I'm not a female, but this seems to me an obvious competition within sexes. Assuming that most people are heterosexual, for a female a competitor is another female, just like for a male a competitor is another male. So the ability to put down members of the same sex is an evolutionarily selected trait. (Of course it is not the only evolutionarily selected trait; in other contexts a cooperation is rewarded.) Males and females use somehow different methods of putting down their competitor, probably the ones better suited to their comparative advantages: males will try to put down other males by physical attacks or threats, females prefer mental and social attacks.
However, a reason why some behavior is evolutionarily selected is not the same as a mental process by which it is started. (For example a desire to have sex is selected because it leads to reproduction, but we do it for pleasure.) A psychological trigger that starts the intra-sex competition may be a feeling of insecurity. Which, among other things, may be also triggered by a presence of a competitor.
In my understanding, few men would notice if I wore the same dress twice. Out-competing other women for men only makes sense if the men notice. The level of attention to dress that will impress most men is lower than the level that will impress most other women. So to the extent that women are dressing carefully to impress men, it's largely mediated through other women. Women may snark to men, or to other women in men's hearing, about other women in order to jockey for status. (I realize this isn't an Austen crowd, but think Miss Bingley snarking to Mr. Darcy about Elizabeth Bennet.)
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?