We should count by people. We should add up all the utility we predict each person will experience over their whole lifetime, and then divide by the number of people there are.
If we don’t do this we get weird suggestions like (as you said) we should be more willing to harm the long-lived.
Also, we need to add another patch: If the average utility is highly negative (say −50) it is not good to add a miserable person with a horrible life that is slightly above the average (say a person with a utility of −45). That will technically raise the average, but is still obviously bad. Only adding people with positive lifetime utility is good (and not always even then), adding someone with negative utility is always bad.
We should count by people. We should add up all the utility we predict each person will experience over their whole lifetime, and then divide by the number of people there are.
If we don’t do this we get weird suggestions like (as you said) we should be more willing to harm the long-lived.
Also, we need to add another patch: If the average utility is highly negative (say −50) it is not good to add a miserable person with a horrible life that is slightly above the average (say a person with a utility of −45). That will technically raise the average, but is still obviously bad. Only adding people with positive lifetime utility is good (and not always even then), adding someone with negative utility is always bad.