Most ethical thinking supposes a fixed population, and considers such things as the distribution of resources amongst them. If population is itself made a matter of decision, then further problems arise. Do numbers matter by themselves, with it being a better world if more people live lives of some positive happiness? Or does only the average level of welfare matter? If a mother conceives in a way that she knows is likely to bring about the existence of a handicapped child, when she could have acted differently and avoided the risk, why has she done wrong? Nobody would have been better off had she acted otherwise, for the only child that does exist would not have existed. These and other questions are subtly explored in Parfit's Reasons and Persons (1984). See also Malthus.
Philosophy dictionary. Academic. 2011.