This week, I’ve been doing research into the characteristics of tremors that indicate a range of neurological and physiological conditions. I’ve looked up everything from a Parkinson’s Disease shake to the kind of quiver that I get when I’ve misjudged my daily caffeine quota. It’s been fascinating to learn about the features that help doctors to distinguish one medical disorder from another. The diagnosis process can be tricky; certain disorders share symptoms, so the doctor becomes a detective, piecing together clues. There is often a series of tests in clinic, which aim to capture the full array of cognitive and physical symptoms.
Analysing tremors in the writing of historical people is especially difficult, because the person is missing – long dead. The tell-tale visible signs are absent and we cannot quiz the ‘patient’ or their relatives about any changes that they have noticed. The tremor of Parkinson’s Disease is often most noticeable when the person is at rest. In contrast, a condition called Essential Tremor is often an action tremor, so manifests itself with movement. A shake can be exacerbated by stress or exhaustion or relieved slightly by alcohol. It can become more or less obvious at different times of the day. It may extend to the head, vocal cords, legs and arms. Looking at a sample of medieval handwriting by a scribe who had a tremor, we remain clueless about his head shakes, his consumption of alcohol, whether he was anxious, or whether he shook when he was resting. Instead, we have to rely on his handwriting alone as a historical record of the way in which his oscillating muscles affected his movement.
As I delved deeper and deeper into types of tremor, the phrase ‘forger’s tremor’ began to recur. This tremor, sometimes referred to as a ‘criminal’s tremor’, is the tiny shake in writing that is indicative of forgery. It seemed like the stuff of a medieval mystery novel in the making, so I decided to investigate further. When querying the word ‘tremor’, ‘quake’ or ‘shake’ in the Middle English Dictionary, I found that (in humans) it often referred to the palpitations caused by a) physical infirmity and the cold, and, most vividly, b) fear or anger. See below, from Book Five of Geoffrey Chaucer’s Troilus and Criseyde:
“…swich a tremour fele aboute his herte, That of the fere his body sholde quake.”
(“…he would feel such a tremor in his heart, that his body would quake with fear.”)
This is an extreme bodily reaction to fear: Troilus’s body shakes all over as he dreams of falling into the hands of his enemies. This is Troilus displaying the appropriate level of emotional response to his separation from Criseyde and his horrific dreams. His emotional anxiety is expressed physiologically through tremors and quakes. In John Lydgate’s Fall of Princes we see these great shakes toned down to a more understated tremble: ‘I do presente this book with hand shaking’. The poet’s hands quake at the thought of presenting his work to the patron who has commissioned it. The shaking is part of a convention of authorial modesty, the medieval ‘humblebrag’ as it were. The author makes a self-effacing, but self-promoting, statement, dismissing himself as inferior in the hope or expectation that the reader will judge him otherwise. A few lines earlier, Lydgate even describes the book itself trembling before its patron: ‘go litel book trembling’. Lydgate’s authorial tremor – whether real or figurative – is a sign of his anxiety, of his deference towards his patron, and his investment in his work.
A forger, aware of the misdeeds she is about to commit, might have flutters of anxiety. She may experience fear of being caught, or nervous twitches. Thus, the instability of her writing may be partially caused by psychological stress. However, the forger’s tremor is primarily down to writing with too much care. The forger’s tremor is different from the tremors caused by neurological disorder, anxiety or caffeine overdose, because it is often simply the result of writing slowly. As the forger’s hand slows down to copy a victim’s most idiosyncratic features, it makes involuntary jerking movements. It struggles with gravity and its own weight. The forger’s hand pauses unnaturally as she checks the shape of the writing and corrects herself. It’s kind of like riding a bike: move swiftly and you glide smoothly, ride too cautiously and you wobble and fall off. The effects of the forger’s shudders are most obvious in signatures, as genuine signatures are usually made in sweeping, almost automatic, movements.
In the case of a contested will, then, the presence of a forger’s tremor would be compelling evidence that the writing was not genuine. The potentially fraudulent writing would be compared with a ‘known’ sample – one that has been confirmed as genuine. As well as the presence or absence of a forger’s tremor, the pen pressure, alignment, stroke formation and slant would be compared in minute detail. With advances in digital technology, this procedure is becoming more and more precise – it is now possible to aid the human eye by running the two samples through software to determine how closely they match.
Intriguingly, Michael Caligiuri and Linton A. Mohammed, in their book on Forensic Document Examination, point out a situation where this comparison might go entirely awry.  What happens if the fraud victim also had a tremor – if he or she was in an advanced stage of Parkinson’s Disease, for example? How, then, do we distinguish between the genuine signature and the forger’s? Parkinson’s Disease makes life very difficult for writing analysts, as it is entirely impossible to find a standard handwriting sample with which to compare the forger’s efforts. The tremors render each stroke different, and micrographia – handwriting that gets progressively smaller – means that the sample will lack a consistent size. In addition, Parkinson’s Disease is a progressive condition, so the features of the sample will differ depending on the age of the writer when he wrote it. Nightmare.
Caligiuri and Mohammed took up the challenge of devising a way to distinguish between the writing of a person with a progressive tremor and a forger’s copy. Using digitising tablets, they found that when a person pretending to be a fraudster tried to copy a Parkinsonian resting tremor, they could not achieve the dysfluency of the stroke – their work was just too smooth. Conversely, when they attempted the action tremor of Essential Tremor, they tried too hard and the tremor was too dysfluent! When they tried to copy micrographic handwriting, they also failed – it is tough for the healthy brain to reproduce the effects of the perception and motor difficulties caused by Parkinson’s Disease. Caligiuri and Linton found that there were many more ways in which forgers fell short of a perfect copy, many of them stemming from the healthy brain’s adaptive mechanisms (ie. the things it does to cope with a strange task). For example, the hands of a person with a tremor experience movement oscillations at a higher frequency than is usual during writing. These high frequency movements are involuntary in the person with the tremor. However, for a forger to replicate them voluntarily, his brain has to reduce the extent of his hand movements, resulting in a signature with smaller surface area and size. Much earlier studies, by A.S. Osbarn (1929) and then Judie Walters (1997) also found that the forger usually fails to master the pattern of neurological tremor: the forger may, for example, put shakes in the wrong portion of the letter. Caligiuri and Linton only tested four pairs of writers, so admit that their findings are preliminary – but they are a great incentive for more research in this area.
So it appears that when a forger attempts the handwriting of a person without a tremor, their body betrays them with a ‘forger’s tremor’. Yet, when they attempt to replicate a neurological tremor, it is their brain that unmasks them. Interesting!
 Michael P. Caligiuri, Linton A. Mohammed, The Neuroscience of Handwriting: Applications for Forensic Document Examination (CRC Press, 2012)
 Judie Walters, ‘Handwriting Changes Due to Parkinson’s syndrome’, Forensic Science International, 88 (1997)