But unless your population ethics are “fine-tuned” to make Plus and Minus equally cost-effective, one of them will be clearly better (more cost-effective) than the other. If you think Minus is better than Plus, then Minus is better than Plus+Minus, which is better than Growth, so you should donate exclusively to Minus.
I don’t think this follows. If you think that Minus is better than Plus, it does not follow that Minus is better than Plus+Minus. Likewise, if you think that Plus is better than Minus, it does not follow that Plus is better than Plus+Minus. The key is that with certain population ethics, the change caused by Plus or Minus is context-dependent.
For instance, suppose you have average utilitarianism and utility is proportional to income per year. Also assume an existing population of two people each earning $500 in a poor country.
Minus causes one less person to be born in the poor country. This doesn’t change the average, so with Minus you have spent $1000 for no change in utility at all.
Plus costs $6000 and causes there to now be 3 people. Overall utility is (5000 + 500 + 500)/3 = 2000
Minus + plus costs $7000 and causes there to be 2 people with overall utility of (5000 + 500)/2 = 2750
Plus has a cost per added utilon of 6000 / (2000 − 500) = 4.
Minus+plus has a cost per added utilon of 7000 / (2750 − 500) = 3 + change
Therefore plus+minus is better than plus which is better than minus.
I assumed that the effect of the intervention is “small enough relative to the world” for your population ethics to be smooth. For average utilitarianism in particular, this corresponds to the percentage change in population being small. In your scenario, this isn’t true, since there are only 2 people, but in the real world it holds up very well. Just 1% of the world population is 70 million people, and virtually no intervention (except for things like existential risk reduction) could cause such a large population change.
I don’t think this follows. If you think that Minus is better than Plus, it does not follow that Minus is better than Plus+Minus. Likewise, if you think that Plus is better than Minus, it does not follow that Plus is better than Plus+Minus. The key is that with certain population ethics, the change caused by Plus or Minus is context-dependent.
For instance, suppose you have average utilitarianism and utility is proportional to income per year. Also assume an existing population of two people each earning $500 in a poor country.
Minus causes one less person to be born in the poor country. This doesn’t change the average, so with Minus you have spent $1000 for no change in utility at all.
Plus costs $6000 and causes there to now be 3 people. Overall utility is (5000 + 500 + 500)/3 = 2000
Minus + plus costs $7000 and causes there to be 2 people with overall utility of (5000 + 500)/2 = 2750
Plus has a cost per added utilon of 6000 / (2000 − 500) = 4. Minus+plus has a cost per added utilon of 7000 / (2750 − 500) = 3 + change
Therefore plus+minus is better than plus which is better than minus.
I assumed that the effect of the intervention is “small enough relative to the world” for your population ethics to be smooth. For average utilitarianism in particular, this corresponds to the percentage change in population being small. In your scenario, this isn’t true, since there are only 2 people, but in the real world it holds up very well. Just 1% of the world population is 70 million people, and virtually no intervention (except for things like existential risk reduction) could cause such a large population change.