Between 1986 and 1992, 154 prospective lesbian mothers volunteered for a study that was designed to follow planned lesbian families from the index children's conception until they reached adulthood. Data for the current report were gathered through interviews and questionnaires that were completed by 78 index offspring when they were 10 and 17 years old and through interviews and Child Behavior Checklists that were completed by their mothers at corresponding times. The study is ongoing, with a 93% retention rate to date.
According to their mothers' reports, the 17-year-old daughters and sons of lesbian mothers were rated significantly higher in social, school/academic, and total competence and significantly lower in social problems, rule-breaking, aggressive, and externalizing problem behavior than their age-matched counterparts in Achenbach's normative sample of American youth. Within the lesbian family sample, no Child Behavior Checklist differences were found among adolescent offspring who were conceived by known, as-yet-unknown, and permanently unknown donors or between offspring whose mothers were still together and offspring whose mothers had separated.
I can't give you a similar study for gay parents. It would be interesting to see what the case is there.