When it comes to assessing facial attractiveness, ‘Facial Attractiveness: Evolutionary Based Research ‘ by AC Little has proposed three tenants: Symmetry, Dimorphism and Averageness; it is the last point that is seldom discussed and even less understood.
An averaged face is one produced as a composite of many other faces. If we were to get many faces, all of a similar ethnic or racial group and overlay their faces together, then the Averageness Hypothesis suggests that the produced face will be more attractive than the sum of its parts, i.e. more attractive than those faces by themselves. This is because averaged faces have greater symmetry and clearer skin, but then the real question to answer becomes is averageness (koinophilia) desired because it is attractive by itself or is it desired because it makes the faces more symmetric (is symmetry the real driving force for why we prefer averageness)?
Tim Valentine; Stephen Darling; Mary Donnelly (2004). Why are average faces attractive? The effect of view and averageness on the attractiveness of female faces. , 11(3), 482–487.