"The most simple one, that women engage in far fewer numbers explains all observable effects in my opinion." > Well yes. But why do you think that is? If OSS is, in general, hostile to woman (and I would suggest that it is) than obviously fewer of them engage with it. Of course that same problem is reflected all the way up the chain - fewer woman become programmers because computing is hostile to them - a trend which starts at school and has been the case since about the 1980s.