Comment Re:Girls, girls, girls... (Score 1) 333
It's biology.
The studies were done with babies less than a year old as to rule out cultural influence.
It is also the fact that different hormones affect the human brain differently, a brain awash with testosterone will be structurally different than one awash in estrogen and progesterone.
Keep in mind just a change in the level of serotonin can radically change someone's behavior.