The day you were born, you brought both costs and benefits into this world. The costs include the demands you made (and continue to make) on the world's resources. The benefits include your ongoing contributions to the world's stock of ideas, love, friendship, and diversity.
Do the costs outweigh the benefits, or vice versa? In other words: Should the rest of us consider your birth (or any child's birth) a blessing or a curse?
Let's not try to settle this by listing all the costs and benefits of sharing the world with other people. After an evening stuck in summer traffic, you'll remember that the driver in front of you imposed a cost, but you might forget that the guy who invented your car's air conditioner conferred a benefit. New Yorkers remember to complain about the crowds, but sometimes forget that without the crowds, New York would be Cedar Rapids.
Instead of making a list, let's think about the decision your parents faced when they were considering whether to conceive a child. Is it more likely that they undercounted costs or that they undercounted benefits?
I'll start with benefits. The clearest benefit of your birth is that it gave your parents a child to love; they certainly counted that one. But the other benefits are spread far and wide. If you build a better mousetrap, millions will be in your debt. If all you do is smile, you'll still brighten thousands of days. We don't know how to list those benefits, but we do know that many of them fall on total strangers. That makes it unlikely that your parents took them fully into account.
Now let's look at costs. The costs of your existence fall into two categories. First, you consume privately owned resources like food and land. Second, you might consume resources to which you have no clear property right--for example, you might open a factory that pollutes the air I breathe, or you might become a burglar who steals my stereo system.
(You might imagine that there are also costs associated with your competing in the marketplace, bidding some prices up and others down, applying for the job I wanted, and so forth. But each of those costs has an offsetting benefit. If you bid up the price of cars, sellers will gain as much as buyers lose. If you prove a stronger job candidate than I do, my loss is the employer's gain.)
Stealing and polluting clearly impose costs on strangers. But if you're at all typical, your consumption of staples like food and land will far exceed your consumption of other people's air and other people's property. In other words, for most people, the first category of costs is the big one. So let's concentrate on that.
Where do you get all those resources you own and consume? Some you create; those don't cost anybody anything. Some you trade for; again, those don't cost anybody anything. The rest you inherit; and those come from your siblings' share. That means your siblings--not strangers--bear most of the costs of your birth.
That's a point that's often missed. When people think about overcrowding or overpopulation, they typically imagine that if, for example, I had not been born, everyone else would have a slightly bigger share of the pie. But that's not right. If I had not been born, both my sisters would have substantially bigger shares of the pie, and everybody else's share would be exactly what it is now.
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.