Too many factors involved to say one or the other, imo. Girls + estrogen and boys + testosterone during that timeframe is going to have biological side effects because of hormonal surges, but culture and individual environment has an impact on aggressiveness/expressiveness. It's hard to look at a culture where expressing certain emotions is unacceptable, or one sex is expected to act a certain way while the other isn't, etc.
Then you have to consider factors like nutrition, education, the hormones and crap added to our food, the list of things that could cause behaviors goes on forever.
I think in the end it's mostly going to come down to fluctuating hormones that affect emotions and still being in formative years mentally where they may feel indestructible and knowledgeable about things that they don't actually have enough experience with.
Although I'm still a "smart-ass 11 year old" who thinks she's never wrong, so maybe you shouldn't listen to me
