A self-effacing attitude, or the semblance of it, is socially necessary, because while young women are allowed to be Smart, they are not allowed to be Arrogant.
This gibes with what I've heard about the difficulties of being a female professor in a traditional male-dominated environment of academia.
The Trouble with Bright Girls (article @ the Huffington Post)
Excerpt:
The topic of this article seems to relate to several common Less Wrong issues: the nature of human intelligence, and the gender imbalance among LW readers.
I'm not sure how much credence I give to the proposed explanation of the difference in mindsets. It may well have to do with socialization and feedback, but the specific description of feedback that is presented seems a bit too much of a "just-so story" to me. The difference itself is fascinating, though, and I hope more is done to further our understanding of it.