Most men, by the time they are men, have had at least one experience of being punched in the nose. At the very least, they've had to face the very real threat of being punched in the nose. Talking to the wrong guys girlfriend, making the wrong comment or any perceived slight in a bar can produce a threat of physical violence.
Most women have never had to face the same threat. Girls who are physically aggressive tend to have the run of the play ground as far as physical violence among girls goes. Most girls will avoid any possibility of physical confrntation so the physically aggressive female rarely get's to practice her aggression on her peers in fair combat.
That is why, in the adult world, I find aggressive women are far more emboldened and unrestrained than aggressive men. Men have had to face serious corrective consequences to misplaced or overzealous aggression in the form of very serious rivals and therefore have had to learn to restrain their aggression. Even a well matched fight usually leads to risk of serious injury to both parties.
So, the aggressive adult woman believes that aggression solves problems while adult men understand that unchecked aggression can lead to worse problems. So, while men are on average, more aggressive - especially in the younger years before the lessons have sunk in - the women who are aggressive are much more adversarial and dangerous.
For example, watch the aggressive female driver. She's a mom of two with the pedal to the metal from traffic light to traffic light. The aggressive sports moms I've seen are far more vocal and confrontational than sports dads. Most dads are laid back and relaxed while the odd mom who gives a damn about sports is shouting from the sidelines.
This is just a trend I've noticed and my theory of such a trend. To test the theory, perhaps one could test whether physical confrontation, or threat thereof, at a young age correlate to greater social caution in middle age? Separating the thugs, soldiers and MMA fans that just enjoy blood from the test sample would be necessary.