I'm sure there's some of both. Traditionally there haven't been a lot of other women to paddle with so women had to act and paddle more like men if they wanted to get better and paddle harder stuff. Now that there are more women paddlers out there and workshops and resources for women I think women are building more confidence by paddling with other women. In my experience, the majority of female paddlers have an easier time pushing their limits when they're paddling with other women or with men that they really trust. Then again, you have some women who are more aggressive and just go for it.
In my opinion, I think the fact that women have traditionally had to act or approach kayaking like men has not been to our advantage because that only works for a very few women (which is why there have been so few of us). That's one of the reasons I started Girls at Play, because I wasn't one of the women who was very good at or liked approaching whitewater like the guys. I like to have support and I really enjoy the social aspect of kayaking as well.
The stress response thing applies to most women -- it's not something that can change just because you're paddling with guys. It's the way we're wired. Instead of fight or flight we're more like tend and befriend. Notice that when you paddle with women they like to take their time, they like to scout a little longer and they like to discuss their options for running rapids. They like the social support. Most guys can make up their minds really quickly as to whether or not they're going to run a rapid and are more indepdendent about it and they love to go fast! Of course, I'm making generalizations here and I realize that not every man and woman falls into these behavior patterns, but a lot do.