ABSTRACT
Recognising the identities of people we know is fundamental to being able to interact with them in terms of our past experience of them as individuals, so recognition from the face is an ability at which we become very skilled as we grow up. Nonetheless it is puzzling how we achieve this, given the constraints on the underlying biological structure of a human face (see Chapter 1). If all faces must essentially fit a common overall template, how do they also convey so accurately our individual identities? As long ago as 1883, Sir Francis Galton expressed the problem as follows:
The difference in human features must be reckoned great, inasmuch as they enable us to distinguish a single known face among those of thousands of strangers, though they are mostly too minute for measurement. At the same time, they are exceedingly numerous. The general expression of a face is the sum of a multitude of small details, which are viewed in such rapid succession that we seem to perceive them all at a single glance. If any one of them disagrees with the recollected traits of a known face, the eye is quick at observing it, and it dwells upon the difference. One small discordance over-weighs a multitude of similarities and suggests a general unlikeness.
(Galton, 1883, p. 3)