 2006). These post-traumatic models of dissociation are contentious in the dissociation literature and alternative accounts have been proposed to account for the apparent relationship between dissociation and self-reports of traumatic experiences, including the impact of iatrogenic and sociocultural factors (e.g., Lynn et al., 2012a, Lynn et al., 2012b, Lynn et al., 2014) as well as the high levels of fantasy proneness and suggestibility that have been sometimes observed in individuals with dissociative experiences (e.g., Giesbrecht, Lynn, Lilienfeld, & Merckelbach, 2008). The findings of a recent systematic review and meta-analysis aiming to evaluate both post-traumatic accounts and the above “fantasy models” of dissociation have strongly supported the former (Dalenberg et al., 2012, Dalenberg et al., 2014), highlighting (amongst other converging evidence) the existence of a reliable association between trauma and dissociation in both clinical and non-clinical samples, including studies employing objective measures of trauma (e.g., protective agency and parent/carers' reports of trauma).
A growing number of theoretical and empirical studies have started to examine dissociation as a potential contributor to the predisposition to experience psychotic symptoms, and in particular voice hearing. For example, Allen, Coyne, and Console (1997) propose that the experience of trauma-induced dissociation could increase an individual's vulnerability to developing psychotic symptoms, including voices. They argued that dissociative detachment deprives individuals of “internal and external anchors”, therefore increasing the individual's sense of feeling disconnected from the world, interpersonal relationships and within their intrapersonal self (i.e., individuals describe feeling a sense of disconnection from their body, thought processes and actions). This is assumed to lead to impaired reality-testing, severe confusion, disorganisation, and disorientation, which appears to mirror, or increase proneness to, psychotic experiences. When compared to other symptoms of psychosis, voice-hearing in particular seems to be associated with dissociative experiences (e.g., Kilcommons & Morrison, 2005). These seemingly strong associations, and the body of literature suggesting that dissociation may represent a post-traumatic sequelae or a regulatory response to trauma (for a review Dalenberg et al., 2012), has led researchers to examine dissociative processes as possible explanatory or mediating mechanisms of the robust and well-replicated relationship between adverse life experiences and the risk to experience psychotic symptoms, including AVHs (e.g., Bentall et al., 2012, Bentall et al., 2014, McCarthy-Jones, 2011, Shelvin et al., 2007, Varese et al., 2012). For example, Moskowitz and Corstens (2008) proposed that AVHs might be regarded as trauma-induced dissociative experiences. Similarly Longden, Madill, and Waterman (2012) also proposed that voices could be conceptualised as dissociated or disowned components of the self, arising from the failure to integrate adverse and traumatic sensory and psychological experiences into the context of the self.
An increasing number of studies have examined the association between voice-hearing and dissociative experiences across clinical and non-clinical samples, as well as different diagnostic groups. Rather than employing experimental methods, the literature which examined possible associations with hearing voices has employed a range of self-report instruments to measure dissociation, in particular (but not exclusively) the Dissociative Experiences Scale (DES; Bernstein & Putnam, 1986), the most commonly employed questionnaire to measure dissociation across different clinical and non-clinical populations (Holmes et al., 2005, van IJzendoorn and Schuengel, 1996). The DES assesses three distinct areas of dissociation including absorption (i.e., the experience of losing contact with one's present moment experience and becoming immersed in internal events such as thoughts and imagery; Waller et al., 1996, Waller and Ross, 1997), depersonalisation/derealisation (i.e., experiencing a sense of unreality, detachment or disconnection in relation to one's body and surroundings; Hunter, Sierra, & David, 2004) and dissociative amnesia (i.e., the inability, distinct from ordinary forgetfulness, to consciously retrieve autobiographical, personal information that would ordinarily be readily accessible to recall; Spiegel et al., 2011).
At present, empirical studies have not been systematically reviewed and synthesised, and an assessment of the quality of evidence to support the proposed association has not been carried out. Therefore, the aims of the current study are to: 1) provide a comprehensive systematic review of the quantitative literature investigating the relationship between voice-hearing and dissociation; 2) evaluate the quality of this empirical evidence using specific quality assessment scales; and 3) synthesise and evaluate the magnitude of the relationship between voice-hearing and dissociation using meta-analytic methods. More specifically, we aimed to use meta-analysis to examine the overall magnitude of association between voice-hearing and dissociation, the consistency of the relationship across clinical and non-clinical studies, and across the most widely investigated subtypes of dissociative experiences (i.e., absorption, depersonalisation and dissociative amnesia).
Section snippets
Inclusion and exclusion criteria
As the first valid and widely employed measure of dissociation (the DES) was published in 1986, studies were reviewed from 1986 up to and including March, 2014. Studies meeting the following criteria were included in the review: 1) use of a self-report measure of dissociation; 2) use of a self-report measure of voices and/or hallucination proneness; 3) quantitative methodology; 4) published in a peer review journal; and 5) written in the English language. Studies were excluded if: 1) the study
Overall summary of studies
Thirty-one studies were included in the systematic review. Table 1 provides a summary of each study, including a brief overview of relevant study characteristics and research findings.
Demographic characteristics
A total of 3449 participants took part in the studies included in the review. The female to male ratio was 1693:1531 (however, two studies did not report sex/gender demographics). The mean age was 35.4 years, ranging between 8–82 (with information regarding age not available for two studies). Twelve involved
Procedure
The studies included in the systematic review were further examined for inclusion in the meta-analysis. The meta-analysis eligibility criteria (Section 4.2 below) are presented in addition to the systematic literature review criteria (Section 2.1).
Meta-analysis inclusion and exclusion criteria
Studies that met the following criteria were included in the meta-analysis: 1) the study statistically examined the relationship between dissociation and voices and/or hallucination-proneness; 2) the measures of dissociation, voices and/or
Statistical analysis for the overall sample
The results of the summary effect size for the overall sample analysis for the relationship between voices and total dissociation scores are presented in Fig. 2. The analysis showed a significant association, with a summary effect of r = .51 (95% CI [.39–.63], p < .001 after the exclusion of one outlier; see Section 5.4). These results are suggestive of a large association between voices and dissociation, based on Cohen's (1988) criteria (i.e., r ≥ .40 indicates a ‘large’ effect).
Statistical analysis for clinical and non-clinical groups
Results for the
Discussion
The aims of the current review were to: 1) systematically investigate the relationship between voice-hearing and dissociation; 2) evaluate the quality of the evidence; and 3) assess the magnitude of the suggested relationship using meta-analytic methods. The review identified 31 studies that examined the association between voices and/or hallucination proneness and dissociation and 19 studies for which it was possible to extract effect sizes pertaining to this relationship.
Despite large
Conclusion
In summary, the current review is the first in the area of dissociation and voice-hearing which has aimed to summarise a broad and varied range of studies. The findings are presented with caution, as the research thus far is in early stages of development. We outline a number of theoretical and methodological issues that may have biased the currently available literature, and propose pertinent implications for future research and clinical practice. Nonetheless, the evidence synthesised within
Role of funding sources
No funding source involvement.
Contributors
Authors MP, KB and SB designed the study. MP wrote the protocol and conducted literature searches. Authors MP, KB and SB reviewed studies for inclusion/exclusion criteria. MP conducted the quality assessment. MP and FV conducted the meta-analysis. MP wrote the first draft of the manuscript and all authors contributed to and have approved the final manuscript.
Conflict of interest
All authors declare that they have no conflicts of interest.
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