Generalized Anxiety Disorder (GAD) is a chronic condition that is characterized by excessive and uncontrollable worry and anxiety. In Canada, 3 to 4% of the population suffer from GAD at any point in time. These individuals have a lowered quality of life and are at risk for many medical conditions such as coronary heart disease and cancer. Research suggests that both pharmacological and psychological approaches are effective for treating GAD in the short-term; however, psychological treatments appear to offer the greatest long-term benefits. There exist a number of effective psychological treatments for GAD, most of which fall into the category of cognitive-behavioural therapy or CBT. In the 1990s, a group of Canadian investigators developed a CBT protocol for GAD that included four components. Data from five clinical trials suggest that one of the four components is particularly important for treatment success: experiencing uncertainty rather than avoiding it in everyday life. Stated differently, learning to tolerate and deal with uncertainty appears to be the key to decreasing worry and anxiety. Given this finding, the investigators have developed a new treatment that exclusively targets intolerance of uncertainty: Behavioural Experiments for Intolerance of Uncertainty or BE-IU. The goal of the current proposal is to test the efficacy of BE-IU (6 weekly treatment sessions) by comparing it to a Waiting List (WL) control condition (6 weeks). A total of 50 participants with a primary diagnosis of GAD will be randomly assigned to either BE-IU or WL and will be assessed at 4 time points ranging from pre-treatment to 12-month follow-up. The conditions will be compared in terms of treatment efficacy and mechanisms. The investigators will also examine the predictors of change during the 12-months following treatment. The proposed study will produce data on the efficacy and mechanisms of a treatment for GAD that is less costly, less complex and easier to disseminate than treatments that are currently available.

Inclusion Criteria:

1. at least 18 years of age;
2. principal diagnosis of GAD;
3. no change in medication type or dose in 4 to 12 weeks before study entry (4 weeks for benzodiazepines, 12 weeks for antidepressants and hypnotics);
4. willingness to keep medication status stable while participating in the study;
5. no use of herbal products known to have CNS effects in the 2 weeks before study entry;
6. no evidence of suicidal intent (based on clinical judgement);
7. no evidence of current substance abuse, current or past schizophrenia, bipolar disorder or organic mental disorder;
8. no current participation in other trials;
9. no concurrent psychotherapy during treatment phase of trial;
10. no evidence of anxiety symptoms due to a general medical condition based on clinical judgement (e.g., clinical hyperthyroidism, hypoglycaemia, anaemia).

Exclusion Criteria:

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