Men's defense of their prototypicality undermines the success of women in STEM initiatives

•Men are prototypical (representative and normative) in STEM.•Men believed that efforts to increase the % of women were working (vs. not).•They reacted to successful initiatives by opposing them.•Opposition was explained by men's concern about no longer representing STEM.•Relationship held only among those who believed that men should be STEM prototype.

Two studies tested the prediction that men in STEM (Science, Technology, Engineering, & Math; students in Study 1; professionals in Study 2), who believed that initiatives to increase women's representation in these fields were effective would experience prototypicality threat (men's concern that they would no longer be the gender group that best represents what it means to be a member of the STEM community). Among those who believed it is legitimate for men to represent STEM, men's prototypicality threat mediated the relationship between perceptions that more women were entering their field and resistance toward this change (i.e., opposing women in STEM initiatives, wanting women to conform to the field's traditional norms, and expressing exclusionary intentions toward women peers). The opposite pattern was observed among those who rejected the idea that men's claim to represent STEM was legitimate. This work highlights how diversity initiatives in STEM, if successful, can be undermined by triggering prototypicality threat among men.