<p>Individuals sometimes misjudge the views of others in their group, believing that their own attitudes differ from those held by the majority. Across five studies (<i>N</i> = 1159) conducted in Australia and the United Kingdom, we assessed the similarity between cisgender women’s own beliefs and their perceptions of the group norm (i.e., other cisgender women) regarding the inclusion of transgender women within the boundaries of womanhood. We found robust evidence that cisgender women in our sample underestimated the level of support for inclusion of trans women among their group relative to their own beliefs. Brief interventions that communicated norm information were shown to be effective in increasing participants’ perceptions of the norm. However, overall, the intervention did not affect willingness to express inclusion opinions or engage in actions to support trans women’s rights and their social and legal inclusion into the group ‘women.’ These findings highlight the extent to which group norms surrounding trans inclusion can be misperceived and suggest that correcting these misperceptions may help counter the influence of exclusionary minority voices.</p>