"[W]e might think of the creation of an unsatisfied preference as putting a debit in a kind of moral ledger of debits and credits. The satisfaction of the preference merely cancels out the debit. This ‘debit model’ of the ethical significance of preferences has the advantage of explaining the puzzling asymmetry in our obligations regarding bringing children into existence [...] We consider it wrong to bring into existence a child who, because of a genetic defect, will lead a thoroughly miserable existence for a year or two and then die; yet we do not consider it good or obligatory to bring into existence a child who, in all probability, will lead a happy life."