The human sense of fairness is an evolutionary puzzle. Why do we put so much value on what we receive relative to others? Examining other species’ reactions to different reward distributions reveals that humans are not alone in disliking inequity. However, a full sense of fairness also requires that individuals notice and seek to equalize outcomes that advantage them—a response that has been documented in our closest relatives, the apes. Dr. Sarah Brosnan will explore how the evolution of this response, combined with advanced abilities at inhibition and planning, allowed for the development of a complete sense of fairness in humans.