The Science Behind Forensic Handwriting Examination

Handwriting represents an individual’s language through graphic signs. It originates in higher brain centers and is expressed via neuromuscular actions of the arm. This process relies on sensory organs and psycho-physical abilities, forming a complex and delicately balanced system. Even slight disruptions in this system affect handwriting.

Given this complexity, altering one’s handwriting is challenging, which makes it possible to distinguish genuine handwritten documents from simulated or falsified ones. 

This is exactly what forensic handwriting examination is designed for—to determine whether there is evidence supporting a common origin for two or more handwriting samples.

Handwriting’s dynamic nature prevents identical strokes. What’s more, attempts to imitate or disguise handwriting will always reveal inherent personal traits. 

For this reason, forensic handwriting examiners (FHEs) focus on the similarity and compatibility rather than the absolute identity of samples under examination. During analysis, they assess both similarities and differences in the visual characteristics of the questioned handwriting and the comparison sample(s).

To minimize cognitive and confirmation biases, handwriting examination typically involves five distinct and independent steps.

The process begins with the formulation of hypotheses. It’s crucial to have at least two competing and non-overlapping hypotheses to ensure that all possibilities are fairly explored. For example: 

• “The questioned text/signature is genuine" versus "The questioned text/signature is not genuine."

• "Subject A authored the questioned text/signature" versus "A subject other than A authored the questioned text/signature."

The remaining four steps focus on the technical procedure. These are: Analysis, Comparison, Evaluation, and Verification, known as the ACE-V framework. 

In the Analysis (A) phase, the expert independently examines all features of the questioned and reference handwriting. 

All characteristics of the analyzed materials are documented to ensure that every aspect of the handwriting is considered, resulting in a more accurate and scientifically valid final evaluation. Detailed descriptions also allow the expert to ensure the comparability of specific handwriting characteristics. 

The analyzed features primarily focus on pictorial characteristics and the dynamics of execution. These include:

While examining signatures, the expert must also evaluate aesthetic qualities, including personalization and complexity. These elements indicate the author’s graphic skill at making the signature identifiable.

It is recommended to document these observations in a laboratory case note for future reference and report generation.

After determining and recording the handwriting characteristics, the expert proceeds to the Comparison (C) phase. They assess the similarities, compatibilities, and discrepancies between the samples, assigning specific values to each observation.

The Evaluation (E) process is the final step prior to reaching a conclusive judgment. In this phase, experts assess the hypotheses formulated earlier by analyzing their strengths and weaknesses. 

Recent research in forensic science, covering technical aspects and evidence evaluation methodologies, highlights the Bayesian approach as the most effective. In this approach, some hypotheses fall within the competence of the FHE, while others are preferably excluded from their consideration.

The “a priori probabilities” (i.e., “classical probabilities”) typically relate to factors outside handwriting analysis, such as investigation circumstances, witness statements, and other case evidence. These factors can influence the initial probability of each hypothesis in forensic analysis and should remain unknown to the FHE to prevent bias in their technical evaluation.  

Assigning and evaluating classical probabilities is the sole responsibility of the judicial authority and the investigation commissioner, who consider the overall context and supplement the FHE’s technical assessments with additional evidence.

The “a posteriori probability” is obtained by applying the Bayesian formula, which allows one to combine different probabilities by integrating the “a priori probability” with that derived from the observed data. This approach allows the initial probability of a hypothesis to be updated in the light of newly available evidence.

Assigning a probability of occurrence to the results obtained for each hypothesis quantifies the evidence and assesses how well each hypothesis is supported by the data collected during the analysis. The ratio of these probabilities yields the likelihood ratio (LR), reflecting the strength of the forensic expert’s statement regarding the relationship between the analyzed handwriting and the formulated hypotheses.  

The LR can be expressed numerically, providing a quantitative measure of evidence strength, or verbally, using a descriptive scale to communicate observations clearly to non-specialists. For example, in numerical representation, an LR greater than 1 indicates that the observed data are more consistent with the initial hypothesis than the alternative, while a value less than 1 suggests the opposite.

