Twenty-eight studies (n=1,839) were included in the review.
In terms of quality, of the 28 included studies, 12 were randomised trials, 21 were controlled, and 16 had a sample size over 40.
Effectiveness.
The overall mean weighted effect size (d) of any therapy was 0.72 (standard error 0.02; range: -0.20, 2.08, p<0.001). There was significant heterogeneity between the studies (p<0.001). No significant effects of study quality, coder or coding time on overall effect size was found.
There was a significant difference in the mean weighted effect size between treatment groups (d=0.74) and no treatment groups (d=0.46; p<0.001). Significant heterogeneity (p<0.001) was found for both treatment and no treatment groups.
There were moderate mean effect sizes for social functioning (d=0.48) and large effects for behaviour, psychological distress, self-concept and 'other' outcomes (including academic functioning and risk assessment) (d=1.60, d=1.05, d=0.71 and d=1.49, respectively). For each of the outcome measures, significant heterogeneity remained (p<0.01 to 0.001).
Effect sizes (d) above 0.8 were achieved by cognitive-behavioural therapy, play therapy, supportive therapy, group therapy and abuse-specific therapy.
Effect sizes over 1 were reported for behaviour outcomes (supportive, group and abuse-specific therapies), psychological distress outcomes (cognitive-behavioural therapy, abuse-specific therapy, individual therapy and family therapy), self-concept outcomes (group therapy and no treatment) and other outcomes (play therapy, supportive therapy, group therapy and abuse-specific therapy). For social functioning, the largest effect size was achieved by play therapy (d=0.72).
Moderator analyses.
No significant effects were found for age or gender of the participants or therapist training. Significant moderator effects were reported for ethnicity (effect sizes increased with an increasing proportion of non-Caucasians; p<0.001), intrafamilial abuse (effect sizes smaller with a greater proportion of intrafamilial abuse; p<0.002), and numbers of therapy sessions and duration of treatment (greater effects were seen with more sessions and longer duration; p<0.001).
No significant funnel plot asymmetry was found. The number of non significant studies needed to reduce the overall effect size to below statistical significance was 22.