The world has become prosperous. Population has stabilized at 10 billion. All is well in every way. And now scientists are working on a cure for aging, a pill that will increase average life span from 75 to 150. If people live longer, they will have to produce half as many children (to avoid a drain on resources). But this helps a bit: the pill will have a secondary effect of making fertility last to age 50. And we are to assume that the super-elderly would retain the abilities and vigor of people in their 60s and 70s.
Should the anti-aging pill be pursued? There are a couple of crucial questions. (1) What's better, a 75/2 life or a 150/1 life? (2) What of the fact that there are half as many people in 150/1? Should halving the world's population concern us?
Blackford agrees with Singer that the 150/1 life is better, but disagrees with him about the significance of halving the world's population. So (2) is where the action is. Because they answer (2) differently, they come to different conclusions about the whole question. Singer: no anti-aging pill. Blackford: let's do it.
But let's not rush to (2). (1) is interesting...and puzzling.
Up to age 75, Blackford says a 75/2 life will be just as happy as a 150/1 life. No argument, it's just supposed to be obvious. But how so? Can it make no difference to happiness whether we have one child or two?
Also there's this: the 150/1 people will have no siblings. Will that make no difference to happiness? (Happiness isn't really all that counts, as I argue here, but let's pretend it is to keep things simple.)
After age 75, Blackford assumes there will be just a small reduction in average happiness for the 150/1s. He draws support from positive psychology research that shows that people in their 70s and 80s tend to be quite happy. But how much can that tell us about what it's like to be 135?
At any rate, both Blackford and Singer assume that average happiness for 150/1s is a little lower than for 75/2s, because of that late life decline. Maybe it's just 9 for 150/1s and 10 for 75/2s. Yet they both assume 150/1 is the better life. Do the math, they say: 150 x 9 = 1350. 75 x 10 = 750.
It seems as if they must be using the right formula, but not necessarily. Suppose you are offered a gift of 150 good books (9 points each) or 75 great books (10 points each). If you multiply, the total value of the first collection seems greater than the total value of the second. Yet, it might be rational to choose the 75 great books, if that's the only way you're going to get Tolstoy and Dickens.
Likewise, it's perfectly reasonable to want the shorter life, even if it yields less total happiness. Why? Because just like you want Tolstoy and Dickens, you might want the high highs you'd have with a second child and with siblings.
I think 75/2 may be the better life. So I don't have to worry about the conundrum whether a world of 150/1s should be preferred, since it contains better lives but fewer of them. It doesn't necessarily contain better lives. But "what-me not worry?" (to mangle Alfred E. Neuman). So (perhaps) more on that later.