You're entering a train car, a restaurant, a local store. As you step inside, you scan the people there. You don't know any of them, yet in seconds you register impressions of them all. He looks friendly, she appears evasive, that teenager seems threatening. Even as you're assessing the factual cues of their bodies--gender, skin color, height, age--you already seem to know whom you perceive as likable and whom you should avoid.