Just because culture and income have some effect doesn't mean that skin color doesn't have a large effect. We can ask 'what if' forever, bringing up endless objections, but it's just a form of propaganda (intentionally or not) and doesn't change the obvious facts.
You are opposing "race" and "culture" when race is in large part a cultural construct.
The question I ask is: Why struggle? What is the motivation for denying these groups justice?
But to clarify one important point: Most discrimination is ignorance, IMHO, not intentional. People don't see their own cultural bias - e.g., instinctively trusting the person more like what they are already familiar with (which becomes self-fulfilling) - or overlook their advantages, such as the fact that the culture of their college and workplace are so natural to them that they don't even notice that there is a culture, but that it's alien and exclusive for others. They don't imagine what it's like to be the only woman or dark-skinned person in the room, all day every day, for the rest of their lives; never sure if it's going to cost you a promotion, a job, a client, etc.
That was not my intent, as I certainly don't beleive that. Rather, I wanted to disambiguate the GP question (which could be taken either way).
Perhaps I should have shortened my request for clarification to "which culture and what effect are you asking about?"
Anyway, over/under representation happens through many avenues, some through overt and intentional discrimination, and much more through secondary and tertiary effects, many of which reinforce each other through positive feedback loops (not to mention reinforcing the overt and intentional sort as well).
eg. once you control for sibling order, parents' educational background and income, etc., it is clear that on average girls get their first computer at a later age than boys by several years. If a computer is supposed to be shared between siblings and it isn't in a common area it will tend to go into the boy's room. and so on.
Parents aren't deliberately reinforcing gender stereotypes to reduce their daughters' opportunities to discourage them from using computers, but that's exactly what is happening, and this is entirely a cultural phenomenon.
You can in many other cases measure a discouraging effect from within the culture of an underrepresented group, but just because it is worth noting however and whenever a group is complicit in their underrepresentation, it does not magically shift responsibility away from members of an over-represented group to accept changes to their culture to stop discouraging and excluding others, most specifically in this case any beleifs that participation in Free Software is entirely based on merit.
What leads me to believe culture has more of an affect nowadays than race is the fact that Asians have higher average incomes than whites, even though they were also historically subjected to racism.