How Many Girls for Every Boy?

**skeptic2** · Jun15-09, 08:45 PM

A society has the rule that if a couple gives birth to a boy, they may not have any more children. If they have a girl they may continue having girls until a boy is born. The sex of the babies is purely random and overall the chances of having a boy or a girl are exactly equal. What is the percentage of girls in the population assuming both sexes live to the same age?

**humanino** · Jun15-09, 11:09 PM

Half and half. Once I asked this here, and jimmysnyder found it completely obvious.

If parents voluntarily stop having children, it would have no effect on the ratio.
If parents who give birth on Tuesdays are not allowed to have more children, it would have no effect on the ratio.
If you pick parents at random and tell them that them must stop having children, it would have no effect on the ratio.

**jimmysnyder** · Jun16-09, 04:08 AM

To put it in even simpler terms, the ratio of boys to girls is determined by the people who have children, not by the people who don't. The people who do will produce them in the ratio 50:50.

**Borek** · Jun16-09, 04:16 AM

http://www.physicsforums.com/showthread.php?t=233255