Two reasons: FUD and the fact that women are more rational (as someone said "more sensible"). Purely statistically speaking, you are more likely to do work that

Two reasons: FUD and the fact that women are more rational (as someone said "more sensible"). Purely statistically speaking, you are more likely to do work that's both interesting and well paid if you go into medicine versus software engineering. Medicine takes much longer and steeper learning curve, but once you climb that an average job requiring an MD is more interesting than an average job requiring a BS in CS and pays better (while if you fail to get an MD, an average job requiring a BS/MS in Bio-Chemistry is at least as interesting and still decently paying). The FUD part is pretty self explanatory: introversion doesn't mean lack of social skills (the latter are learned and eventually most geeks do end up learning them one way or another) and given the amount of team work involved in any real engineering projects there are plenty opportunities for extroverts to re-charge. The way to encourage women to go into these fields is to provide dual degree programs (no opportunity cost for seeking a CS degree and applying to engineering jobs as an undergraduate) and combating the FUD.