The King's Law
Deciding that his kingdom needed more women, the king proclaimed a new law:
"Henceforth, upon the birth of a son a family shall bear children no more!"
His thought was, of course, that a family could have as many girls as
they wanted before having to call it quits, which should raise the ratio
of girls.
Did his plan work?
It did not.
The chance of a boy or girl being born is equal, so each birth can be considered as being half a boy.
In other words, this cannot change the boy:girl ratio.