Given the worldwide pattern that women leaders are often constrained by expectations to demonstrate proof of masculinity to be considered effective, this study explores whether it is possible for these expectations—and the leadership styles they promote—to change over time. I develop a new methodological tool to measure stereotypically masculine leadership style called the Ferrous Scale and apply the Scale to the three female prime ministers of New Zealand, where I find that Ferrous scores decline with each female prime minister. I conduct interviews with the three women leaders and examine a set of hypotheses on the causal mechanisms behind the apparent erosion of the association between executive office and masculinity in New Zealand, building a theory called the Succession Effect: that a succession of female prime ministers—rather than societal gender progressiveness or partisan affiliation—was the prerequisite in New Zealand to loosen the association between executive office and masculinity.