Pre-print, Working paper: We here address the causal relationship between maternal depression and child human capital using UK cohort data. We exploit the conditionally-exogenous variation in mothers' genomes in an instrumental-variable approach, and describe the conditions under which mother's genetic variants can be used as valid instruments. An additional episode of maternal depression between the child's birth up to age nine reduces both their cognitive and non-cognitive skills by 20 to 45% of a SD throughout adolescence. Our results are robust to a battery of sensitivity tests addressing, among others, concerns about pleiotropy and the maternal transmission of genes to her child.