Great topic, i found the fact that population growth is fueled more by the reduction in death rates rather then the changes in birth rates. We have not even reached the upper limit of the average human life expectancy.  The population will only increase faster even with the life expectancy staying where it is at now.   Reading about the ZPG would only effect the population if it limited females to 2 kids for multiple generations shows how fast the population is expanding and what it would really take to just stabalize it.  I don’t see an off earth migration happening any time in our future, but that is my opinion.   We should focus on saving and sustaining our planet before we go and take over another.  Smail makes a great argument about this population growth topic.