A debate has emerged in the bioethics literature about the use of biotechnology to modify human nature. A failure to define humanness has produced conceptual confusion in this debate. We draw upon recent social psychological work on folk concepts of humanness and dehumanization to analyse the understandings of humanness that underpin the rival positions. We argue that advocates and opponents of human nature modification employ distinct conceptions of humanness, and that their differing evaluations of modification make sense in light of these conceptions. Advocates view modification as the enhancement of a non-essentialist sense of humanness that takes us further from animal nature. Opponents view it as the loss of an essentialist sense of humanness that takes us closer to a robotic state. Recognition that humanness has multiple senses implies that there is no mutually exclusive choice between seeing the outcome of modification as a quantitative gain in humanness or a fundamental, qualitative loss of it.