I really wish I could find the original paper I read on the subject for you. I just went looking for it again and I didn't find it. It was something I read randomly in my travels, it was really long and really well researched with lots of data. I wish i bookmarked it. I found some more interesting stuff though.

http://www.issues.org/29.4/hal.html

this one has more of an "agenda" behind it

http://www.vdare.com/articles/national-data-stem-shortage-sh...