Abstract: Many people believe that all human life is of equal value. Most of them also believe that all human beings have a moral status superior to that of nonhuman animals. But how are these beliefs to be defended? The mere difference of species cannot in itself determine moral status. The most obvious candidate for regarding human beings as having a higher moral status than animals is the superior cognitive capacity of humans. People with profound mental retardation pose a problem for this set of beliefs, because their cognitive capacities are not superior to those of many animals. I argue that we should drop the belief in the equal value of human life, replacing it with a graduated view that applies to animals as well as to humans.