Attempts to determine or to select what kind of person or people to bring into existence are controversial. This is particularly true of “negative selection” or “selecting against” a certain type of person—that is, the attempt to prevent a person of a certain type, or people of that type, from existing. Virtually everyone agrees that some instances of negative selection are objectionable—for example, that selection against healthy people would be wrong, particularly if this were combined with positive selection of people with serious diseases. But some people believe that all negative selection is objectionable and therefore that all “selection for existence,” whether positive or negative, is objectionable. For if negative selection is objectionable, it seems to follow that positive selection is as well, since the attempt to bring a person of a certain type into existence is simultaneously an attempt not to bring into existence a person who is not of that type. In short, positive selection is implicitly negative as well.

Why would someone believe that, for example, the attempt to avoid having a child with Tay-Sachs disease is objectionable? The reasons given vary but usually appeal to the idea that to decide that certain people ought not to exist is to discriminate against people of a certain type on the basis of values that are contested or not universally valid. I will not discuss the view that all selection is wrong, which is in any case the view of only a small minority. 1 Most people believe that some forms of se-