nature vs. nurture?

I'm sure I've handed the girls (and boys) baby dolls and taught them how to give "Ahhhhhhs" with hugs and little wet kisses. But never (and I'm sure about this) have I pointed an index finger (let alone a real gun) and made a shooting sound. Where do boys learn this?! Is it genetic? The girls don't routinely do this...but Frank (and all the little men for that matter) are packin heat with their index-finger-pistol from the moment they crawl out of bed in the morning. Frank takes aim and chooses his breakfast cereal with one eye closed and the trigger pulled. "Pow" (apparently he's having Fruit Loops, partner.)