There's a whole number of reasons why women might not have been in the labour force. Women, of course, are more likely than men to be at home taking care of the kids, for example. We know that women are enrolled in school at a higher rate now than men are. That could have another impact. I'm guessing, though, that it's child-rearing that has the bigger impact on that.
On March 3rd, 2009. See this statement in context.