In her book The Moral of the Story: An Introduction to Ethics, Nina Rosenstand discusses a study published in 2007 by Antonio Damasio (a neuroscientist) and others working in the scientific field. The study appeared in the journal Nature and I have read similar observations in Damasio's books.
The study in Nature continues Damasio's work on individuals who have suffered ventromedial frontal lobe damage. It concludes that damage to one's frontal lobes usually makes it difficult for humans to make decisions that involve the lives of others. We evidently have "an emotional reluctance" (Rosenstand, 22) to make decisions that will bring about the death of others. However, persons who have experienced damage to the ventromedial frontal lobes evidently do not hesitate to make decisions that possibly save numerous lives but also bring it about that one or at least a few humans die.
The upshot of this study is supposed to be that we have a center in the brain that is responsible for providing a "moral compass" (ibid). In the words of Rosenstand, "we do appear to have been equipped with some sort of moral intuition from birth" (ibid). Furthermore, the study by Damasio (et. al.) indicates that our moral decisions are probably based on both emotional and rational factors. Rosenstand points out that the scientific study of where moral decisions are made does not exhaust ethics. Moreover, there are admittedly other studies that balance the one undertaken by Damasio and his colleagues. But I think that Damasio, other scientists and philosophers have made a good case for viewing human conscience or moral intuition as a neurobiological phenomenon.