Finding a similarity between two signatures is easy.
Finding many similarities is also not particularly difficult.
The real question is:
When do similarities become evidence?
Professional forensic handwriting examination is not based upon isolated similarities or isolated differences.
It is based upon the interpretation of similar combinations of habitual writing characteristics.
What Is a Similar Combination?
Every person's handwriting is composed of numerous habitual writing patterns.
These include letter formations, connecting strokes, movement, rhythm, pressure patterns, spacing, proportions, line quality and countless other characteristics that have become automatic through years of repetition.
Individually, many of these characteristics may also appear in the handwriting of other writers.
The significance lies not in one characteristic alone.
It lies in the combination.
When several independent habitual characteristics consistently occur together in two writings, their evidential value becomes considerably greater than that of any single characteristic viewed in isolation.
Habitual Patterns
The formation of letters, connecting strokes, symbols and signature movements are physical expressions of deeply established habitual patterns.
These habits are repeated so frequently that they become automatic and unconscious.
The writer is generally unaware of them.
Because they are unconscious, they cannot easily be altered intentionally.
Nor can they easily be copied by another person.
This is why habitual patterns become such valuable identifying features.
Why Forgers Fail
A person attempting to imitate another person's signature naturally concentrates on what is visible.
The shape.
The size.
The overall appearance.
Yet every imitation unintentionally adds some movements and omits others.
The result may resemble the genuine signature in appearance while failing to reproduce the writer's unique combination of habitual characteristics.
Even when the imitation appears convincing, the unconscious writing habits usually reveal a different writing process.
Similarity Alone Is Never Enough
One similarity rarely identifies a writer.
Nor does one difference necessarily eliminate a writer.
Professional handwriting identification depends upon evaluating the significance of numerous independent characteristics occurring together.
It is the combination that matters.
A similar combination of habitual writing characteristics often possesses far greater evidential value than any isolated feature considered on its own.
Looking Beyond the Obvious
The untrained observer often asks:
"How many similarities did you find?"
The experienced examiner asks a different question:
"Which similarities are significant, and do they form a consistent habitual combination?"
That distinction is fundamental.
Professional forensic document examination is not a matter of counting similarities.
It is a matter of determining the evidential significance of the combination they create.
Beyond the Obvious
The obvious statement is:
"I found several similarities."
The professional statement is:
"I determined whether those similarities formed a significant combination of habitual writing characteristics."
An isolated handwriting characteristic is simply an observation.
A significant combination of habitual characteristics becomes evidence.
Looking Beyond the Obvious begins when individual observations are interpreted collectively rather than separately.