If there are not enough resources to go around, people will be forcefully shown that they shouldnt procreate at an uncontrolled rate. This is currently whats happening in Africa. There isnt enough food to go around and they dont exactly try very hard to grow their own, so they all starve and die. Granted, not everyone there is at fault because of wars and such, but if I was in a place like that I would be trying my hardest to move elsewhere. They just keep having children though and stay put.
An interesting consequence of the above is that generations are passing more quickly with people having children at a younger age, this is going to lead to faster evolution and likely a quicker maturation rate eventually.