NobodyReally
Superstar
Females are partly programmed to do it,” explains Corinne Sweet, a relationship psychotherapist and author of Change Your Life with CBT. “Firstly it’s only natural to compare yourself as it gives you a point of reference which can be reassuring. However, the harsh reality is that it’s a cattle market out there and the commodity is male attention. Women are checking out the competition and identifying who the alpha female in the pack is. Women subconsciously put themselves in a hierarchy,” she claims.
http://www.telegraph.co.uk/women/womens-life/10272172/Why-do-girls-check-out-other-girls.html
I mean,I'm sure that's true for many women. But it doesn't mean it's innate or a given for every single woman you see. I have a man, are you telling me that when I'm out shopping with my girlfriends, I'm sizing up every woman, even though my man isn't even with me? That doesn't make any sense.
Also, that's one woman's theory. Not a law of science.