For hundreds of years people have debated what a human is. Some claim humans are those with human DNA. Some claim humans are those with certain traits like rationality. Others say humans are those who are made in the image of God. Scholars in this debate think that if society accepts the wrong definition of a human, people will look at their neighbor as more of an animal, object, or machine-making maltreatment more likely. Despite their seriousness, these claims have never been empirically investigated. This book focuses on human rights as exemplary treatment, and shows that the definitions of a human promoted by biologists and philosophers actually are associated with less support for human rights. Those members of the public who agree with these definitions are less willing to sacrifice to stop genocides, and are more supportive of buying organs from poor people, experimenting on prisoners against their will, torturing people to potentially save lives, and having terminally ill people commit suicide to save money. It appears that the critics are right. However, few Americans agree with these definitions of a human, and looking at how most of the public defines a human, we see a much more nuanced picture, and the presently dominant definitions of a human are unlikely to lead to human rights abuses. Therefore, the critics are right about the definitions of a human promoted by academic biologists and philosophers, but because few Americans agree with these views, concern about widespread maltreatment is overblown.