I am teaching a bit of probability in my mathematical methods class. One of my favorite intro problems is this:
China has a "one child" policy. This policy is problematic (and, at times, downright murderous) in a society that values sons over daughters. It has been suggested that a more popular and humane policy would be a "one son" rule. With this policy, every married couple could keep having children until they had their first son, then they could have no more.
Assuming that for any pregnancy a son or a daughter is equally probable, what would be the long term effect of this policy on the distribution of boys and girls in China?