[Morendil]

Recommended reading on this topic: Ensmenger's "The Computer Boys Take Over", or the more scholarly "Gender Codes" edited by Thomas Misa.

The latter in particular gives lots of detail, not just in describing the situation (the imbalance isn't the same everywhere, in every company or in every job description) but also in in explaining how (historically) and why (causally) the situation became what it is today.