Charles Crawford is an evolutionary psychologist at Simon Fraser University in Burnaby, Canada and he has an interest in teenage girls. No not that sort of interest! He is interested in the catty remarks they make about one another, particularly in respect to physical appearance.

Studies by Charles and by Maryanne Fisher of York University in Toronto indicate that whilst competition amongst men tends to focus more on aggression or displays of wealth or physical fitness, women's competition tends to focus on physical beauty. Yes, hard as it is to imagine in todays enlightened feminist utopia, there are still cat fights amongst teenage girls and it is physcial appearance that is the weapon of choice.

What is more it appears that they are at their worst when the girls are in a fertile period of their monthly cycle. Asked to rate the physical beauty of other girls, girls consistently rated them lower when they were fertile compared with when they were not.

You can read more in Nature here.