Marrying at 18 often prevents people from getting a good education and high-paying jobs. People living in poverty are forced to have less children, This is why the Arab birth rate goes down: Arab women do not want to turn their wombs into demographic bombs even though old terrorist Arafat wanted them to. They want a better life for them and their kids. 

Arab birth rate is going down. Hamas knows that and forbids UN family planning services, trying to stop the trend.