Group Transdiagnostic Cognitive Behavioural Therapy for Intolerance of Uncertainty Versus Applied Relaxation

Many people find uncertainty distressing, but for some, it may become overwhelming and drive persistent anxiety, worry, and avoidance. This tendency is called intolerance of uncertainty (IU), and appears across many mental health conditions, including generalised anxiety, social anxiety, depression, and obsessive-compulsive disorder, which is why researchers describe it as a transdiagnostic feature. A type of therapy called Cognitive Behavioural Therapy for Intolerance of Uncertainty (CBT-IU) has been shown to reduce anxiety in previous trials. What remains to be known is how uncertainty changes when targeted directly, and whether the same mechanism applies across diagnoses. IU has mostly been measured using questionnaires, which indicate whether people find uncertainty difficult, but not what is actually happening when they encounter it. Recent research suggests that IU is not a single trait but a set of differences in how people estimate different kinds of uncertainty. Therefore, in addition to standard questionnaires, computer-based tasks will be used to explore an "uncertainty fingerprint" for each participant, capturing different aspects of uncertainty processing in a way that self-report measures cannot. In this study, the investigators are comparing CBT-IU, adapted to work across disorders, against Applied Relaxation. Applied relaxation is a well-established therapy that teaches people to notice and release physical tension but does not focus on uncertainty. Adults with high IU and anxiety symptoms (regardless of formal diagnosis) will be randomly allocated to one of the two 12-session group therapies. The investigators will measure anxiety and IU using standard questionnaires as the primary outcomes. Additionally, cognitive tasks will be used to measure the uncertainty fingerprint before therapy, immediately after, and three months later. The aim is to test whether reductions in IU are specific to CBT-IU rather than shared with an equally credible therapy that does not target uncertainty. Going beyond self-reports, the trial will then begin to resolve which specific aspects of uncertainty processing change, so that future treatments can be more precisely targeted and better matched to each person.

Inclusion Criteria: * Normal or corrected-to-normal vision * Fluency in English * Capable of giving informed consent * Live locally/ within commutable distance to Cambridge * Intolerance of Uncertainty Scale - Short form (IUS-12) score \> 28 * Generalized Anxiety Disorder scale (GAD-7) score \>= 8 * Medication-free, or on a stable medication regimen prior to and throughout the study Exclusion Criteria: * High risk of suicidal intent or self-harm (defined as an affirmative response on item 9 of the PHQ-9 above the clinical threshold, and/or corresponding clinical risk indicators during the SCID-5 interview) * Engagement with any other psychological treatment within the 3 months prior to the study or during the study period * Current diagnosis of obsessive-compulsive disorder (OCD) * Current substance use disorder * Acute psychosis * History of traumatic brain injury * Learning disability * Epilepsy/seizures * Severe cognitive impairment