no it's not Y+A=    No it's not zero population growth thats absurd. WOW! no wonder trump won. and if there kids have children that do the same after 15 to 25 years of life you now have 3rd gen while most are still alive will have 2+2+4+8 now instead of 2 parents you started with.Now  you have 16 and they are all hopefully still in good health how is that not population growth? when the grand parents die at approx 80 you subtract them and add the next generation this time 8 times 2 now you have 30 people all from 2 after they die.