When there's a suspect in a crime and the evidence includes a handwritten note, investigators may call in handwriting experts to see if there's a match. In some cases, it might be the one piece of evidence that gets a suspect charged and eventually convicted. But what if it's a false match? How exactly do experts go about analyzing someone's handwriting?
In the world of forensic analysis, which includes crime scene investigation, DNA testing, fiber analysis, fingerprint analysis, voice identification and narcotics analysis, to name just a few of the disciplines, handwriting analysis fits into the area of questioned documents. Questioned document examiners (QDEs) analyze documents for signs of alteration, forgery and, when sample documents are available, handwriting or typing comparisons to determine or rule out authorship (and/or tie a document to a specific machine in the case of typing). Handwriting analysis is a tedious and methodical process that relies on extensive knowledge of the way people form letters, which characteristics of letter formation are unique and the physiological processes behind writing - the ways in which a person's fine-motor skills can affect his or her handwriting and leave clues about the author's identity.
The primary basis of handwriting analysis as a science is that every person in the world has a unique way of writing. When we were all kids in primary school, we learned to write based on a particular copybook - a style of writing. Which copybook our handwriting is based on depends on when and where we grew up (see Handwriting-L: Copybook Examples for good examples of copybooks from different countries and eras). So at first, we all probably wrote in a similar way to kids of our own age and location. But with the passing of time, those writing characteristics we learned in school - our style characteristics - became only the underlying method of our handwriting. We developed individual characteristics that are unique to us and distinguish our handwriting from someone else's. Most of us don't write the way we did in first or second grade. And while two or more people may share a couple of individual characteristics, the chance of those people sharing 20 or 30 individual characteristics is so unlikely that many handwriting analysts would say it's impossible.
First and foremost, handwriting analysts must be able to accurately distinguish between style characteristics and individual characteristics, which takes a lot of training. They can pretty much ignore the style characteristics, which are only useful for determining with a fair degree of certainty which copybook the writer learned from. The individual characteristics are what matter the most in determining authorship.
So the process of handwriting analysis when comparing two documents - one by a known author, one by an unknown author - starts not with checking for similarities, which any of us could do with a fair degree of accuracy, but instead with checking for differences. It's the differences that initially determine if it's possible that the same person wrote both pieces of text. If there are key differences in enough individual characteristics, and those differences do not appear to be the result of simulation (an attempt to disguise one's handwriting or copy someone else's), then the two documents were not written by the same person. Simulation has its own telltale characteristics, which we'll discuss in the next section. However, if the differences don't rule out a match, and there are significant similarities in the individual traits in the two documents, singular authorship becomes a possibility.
Moving from possibility to probability is where the heavy lifting comes in.