It's not that they are specifically "girl games", but rather games that girls would like over say Gears of War or Hitman. Nowadays what is considered a girl games is just one with less violence, usually puzzle games (Picross, Puzzle Quest, and even Zelda fits in there)or management games (Nintendogs, Cake Mania and Animal Crossing).
And the DS and Nintendo overall are really making leaps and bounds to include games like that with they're new casual gamer demographic, which isn't just girls, but younger kids, our parents and the like, which means that there are an abundance of games coming out that aren't all about the killing.
I think what Will was referring to however were the "Girl Games" of the 90's, which were exclusively the pink/cute/frilly type. The idea still holds some water now, but in a different sort of way. Back then, game developers were legitimately trying to expand they're demographic to include girls by appealing to they type of toys they had; Barbie, My Little Pony, even an Easy Bake Over computer game, while at the same time being an obvious money grab. Nowadays, it's just a money grab scheme by franchises (mostly Disney), slapping a brand name on mediocre adventure/platforming games based on popular girl TV shows; Dora, That's So Raven, Hannah Montana, High School Musical...
All that to say that we girls all don't like a good action shooter game every once in a while. We're just in the minority; though the general thought that girls can't be hardcore gamers is slowly disapearing, it's not gone yet.
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This looks like a job for Science!