Handwriting Examination: Meeting the Challenges of Science and the Law
Handwriting is a complex motor skill that is the combination of sensory, neurological, and physiological impulses. Factors such as visual perception and acuity, comprehension of form, central nervous system pathways, and the anatomy and physiology of the bones and muscles of the hand and arm all combine to produce the desired output (Hilton 1982; Huber 1999).
Most people learn to write by copying letter formations from a copybook at a young age. The ability to reproduce the letter formations varies from one person to the next and is based on each writer's perception of the image and his or her ability (motor skills) to reproduce that visual perception. The act of handwriting is mastered through practice and repetition. Once this occurs, writers focus on the subject matter rather than the physical act of writing and deviate from the copybook forms, interjecting their own individual characteristics. The writing becomes a pattern of subconscious, habitual formations that are repeated from one writing to the next (Hilton 1982; Huber 1999).
The comparison and evaluation of these individual features or habits enable forensic document examiners to identify or exclude, if possible, a known writer as the source for any questioned writing. Lay people may recognize the handwriting of an individual and differentiate between individuals to some degree; however, they observe only the gross features of the handwriting, such as letter formation, size, or slope of the handwriting. Lay people typically do not consider the subtleties in the writing that may differentiate it from other very similar writing. In contrast, document examiners analyze and can differentiate both the gross features and the less conspicuous elements in the writing.
Handwriting features that examiners evaluate include the size and slope of the writing, pen pressure, pen lifts, the spacing between words and letters, the position of the writing on the baseline (the position of the character in relation to the ruled or imaginary line), height relationships, beginning and ending strokes, and line quality. A writer's identity cannot be established through a single individual feature in the writing. Rather, identity is established through a combination of the significant features between the writings, with no significant differences.
Link: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/oct2009/Zbackup/Review/2009_10_review02.htm