Cruelty is widely regarded to be a uniquely human trait. This follows from a standard definition of cruelty as involving the deliberate infliction of suffering together with the empirical claim that humans are unique in their ability to attribute suffering (or any mental state) to other creatures. In this paper I argue that this definition is not optimum for the purposes of scientific inquiry. I suggest that its intuitive appeal stems from our abhorrence of cruelty, and our corresponding desire to (...) define cruelty in such a way that it is almost always morally wrong. Scientifically speaking this is an arbitrary condition that inhibits our attempt to study cruelty as a natural phenomenon. I propose a fully naturalized definition of cruelty, one that considerably expands the range of creatures and behaviors that may be conceived as cruel. (shrink)