Abstract: The review includes 17 studies from six countries (USA, UK, Australia, Canada, Thailand and China), which recruited 919 children with ASD. Not all 17 studies could be compared directly or combined in meta-analyses due to differences in the theoretical basis underpinning interventions, the duration and intensity of interventions, and the outcome measurement tools used. Data from subsets of 10 studies that evaluated interventions to enhance parent interaction style and thereby facilitate children's communication were included in meta-analyses. The largest meta-analysis combined data from 316 participants in six studies and the smallest combined data from 55 participants in two studies. Findings from the remaining seven studies were reported narratively. High risk of bias was evident in the studies in relation to allocation concealment and incomplete outcome data; blinding of participants was not possible. Overall, we did not find statistical evidence of gains from parent-mediated approaches in most of the primary outcomes assessed (most aspects of language and communication - whether directly assessed or reported; frequency of child initiations in observed parent-child interaction; child adaptive behaviour; parents' stress), with findings largely inconclusive and inconsistent across studies. However, the evidence for positive change in patterns of parent-child interaction was strong and statistically significant (shared attention: standardised mean difference (SMD) 0.41; 95% confidence interval (CI) 0.14 to 0.68, P value < 0.05; parent synchrony: SMD 0.90; 95% CI 0.56 to 1.23, P value < 0.05). Furthermore, there is some evidence suggestive of improvement in child language comprehension, reported by parents (vocabulary comprehension: mean difference (MD 36.26; 95% CI 1.31 to 71.20, P value < 0.05). In addition, there was evidence suggesting a reduction in the severity of children's autism characteristics (SMD -0.30, 95% CI -0.52 to -0.08, P value < 0.05). However, this evidence of change in children's skills and difficulties as a consequence of parent-mediated intervention is uncertain, with small effect sizes and wide CIs, and the conclusions are likely to change with future publication of high-quality RCTs. The review finds some evidence for the effectiveness of parent-mediated interventions, most particularly in proximal indicators within parent-child interaction, but also in more distal indicators of child language comprehension and reduction in autism severity. Evidence of whether such interventions may reduce parent stress is inconclusive. The review reinforces the need for attention to be given to early intervention service models that enable parents to contribute skilfully to the treatment of their child with autism. However, practitioners supporting parent-mediated intervention require to monitor levels of parent stress. The ability to draw conclusions from studies would be improved by researchers adopting a common set of outcome measures as the quality of the current evidence is low.

Plain-language summary: The studies varied in the content of what parents were trained to do, and over what length of time parents had contact with professionals. Parents received training either individually with their child or in groups with other parents. In the majority of the studies, the interventions aimed to help parents be more observant and responsive during interactions with their child in order to help their child develop communication skills. In summary, the review finds sufficient evidence that the ways in which parents interacted with their children did change as intended. The review also suggests improvement in child outcomes such as understanding of language and severity of autism characteristics as a result of interventions delivered by parents. However, important outcomes such as other aspects of children's language, children's adaptive skills and parent stress did not show change. The evidence is not yet strong for any outcome and would benefit from researchers measuring effects in the same ways.