From the Williams Institute:
Key findings include:• Child age at placement— or the duration of time in the pre-adoptive context— did not emerge as a significant predictor of child adjustment (likely because all children in the sample were placed under 18 months).• Parents’ level of preparation for the adoption was related to both externalizing and internalizing symptoms, such that parents who were less prepared reported more symptoms in their children.• Parents’ depressive symptoms were also related to externalizing and internalizing symptoms in adopted children, such that more depressed parents reported more symptoms in their children. Depressive symptoms may compromise parents’ emotional availability and ability to parent effectively, which can contribute to child adjustment problems.• Parents who reported more relationship conflict during the early transition phase reported that their children had more internalizing behaviors 2 years later.• Family type (i.e., parent sexual orientation) was unrelated to children’s adjustment.
I'm guessing that this study is far more reliable than the Regnerus study (although that's an incredibly low bar). You can read the full report HERE.