Imagine that an eccentric psychologist accosts you. In his hand is a piece of paper with 20 pictures of roses. One of the pictures shows a rose in the flower bed you just passed, he says, and he asks you to pick its picture out from his lineup. The challenge would seem absurd—but if you were to change the roses to faces, nearly everyone could meet it.
Most of us have a powerful ability to recognize faces, and yet we hardly ever take note of it. We can commit a face to memory with a single viewing, and even if we see that face only once its memory can stay fresh for years. The faces we remember so easily may differ only in subtle tweaks of geometry: the ratio of distances between different landmarks such as the eyes and the mouth, for example.
A small fraction of people, however, cannot recognize faces—even the faces of their parents, spouses, and children. Prosopagnosia, as this condition is known, can affect people from birth or be triggered later in life by injuries to the brain. It strikes an estimated 2 percent of Americans and is often accompanied by other types of recognition impairments, including difficulty recognizing places and objects, such as cars.
