[tajano]

"Women compete, compare, undermine and undercut one another — at least that is the prevailing notion of how we interact."

This is written as if men don't do exactly the same thing --and in my experience they do. Even among my close group of friends, the competition and "playful" undermining is non-stop and often exhausting.

"[Women] express indirect aggression toward other women, and that aggression is a combination of “self-promotion,” making themselves look more attractive, and “derogation of rivals,” being catty about other women."

So for the exact same reasons as men, though men wouldn't normally be described as "catty." Catty means being subtly or indirectly cruel, and though men will act this way, it seems infrequent, perhaps they are more directly combative, or they do it in a more joking way. In any case, it seems that men are equally competitive, for largely the same reasons, though they may express themselves slightly differently.