Just to make it clear, those numbers on the 60/40% ratio males to female gamers came from this demography pdf.
http://www.theesa.com/facts/pdfs/ESA_EF_2008.pdf
It's not really a study, just a presentation of simple numbers. There's nothing in there on how the numbers were acquired or what defines a game or a gamer, but at least it
looks like they've based it on quite a lot of data.