Finally, the FHE determines whether the handwriting samples originate from the same or different sources. An “inconclusive” result is also possible when the examiner can’t make a final decision due to the low quality of available documents, insufficiently comparable data, or other limitations.

The Verification (V) phase is a repetition of the ACE mechanism by a different handwriting examiner. A technical "peer review," this enhances the scientific rigor and robustness of the first expert’s findings and the overall report.

This quote underlines the importance of accurate instrumental analysis during handwriting examination, especially at the Evaluation (E) stage. Using advanced forensic equipment for observing samples’ characteristics helps detect details invisible to the naked eye, ensuring a thorough and objective investigation. 

A handwriting examiner’s toolkit must include:

• a microscope or other high-resolution magnifying instrument to view the smallest details of handwriting and structure of the substrate;

• specialized lighting sources, such as ultraviolet (UV), infrared (IR), transmitted and incident light, to detect any alteration in the document. 

Expert forensic equipment such as video spectral comparators embrace all these features. In this article, we examine samples with a Spectral Luminescent Magnifier Regula 4177-5.

Instrumental methods assist FHEs during the Analysis (A) phase, where handwriting characteristics related to the writer’s style and technique are examined and documented.

Magnification allows experts to evaluate both general and detailed handwriting features, including but not limited to:

• Legibility

• Size and proportions of elements and graphemes

• Spacing between letters and words

• Axial inclination

• Fluidity

• Stroke pressure

• Alignment with the baseline and margins

• Roundness or angularity of letters

• Number of graphic elements per letter

• Quality of connections

• Combination of successive letters 

The images made during investigations can be then included in the report containing final conclusions. 

The same tools prove invaluable during the Comparison (C) phase, where FHEs analyze questioned handwriting against one or more known samples. Using magnification, they can identify and document even subtle differences or confirm similarities between the samples.

Instrumental analysis is helpful for identifying the type of writing instrument used to produce the handwriting—pencil, ballpoint pen, fountain pen, etc. Each of these instruments leaves specific traces that can be investigated when comparing the samples.

Additionally, forensic equipment enables experts to determine the type of ink applied on the document, which may include liquid ink, gel, or even removable ink. Different inks offer significant indications regarding writing techniques and possible manipulations of the document.

During handwriting analysis, alterations in questioned documents can also be identified. Typically, investigating alterations in handwritten documents is a separate process; however, it is essential to account for their potential presence when analyzing handwriting.

The physical and mechanical impacts on the document that can be identified using instrumental methods include:

• substitutions of pages in a contract;

• scrapings to remove parts of text or handwriting;

• creation of “guidelines” to create text and/or signatures similar to those of another document;

• tracing;

• interpolation of text; 

• abuse of signatures on blank sheets on which text is subsequently fraudulently printed.

These types of alterations can be detected and studied using strong magnification and/or different light sources, including but not limited to UV, IR, and IR luminescence. The same techniques are also effective for identifying fraudulent additions to text and chemical alterations.

Instrumental analysis can determine the sequence of strokes, identifying which was made first and whether the same writing instrument was used. This sometimes helps assess whether corrections to the text are legitimate or fraudulent.  

Chemical alterations, carried out through the application of solvents, e.g., to remove parts of numbers or entire words, can be detected using UV sources at different wavelengths (AV-A or UV-B).

An instrumental approach makes handwritten examination impartial and replicable, supporting the expert’s conclusions with visible and measurable evidence. In this way, the applied methodology becomes scientifically grounded, increasing the reliability of the forensic expert’s judgment.

No detail can be overlooked when ensuring that forensic handwriting analysis is credible and admissible in court. The ACE-V method provides a solid basis for interpreting handwriting evidence, enhancing reliability and transparency in conclusions. 

Additional factors to consider in handwriting examination include:

• Bayesian reasoning: Applying this in the Evaluation (E) phase excludes binary yes/no conclusions, which are unscientific and misleading. 

• Verification phase: Far from being redundant, this phase adds another layer of transparency to examination results.

• Instrumental analysis: This is primarily essential for detecting alterations and comparing samples. However, FHEs can effectively analyze questioned and known documents this way to identify handwriting characteristics while avoiding personal judgements that lack evidence, vary over time, and are not objective.