<p>Recent studies cast doubt on the assumption that national gender equality widens the gender gap in preferences and choices for Science, Technology, Engineering, and Mathematics (STEM) majors, as posited by the Gender-Equality Paradox (GEP). Gender-equality effects do not show up within world regions, and the effects cannot be disentangled from other factors found to be higher in “the West.” However, the Affluence Paradox, which describes larger gender STEM gaps in more affluent countries, may be interpreted causally because societal affluence fluctuates, and there are strong reasons for affluence to affect gendered STEM choices. We advance the literature by assessing whether societal affluence is longitudinally associated with gender differences in STEM graduation and by examining this association by study field and a country’s level of development. Using data from the United Nations Educational, Scientific and Cultural Organization (UNESCO) on women’s and men’s STEM graduation patterns over 25 years across more than 100 countries, and applying two-way fixed-effects regression models, we find that societal affluence is positively associated with the gender STEM gap. This association holds across emerging and advanced countries, yet it varies by field of study and — contrary to the economic-need assumption — results from a positive effect of societal affluence on men’s STEM odds. Our findings highlight the role of societal affluence in understanding global patterns of horizontal gender segregation and the need to address its effects.</p>