- anatomical traits - e.g. fingerprint, iris, face, hand geometry, veins ...
- behavioral traits - e.g, handwritten signatures or the rhythm of typing.
Handwritten signatures can be electronically captured in two ways: either the movement (position, velocity and acceleration) of the pen on the pad can be recorded as a function of time, or the signature image itself (x,y coordinates of the pen line on the pad) can be recorded. In either of these "dynamic" or "static" cases, the recorded signal is the result of both the biology (handedness) and the learned and practiced behaviour of the individual.
Signatures clearly have a biological component, as signatures from left handed and right handed people can often be distinguished. Although right handedness may be a learned behaviour, I believe strongly that left handedness is generally the result of a biological predisposition (except in cases of injury or illness).
Signatures are also a behaviour. Celebrities often have one signature for autographs and another for contracts, indicating two different behaviours depending upon context. Signatures can be willfully changed by changing signing behaviour.
Further, signatures can be used automatically (by computers) for human recognition.
Consequently, we can conclude that signatures, whether dynamic or static, are biological and behavioural characteristics that can be used to automatically recognize individuals and thus meet the definition of a biometric method.
Source: James L. Wayman, Ph.D., Principal UK Expert to ISO/IEC JTC1 SC37 WG1 "Biometric Vocabulary Harmonization" and also both the UK and SC37 representative to the JTC1 Information Vocabulary Maintenance Team
Additional information L. Lee, Berger, T. & Aviczer E, "Reliable On-Line Human Signature Verification Systems," IEEE Trans. Pattern Analysis Machine, vol. 18, pp. 643-647, 1996 ISO/IEC JTC1 SC37 WG1 "Biometric Vocabulary Harmonization".