While some of this can be explained by society—in countries like China baby boys are favoured and many unborn girls are electively aborted—there are natural processes at work.

有些时候这种情况可用社会来解释—在中国等国家，男孩比较受欢迎，还未出生的女孩会被选择性地堕胎—不过自然过程也有影响。