Moral behavior and concern for others are sometimes argued to set humans apart from other species. However, there is some evidence that humans are not the only animal species to possess these characteristics. Work from behavioral biology and neuroscience has indicated that some of these traits are present in other species, including other primates. Studying these behaviors in other species can inform us about the evolutionary trajectory of morality, helping us to understand how the behaviors evolved and which environmental characteristics were critical for their emergence. A brief historical look indicates that, while this evolutionary approach to human behavior is not always well received, this line of inquiry is not new. For instance Adam Smith, better known for his economics than his natural history, was clearly sympathetic with the view that moral behaviors are present in species other than humans. This paper focuses on how individuals respond to inequity, which is related to moral behavior. Recent evidence shows that non-human primates distinguish between inequitable and equitable outcomes. However, this is primarily in situations in which inequity hurts the self (e.g. disadvantageous inequity) rather than another (e.g. advantageous inequity). Studying such responses can help us understand the evolutionary basis of moral behavior, which increases our understanding of how our own morality emerged.
