Right, Ive been quite interested in this for a while, to the point of actually reading game design articles about it.
The general consensus is this: Women think differently to men
1)Men look at games as a series of challenges to be overcome, be that beating levels in an RTS, winning rounds of CS:S or whatever. Women see games as a much more "experience" based things; they want to have fun, not necassarily be challenged.
2)Men like to build, expand, improve things, all for a practical purpose. Women like to organise things, and make things....well, nicer. They also like to build and expand, but for more artistic than practical reasons.
3)Women are more social creatures than men.
4)Women physically see things differently. I dont mean in some vague emotional way here, but rather their eyes and visual centers of the brain are built different. Men, due to the evolutional role of being the hunter, have very centralised vision; they concentrate almost entirely on what they are directly looking at. Women, on the other hand, were gatherers, and have a more "dispersed" vision; they are much more aware of things on the periphery of thier vision. As a result, basically, women are more likely to notice the backgrounds in games, while men see more of the central characters, be that the player model or the thing the player is shooting at.
Basically, games are, by and large, made by men. So they cater for men. They involve lots of building things, improving things, shooting things dead. Practical issues take precidence, whilst aesthetic issues either take a back seat or are utterly missing. Social sides of games (mmo's notwithstanding, but i'll get to that in a minuite) tend also to be either a bit rubbish or totally non existant. And graphics concentrate on characters, rather than the settings. If the characters are great, the landscape they are in tends to be merely good. If the characters are rubbish, the landscape will more than likely look like its constructed from lego.
The trouble is, games are made mostly by men, and therefore appeal mostly to men. So the people who want to go into the games industry are mostly men. Viscious Cycle Tiem!
I think the reason MMO's seem to have more women playing them than other types of games is because there is a heavy emphasis on a)interacting socially and b)the aesthetic side of things. Take CoV for example: theres a hell of a lot more options for makeing yourself look cool than there are for actually customising what you can do.
Finally, to pre-empt the inevitable daft resonse that always crops up when you are discussing the differences between two groups of people: All the above is a generalisation. When I say "Women are like this" I actually mean: "Most women are like this", and vise versa. Hell, my girlfriend is a fine example of how women are not all the same: Shes a massive RPG fan. The same goes for games; they are not all like i discussed, but they are *mostly* like this.
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