So when parents are deciding whether to have a third, fourth, or fifth child, they are generally more conscious of the costs than of the benefits. Most of the costs are imposed on their other beloved children, while many of the benefits are dispersed among strangers.
When a decision-maker is more conscious of costs than of benefits, he tends to make decisions that are overly conservative. That almost surely means that parents have fewer children than is socially desirable, and that therefore, the population grows too slowly. My daughter is an only child, which makes me part of the problem.
Somewhere there is a young lady whose life has been impoverished by my failure to sire the son who would someday sweep her off her feet. If I cared as much about that young lady as I do about my own daughter, I'd have produced that son. But because I selfishly acted as if other people's children are less important than my own, I stopped reproducing too soon.
Population growth is like pollution in reverse. The owner of a polluting steel mill weighs all its benefits (that is, his profits) against only a portion of its costs (he counts his expenses, but not the neighbors' health). Therefore, he overproduces. Parents weigh all--or at least most--of the costs of an additional child (resources diverted from their other children) against only a portion of the benefits (they count their own love for their children, but not others' love for their children). Therefore, they underproduce.
This argument seems to suggest that I should have had more children for the sake of strangers. A second, completely separate argument says I should have had more children for the sake of those children themselves. Presumably they'd have been grateful for the gift of life. I'm not sure how far to push that argument. There's obviously nothing close to a consensus on how to assign rights to the unborn, so we can hardly hope for a consensus on how to assign rights to the unconceived. But the second argument does tend to buttress the first.
Personally, I ignored both arguments when I selfishly limited the size of my family. I understand selfishness. But I can't understand encouraging others to be selfish, which is the entire purpose of organizations like Zero Population Growth. Instead, we should look for ways to subsidize reproduction. A world with many people offers more potential friends who share our interests, more small acts of kindness between strangers, and a better chance of finding love. That's the kind of world we owe our children.