Other limitations

Limitations are addressed in the discussion sections of each chapter. However some over-arching limitations will now be discussed. Two of the studies (Chapter 2 and 4) employed cross-sectional designs. The inferences and arguments about causality were justified theoretically and on the basis of previous research evidence. It could still be argued that, since the studies were not designed longitudinally, the

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causal order of events cannot be ascertained, and that other directions of causality cannot be ruled out. The study in Chapter 3 brought more confidence to these inferences, as the associations between attachment insecurity and paranoia were assessed longitudinally. The findings supported the interpretations of causality made in the other studies.

It has been argued that discussing mediation in terms of whether it is ‘full’ and ‘partial’, as was done in Chapter 2, may not be as valuable as once thought. Hayes (2013) argues that there is often a misconception about full mediation, noting that it is often assumed that there is no further need to explore other mediating

mechanisms after one mechanism appears to fully account for X’s effect on Y. Hayes continues by suggesting that researchers often think that, when partial mediation is found, it means the model has been misspecified, and that further investigations are needed to examine other mechanisms. He notes that the difference between partial and full mediation may be sample-size dependent, and that there therefore may not be any value in describing mediation using these terms. This argument strictly pertains to the interpretation of mediation analyses and not the actual statistical analysis. Although in Chapter 2 the results are interpreted using these terms, no claim is make that insecure attachment is the only psychological mechanism implicated in paranoia, and it is important for readers to be cautious in how they interpret these results.

Since evidence is consistent in showing that individuals with psychosis have had experiences of childhood trauma, it could be argued that controlling for post- traumatic stress disorder (PTSD) may be important, especially since PTSD has been reported to be commonly experienced by individuals with psychosis (Resnick, Bond, & Mueser, 2003). However, it has also been suggested that psychosis may increase

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the risk to developing PTSD, for example through experiences such as

hospitalisation (Mueser, Rosenberg, Goodman, & Trumbetta, 2002). In future studies, it will therefore be important to address the issue of PTSD in psychosis by selecting appropriate measures that take this overlap into account.

It has been proposed that specificity could be measured using a different statistical method. In their study van Nierop et al. (2014) used a mixed-effects logistic regression. Their analysis included six variables pertaining to childhood traumatic experiences, a dummy variable coded as 0 for hallucinations and 1 for delusions, and interaction terms between the childhood traumatic experiences and the dummy variable. It was mentioned that interaction terms can be “interpreted as the differential association of each type of trauma on hallucinations versus delusions” (supplementary box 1; van Nierop et al., 2014). The findings revealed that the effect of childhood trauma was not different for delusions and hallucinations, suggesting a lack of specificity. These authors further argued that observing greater effect sizes between specific types of traumas and specific types of psychotic symptoms does not indicate ‘etiopathogenesis’ (p.124). Since specificity research is in its infancy, the best suited statistical method for analysing specificity should be more closely examined.

The Relationship Questionnaire (RQ; Bartholomew & Horowitz, 1991) uses single-item questions to assess attachment style, and it could be argued that this measure may be limited in how sufficiently it can represent a complex construct like attachment (Nunnally & Bernstein, 1994). Despite this, the RQ is one of the most common measures used to assess attachment. It is quick and convenient, it has adequate reliability, and very good discriminant and face validity (Ravitz, Maunder, Hunter, Sthankiya, & Lancee, 2010). In addition, the RQ has shown convergent

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validity with other explicit measures of attachment (Bartholomew & Shaver, 1998), suggesting that the measure is valid.

In Chapter 3 it cannot be ascertained which specific attachment style led to an increase in paranoia. The correlation analysis in Chapter 3 shows that attachment insecurity, as measured by the ESM attachment questions, correlated strongly with the fearful attachment style, as measured by the RQ. An interesting question is whether there really are different types of insecure attachment when looked at from a longitudinal perspective, or whether state attachment can be represented as a mix between attachment anxiety and avoidance and considered as strategies which fluctuate over time.

Although there is no evidence of any participant selection bias, the

individuals who took part in the research studies (Chapter 3 and 4) were referred by a person involved in their care. It could be that the referrer referred only those he/she thought would engage in the research, and only those he/she had a good relationship with. Taking these factors into account the samples in this thesis may not necessarily be representative of a general psychosis population. The study in Chapter 2 used an epidemiological dataset and the issue of participant selection biases may be different in these circumstances; appropriate weights were used in the analysis.

The studies in this thesis did not differentiate between ‘poor me’ and ‘bad me’ paranoia (Trower & Chadwick, 1994). It could be that these two types of

paranoia may be differentially associated with the phenomena tested in these studies. For example, there may be a difference in how these distinctions are associated with childhood traumatic experiences, and attachment representations.

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Finally, it could be argued that current psychotic symptoms may impact on the way that individuals answered self-report questions. However, this is an issue which applies to any psychological study of psychosis and there is no evidence that it compromised the current studies.