So, what was it that people liked about the manipulated faces: a resemblance to their own mother or father or perhaps similarities to themselves? Or was it just that the photos were somehow recognizable? It has long been known that we have a general preference for things we are familiar with. Researchers refer to this as the mere-exposure effect, a phenomenon where people develop a liking to things just because they are familiar with them. Accordingly, faces that are similar to our own generally appear more likable, or sympathetic, to us. This does not necessarily have anything to do with likeness to a parent. In one study of 130 students in a college class, four women posed as students and each attended zero, five, 10 or 15 classes. None of them interacted with the students. Afterwards, students in the class who were shown slides and asked to rate the women on looks found the women who attended a greater number of classes more attractive.
Research has also shown numerous external characteristics are demonstrably more important for emotional evaluation and partner choice. These include youthfulness and health, especially the appearance of skin; gender-typical characteristics such as an angular chin or large eyes; and the absence of negatively perceived characteristics such as strong asymmetry or obesity. When, in a 2015 PLOS ONE study, researchers asked a group of 44 heterosexual males to rate the attractiveness of 266 female Spanish students based on their photos, they found that facial symmetry was deemed attractive. This metric has been found to be perceived as a measure of youthfulness and health, a potential signifier of fertility.