'Pollution makes for more girls' http://www.nature.com/news/2005/051017/full/051017-16.html In polluted areas the ratio of boys and girls being born is skewed towards girls compared to non-polluted areas. That means that more girls are born in a polluted environment. They suspect it might be a survival strategy of the human species. If the environment causes stress on the human species it is a survival mechanism to produce more women since they can produce more offspring (then men). More women, more babies can be born. And maybe the local population can survive more easily. Or are girls just dirty?