> Women especially may not have as much time to contribute, since they are likely getting paid 30% less than their male counterparts and are more likely to be single parents than men.

I don't know if that's it. The undergrad population in the CS department when I was in college was 11% women (already pretty dismal) but when it came to code-heavy advanced courses like writing an operating system it dropped to 0%.

I would guess they either didn't believe they could do it or they were unwilling to put in the extra work, which in a sense was 'unpaid' since it was not a requirement for graduation. None of these women had children or had to worry about pay discrepancies.