When people view pictures of others who have been shown to be either a cooperator or a defector, there is some tendency for people to remember the faces of defectors better than the faces of cooperators, even when they don’t know who is a cooperator and who is a defector. Similarly, people appear to be able to distinguish between honest people and dishonest people simply by looking at their pictures.

If you think about it, this makes perfect evolutionary sense. It is important for our ancestors (and for us as well) to guard against the possibility of deception, because being deceived can only have negative consequences for our survival and reproductive success. As a result, the human brain consists of many evolved psychological mechanisms that protect us from being deceived in our social exchange and interpersonal relationships. The ability to tell potential defectors (bad guys) from potential cooperators (good guys) by their appearance would be tremendously helpful for our ancestors (and us) in guarding against possible fraud.