Men who explain things
This syndrome is something nearly every woman faces every day, within herself too, a belief in her superfluity, an invitation to silence, one from which a fairly nice career as a writer (with a lot of research and facts correctly deployed) has not entirely freed me. After all, there was a moment there when I was willing to believe Mr. Very Important and his overweening confidence over my more shaky certainty.
More extreme versions of this syndrome exist in, for example, those Islamic countries where women's testimony has no legal standing; so that a woman can't testify that she was raped without a male witness to counter the male rapist. Which there rarely is.