Before the 1980s, it was common for incoming college freshmen to join computer science courses without ever having used a computer. If they had, it might have been a school computer; but when schools are getting computer clubs it's also around the time of Title IX so schools aren't supposed to bar girls from computer club or computer usage. So there was no big "reason" for girls to not take CS courses; everyone theoretically started on a level playing field.
But the rise of the personal computer changed that. Margolis interviewed incoming college women and found that they had much less access to personal computers than their male classmates had. Parents were less likely to buy computers for their daughters than they were for their sons, even when the daughters were just as interested in computing as comparable boys. Or there would be a "family computer", but it lived in their brother's bedroom.
It was around this time that college professors started expecting students to have computer experience before they ever took Comp Sci 101. After all, the new personal computer was granting people more and more access than ever before! Unfortunately, that access was not evenly distributed to young women, and they started off their college careers with less real experience and less confidence than their male peers.
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