The study examined the relationship between parental psychiatric conditions and their children's psychiatric, behavioral, and psychosocial outcomes. The research included 15,603 individuals who were children of monozygotic twins born in Sweden between 1970 and 2000. The offspring of parents with any psychiatric illness had a higher risk of all outcomes, with hazard ratios ranging from 1.34 to 1.87. When comparing children exposed to and unexposed to parental psychiatric illness, where both parents were twins, most associations moderated and some ceased to be statistically significant. Associations between any parent psychiatric condition and any offspring psychiatric condition (HR = 1.28), between parent internalizing conditions and offspring psychiatric condition (HR = 1.45), and between parent externalizing conditions and offspring psychiatric condition (HR = 1.27) remained statistically significant. The results suggest that intergenerational transmission depends in part on unmeasured family factors, but there is also evidence of direct causal effects. If the transmission is partly direct, treating the parents' psychological conditions may reduce the risk in the children. The study has limitations, including unmeasured confounders and limited statistical power for some outcomes.