Non-identity cases force us to distinguish harming people in general from harming particular future persons. (Suppose that someone would have lived a happy life, but instead I bring it about that a different, less happy, person comes to exist in their place. This is an impersonally worse outcome -- "bad for humanity", we might say -- though there's no particular person who is worse off than they otherwise would have been.) Frances Kamm further suggests that we need to distinguish harming future persons from harming an already-existing person's future. She claims that some harms are permissible to impose on future persons but not on already existing persons, as the latter have additional rights.
I'm skeptical. But first let me suggest a point of agreement: it may be worse to handicap an existing person than a future person. But this is not because of any difference in the moral status (or rights) of the agents. It's because depriving someone of an ability they already [know they] possess may constitute a greater harm (e.g. by interrupting their life projects, and forcing negative revisions to their self-conception) than preventing them from acquiring the ability in the first place. So, to ensure that we hold fixed the weight of the harm done, let's compare two cases of preventing a future ability from developing.
Kamm discusses a case roughly along the following lines. Suppose that Billy has some gene that will become activated in adolescence, which causes enhanced cognitive development. To keep things simple, let's just say it'll increase his IQ by 20 points. Now suppose (never mind the absurdity) that we can remove this gene and somehow use it to give two other people the benefits instead. Should we do so? Kamm claims that it depends on the timing:
(1) Suppose we have isolated a particular sperm and egg, such that the resulting conceptus will grow into Billy. But Billy doesn't exist yet. In that case, Kamm thinks it would be permissible to extract the gene -- causing the future person Billy to have IQ of 120 rather than 140 -- so that others might benefit in his place.
(2) Suppose that Billy is eight years old (but doesn't know about his genetic potential, and we could magically extract the gene from a distance without anyone ever knowing, so that his two possible futures are exactly identical to those found in the previous case). Kamm claims that Billy, as an existing person, has a right to keep the genetic endowments he now possesses, so that it would be wrong to magically extract the gene.
I don't share Kamm's intuitions here, so I have trouble seeing this as a (fundamental) morally relevant distinction (though of course it may be worth instituting existence-dependent "rights" of this kind for pragmatic reasons). But it's an interesting suggestion, at least. I'd be curious to hear others' thoughts...