Excerp from Wikipedia:
Soft Biometrics traits are physical, behavioural or adhered human characteristics, classifiable in pre–defined human compliant categories. These categories are, unlike in the classical biometric case, established and time–proven by humans with the aim of differentiating individuals. In other words the soft biometric traits instances are created in a natural way, used by humans to distinguish their peers.
The beginnings of Soft Biometrics can be identified as laid by Alphonse Bertillon in the 19th century. He first proposed a personal identification system based on biometric, morphological and anthropometric determinations. The most common traits he introduced were colour of eye, hair, beard and skin; shape and size of the head; body characteristics like height or weight as well as indelible marks such as birth marks, scars or tattoos. A majority of these descriptors presently fall into the category of Soft Biometrics.
Jain lately redefined Soft Biometrics as a set of traits providing information about an individual, though these are not able to individually authenticate the subject because they lack distinctiveness and permanence