When I was in grad school we held an annual dinner for just the women grad students, to welcome the first year students and get to know each other. After going to these for five or six years I noticed that nearly ALL the new incoming women grad students said they were afraid to speak up in seminars, and nearly ALL of them followed that comment up with something to the effect of “But it's not a gender thing, it's just my personal, individual problem.” Year after year I'd hear this.