Recent research has uncovered the historical roots of gender norms about women and their persistent effects on economic development. We find similar long-term effects of masculinity norms: beliefs about the proper conduct of men. We exploit a natural historical experiment in which convict transportation in the 18th and 19th century created a variegated spatial pattern of sex ratios across Australia. We show that in areas that were heavily male-biased in the past (though not the present) more Australians recently voted against same-sex marriage, an institution at odds with traditional masculinity norms. Further evi- dence indicates that these historically male-biased areas also remain characterized by more violence, excessive alcohol consumption, higher rates of male suicide, and occupational gender segregation. We interpret these behaviors as manifestations of masculinity norms that emerged due to intense local male-male competition and that persisted over time.