After much research, I didn't know how I was going to answer this question, so I started compiling a list. I took a little bit from here and a little bit from there to put forth my best explanation.
The facts are that in 2001, only 20% of the earned engineering degrees were by women, and in 2007, only 18% of earned engineering degrees were by women, a decrease. http://www.nsf.gov/statistics/wmpd/pdf/tabc-4.pdf In 2007, only 11% of women made up the engineering workforce. http://www.nsf.gov/statistics/wmpd/figh-1.htm
Why is this? In most other areas of study and skill, with the exception of computer sciences, women are neck and neck with men. One explanation is that girls don't tinker. Tinkering is a stepping stone to engineering. Tinkering enables students to work on something and figure out on their own the whys and hows and to consider what ifs. There's no instruction, just natural curiosity.
So why do boys tinker and girls don't? Is it because boys are naturally curious and girls are often task oriented? Is it because boys are allowed to do things for themselves and girls let others do things for them? Is it because boys learn by doing and girls learn by listening and watching? Is it because men see a broken toaster as a problem that needs to be solved and women ask how the toaster broke?
Another explanation for why there are fewer woman engineers may be as simple as our parent's involvement. A research study shows that the number one influence on a girl's education future is her parents. Parents encourage their daughters to help people, teach people. Engineering isn't often viewed as a way to help people even though that's exactly what it does.
The way engineering is viewed leads into the next explanation. Girls may view the image of engineering as geeky, I think simply because they don't know what else to compare it to except the common stereotype. When someone hears "doctor", people get that even though there are a ton of different kinds of doctors; whether someone is a podiatrist or a cardiologist, he or she helps people get better. When girls hear "engineer", they may not know what that means or what it entails or understand that some kinds of engineers design tools that doctors can use to help make people better.
The word engineer may be too broad, especially when girls can pick careers like nurse, teacher, manager, or therapist and understand what those careers mean by just the name itself. And this may go back to the basics. Boys hear the word engineer, and they question what that is; girls hear the word and listen for someone to tell them what it means. If they don't hear the explanation or aren't introduced to women engineers so that they can have a role model, they probably will decide to do something else.
My last explanation is that socially girls aren't targeted for engineering jobs, just like boys aren't often targeted for nursing jobs. I was watching a commercial for a technical school a couple days ago. It showed images of men drawing up the plans of a house, men gathering the supplies to make the house, and men building the house. The very last scene was of a woman with her kids in her finished home. I had already started planning this blog at the time, so I was more aware of the impression it left me with, but the point is still there.
Have I answered the question? Why are there fewer woman engineers? Probably all of these explanations are contributing factors, and I've probably forgotten a couple more. As women, we've made massive strides and continue to break ground and make history every year. I can't help but be hopeful that soon my question won't be relevant anymore.