  A third problem is in certain respects closely related. It is the  non-identity problem, first discussed in detail in Parfit  1984, 351–379. Suppose a fourteen-year-old girl decides to have a  child. Given her youth, the girl gives her child a bad start in life. If she had waited several years to have a child, that child would  have been better off than the one she had. Our intuition is  that what the girl has done is wrong. But why? The obvious answer  would seem to be that the girl's decision was bad for her  child. But this cannot be right, for that child would not even  have existed were it not for her decision. It is also clear that she  doesn't harm the child that would have existed had she waited, given  that something that does not exist cannot be harmed. It thus turns  out to be very difficult to articulate just what was wrong with the  girl's decision, but as Parfit notes, we do not abandon our belief in the wrongness of what the girl did. Instead, “we cannot defend  this belief in the natural way … suggested” (Parfit  1984, 359). But finding an alternative defense does not look very  promising. And applying the non-identity problem on a grander scale  yields a serious puzzle in population ethics. Suppose governments  around the world continue to ignore the threat of global warming, so  that the lives of people living between 2200 and 2500 are barely  worth living. Contrast this case with the scenario in which the  governments cooperate to enact regulatory measures that slow down the warming trend and render it a non-threat, but in so doing make the  lives of people living between 2010 and 2200 of a lower quality than  they would otherwise have been. Clearly, it seems the second scenario is better, that we ought to bring this scenario about. But on what  basis? Far-reaching governmental policies often play a critical role  in determining the ways and timing in which people get together, so  the identities of the members of the populations in each scenario  would likely be quite different. If we also assume that we do not  harm anyone by bringing him or her into existence, then who is harmed in the former scenario? Who is made worse off? Given that those  people would not have existed had we done something about global  warming, they have not been harmed by our inaction. So what moral  reason can be given in defense of our pursuit of the second scenario? (Parfit 1984, 371–377) These are very difficult issues indeed. (For  further discussion, see DeGrazia 2005, 268–279.)