Here's an interesting article from New Scientist on the environment impact of the children people choose to have. Worst case (if impact continues to rise) is that each child would contribute 8× your personal impact. Assuming children (and their progeny) have the same impact we do today, each child would contribute 6×. And even in the best case scenario of shrinking impact, each child would add 1/3×.
The main point is that it's not zero. Like the other choices one makes in their life -- such as where to live, how to travel, and what to eat -- how many kids to have is an equally profound question from an environmental standpoint.
The main point is that it's not zero. Like the other choices one makes in their life -- such as where to live, how to travel, and what to eat -- how many kids to have is an equally profound question from an environmental standpoint.
This is just one aspect of the larger issue, whether individuals are capable of deciding and acting for the greater good.
Based on my own observations, I see two large hurdles that have to be overcome. One is that self-preservation is an instinct. Children are a means for propagating one's bloodline and, in many cases, a form of financial and emotional support in old age. The global need to have fewer children is trumped by one's individual need for having them.
The other hurdle is the fact that very few of the problems are of an immediate, truck-barrelling-down-on-you type of nature. 100% of pedestrians will instinctively try to get out the way if such a vehicle is bearing down on them, but nobody can actually see the metaphorical truck of overpopulation coming. We can understand it rationally, but it is just not tangible enough for individual members of society to truly grasp.
Combine that with the fact that any one individual's efforts are miniscule on a global scale, and it becomes impossible to expect the world's problems to be solved by leaving them to individual choice to do so.