Abstract
Background Nurturing care, in which children are raised in engaging and safe environments, may reduce child stress and shape hypothalamic-pituitary-adrenal axis functioning. Hence, parent-training programs may impact child cortisol levels, as well as behavioral, social and health outcomes. We conducted a systematic review of the impact of parent-training interventions on children's and caregivers' cortisol levels, and meta-analyzed the results. Methods In January 2020, searches in PubMed, LILACS, ERIC, Web of Science, Scielo, Scopus, PsycNET and POPLINE databases were conducted, and two independent researchers screened the results for eligible studies - randomized trials that assessed the impact of parent-training interventions on child or caregiver cortisol levels. Random effects were used to pool the estimates, separately for children and caregivers, and for children's morning and evening cortisol levels, as well as change across the day. Results A total of 27 eligible studies were found. Data from 19 studies were extracted and included in the meta-analyses, with 18 estimates of child cortisol levels and 5 estimates for caregiver cortisol levels. The pooled effect size (standardized mean difference) for the effects of parent training programs on morning child cortisol was 0.01 (95%CI: - 0.14 to 0.16; I-2: 47.5%), and for caregivers it was 0.04 (95%CI: - 0.22 to 0.30; I-2: 0.0%). Similar null results were observed for child evening cortisol and for the slope between morning and evening child cortisol. No evidence of publication bias was found. Conclusion Existing evidence shows no effect of parent-training interventions on child or caregiver post-intervention cortisol. Researchers are encouraged to adopt standardized protocols to improve evaluation standards, to test for intervention effects on psychosocial outcomes that are theorized to mediate the effects on biomarkers, and to use additional biomarkers for chronic stress.