Systematic reviews of the effect of foster care on mental-health outcomes have consistently indicated a zero-sum game, which makes it unclear whether the intervention is suitable for children in need of out-of-home placements. This thesis took on a meta-analytical approach to examine sources of heterogeneity between studies evaluating the effect of foster care on adaptive functioning, cognitive functioning, externalizing behavior, internalizing behavior, and total problems behavior. The bulk of studies came from two recently published systematic reviews. The searches were replicated to cover studies published until March 31, 2018. From 2943 studies assessed for eligibility, 240 were selected for the analysis covering 25 085 children. A choice of study-related covariates was abstracted, and potential sources of heterogeneity were hypothesized and tested by means of meta-regression. The findings indicated that both the choice of study design and measurement instrument were significantly associated with the variation in effect sizes. These associations were even stronger in child protection-oriented welfare regimes while insignificant in family service-oriented regimes, which instead showed significant associations for outcome type and publication year. The results imply a need to standardize effect studies of foster care, and to further research on sources of heterogeneity in different child welfare regimes.