As you can see from the graph up there, the percentage of woman pursuing computer science related courses at university initially tracked other professions, such as medicine, law and science. Then, around the mid-80s, it nosedived to the largely male dominated field we have today.
The cause wasn't because the discipline appealed to men more, or woman less, say reporters Caitlin Kenney and Steve Henn. In fact their research showed "no grand conspiracy" and "no sign on a door that said, 'Girls, keep out.'". What they found instead was that adverts focused almost exclusively on men: "Ads for personal computers were filled with boys," says Henn. That led to computing being seen as a male activity, and more men getting computers. This "targeted advertising" created a male dominated area, and fostered an advantage in education where men were more likely to own, access or use a computer.
In 1995 a UCLA senior researcher called Jane Margolis looked into Carnegie Mellon's Computer Science degree and found that half of the women in the program ended up quitting, and more than half of those were on the Dean's List – effectively the highest achievers on the course. According to Margolis: "If you're in a culture that is so infused with this belief that men are just better at this, and they fit in better, a lot can shake your confidence". Her research lead to changes in the the program and by 2000 almost half the students were women, while the drop out rate was equal for both genders.
It's an interesting theory, because while we usually look back at those adverts and brush them off as a humorously outdated, it must have sent a very clear message at the time. Obviously there's more to making games than just programming, but it's a key foundation and much of the industry we know today sprung from a purely coding basis before growing into the multiple disciplines we have today.