The reduction in gender gaps observed in the second half of the 20th century seems to have halted and social norms are argued to be a major cause behind the remaining differences in the labour market performance of men and women. This paper seeks to understand heterogeneity in labor market responses to social norms. We focus on two norms: the fact that males should be a household's main breadwinner, and the expectation that women will do most of the childcare. Our results indicate that distinct gender roles have different effects. Using panel data for US couples and an event study approach we estimate the impact of the birth of the first child on labour market trajectories, and we find that motherhood generates very strong responses, in terms of the intensive and the extensive margins of labour supply. Although women who were the main earners have a somewhat smaller reduction in employment than secondary earners, both groups largely conform to the traditional division of roles, pointing towards a misallocation of resources. In contrast, the effect of the male breadwinner norm is highly contextualized. Women of all education levels are affected by it, yet the margin along which they react differs. The results also suggest a hierarchy of norms, as gender roles seem to be less important in households without offspring.