A stone on the beach, we assume, has no moral status. We can kick or hammer the stone, and we have done the stone no harm. Typical adult human beings do have moral status. We shouldn't, without a very good reason, kick a man or woman.
Often, contentious moral issues, such as embryo research, or abortion, or whether to turn off a life-support machine, turn on disagreement about moral status. So the key questions are, who or what has moral status, and why? Jeff McMahan, of Rutgers University, has spent years trying to unravel the answers.