Thirty four studies were included in the review (n=2,134, sample sizes ranged from 10 to 531): 13 controlled studies; and 21 uncontrolled studies.
Effect sizes of controlled studies all favoured GCBT/GCT over the control condition. Effect sizes ranged from 0.1 to 2.87 with an average effect size of 1.11 across studies. Effect sizes were highest (over 1.5) in populations of low socioeconomic status immigrant women, women adjusting to divorce, patients with multiple sclerosis or myotonic muscular dystrophy, and people with low self esteem and mood disturbance. Effect sizes were lower (less than 0.3) in populations of older adults, HIV positive homosexual men and patients with primary dysthymia.
Effect sizes of uncontrolled studies ranged from 0.3 to 3.72 with an average effect size of 1.30 across studies. Effect sizes were highest (over 1.5) in populations with major depressive disorder and in older adults, and were lower (0.3) in a population of nursing home residents.
Compared with other treatments, GCBT/GCT produced larger effect sizes than group behaviour therapy, group supportive insight, assertive skills training, group rational emotive therapy, individual CBT, group interpersonal therapy, group psychodynamic therapy, individual supportive counselling and an educational discussion group. However, GCBT/GCT produced smaller effect sizes than individual cognitive therapy, visual imagery, a mutual support group and comprehensive distancing.
Additional results were reported.