I don't think there's a single culprit. Now, I'm not a sociologist, and I'm simplifying here, but it seems to me that it starts in childhood with the selection of toys and judgments on character that parents and family members apply. Boys get Legos, girls get dolls. This gets reinforced in grade school where perhaps girls are rewarded more for excelling in "creative" activities while boys are subtly steered in a different direction. In high school, science and history books are almost devoid of women. Meanwhile television and movies continue to portray stereotypical views of women. By the time college and career roll around, you've got both subtle and explicit forms of discrimination to contend with -- not to mention a complete lack of role models at the CEO and founder level in Silicon Valley. So the cycle continues.