In many societies, beliefs about differential intellectual ability by gender persist across generations. These societal beliefs can contribute to individual belief formation and thus lead to persistent gender inequality across multiple dimensions. We show evidence of intergenerational transmission of gender norms through peers and how this affects gender gaps in learning. We use nationally representative data from China and the random assignment of children to middle-school classrooms to estimate the effect of being assigned a peer group with a high proportion of parents who believe that boys are innately better than girls at learning mathematics. We find this increases a child's likelihood of holding the belief, with greater effects from peers of the same gender. It also affects the child's demonstrated mathematics ability, generating gains for boys and losses for girls. Our findings highlight how the informational environment in which children grow up can shape their beliefs and academic ability.