I think girls just don't spit in front of other people, but they do it just as much, if we exclude the male tobacco dippers. It's like my Southern Baptists friends who drink from a coffee cup.
sorry SJ but I've never known a female spitter. But I've known hundreds of males who just spit out of nothing but habit, anywhere. anytime. and I've never understood this behavior. It IS just habit, and it IS completely gauche. picking one's nose in public is about the same, as well as grabbing of the omb area. disgusting! and it is definitely a man thing!!!
there are some men however, like SJ and many dude friends in my life, who do not spit - at least not publicly - they just know better, or they never developed the habit. thank God. (I could NOT have married a spitter - which ruled out a huge number of potential prospects). these good men with good manners actually do a little crotch grabbing when they think no one is looking however. and they can leave the toilet seat up occasionally. farting is a given. so, no one's perfect.
do you remember during the Olympics hearing about China (men and women and children) and how they were training Beijing residents not to spit and pick their feet? makes my skin crawl to think of it but apparently, its a very socially acceptable thing and the people spit constantly and everywhere.
I guess we all have our bad habits.

one of my pet peeves is picking your teeth with a toothpick after a meal, in public. kills me. but that's no worse than cell phone etiquette exhibited by most people in public... answering/talking on a cell phone in a restaurant - this ranks right up there, imo.
my bad habits? I'm not telling.