Cathy Chen, now at UMN neuroscience in computational neuroscience with a mouse model, conducted this study for her senior thesis project. We developed the method after focusing on deception as a means of measuring social awareness. Would tamarins make a choice that would ultimately earn them a better reward than the other participant (tamarin or human)? Tamarins were first trained to pick a bowl that had their preferred treat, which they would then receive as a reward. Suddenly, the rules were switched — now, the bowl they picked with the preferred treat went to their cagemate, and they ended up with the reward from the bowl they did not pick. We found that our tamarins were more likely to switch their bowl choice to the lesser preferred treat to be tactically deceitful when the recipient of their choice was a human partner rather than their cage mate. They also opted not to make a choice, or would refuse more often, when the recipient was their monkey friend. The tamarins worked more cooperatively with their cagemates, but maximized their own outcome when their social partner was a human, demonstrating their ability to use tactical deception.