Handwriting analysis is a highly skilled endeavour. What follows has the appearance of a "how to" course, but it will not make you a skilled handwriting analyst! This is simply an outline of methodology.
In order to determine whether two samples of handwriting are by the same person, it is necessary to look for distinguishing features that are common to both, and to ensure that there are no crucial differences. This is a matter of balance: failure to check carefully for differences - to look only for similarities - makes a false match more likely, but being overly scrupulous about differences - which are to some extent inevitable - can mean missing genuine matches.
Only certain characteristics are indicative of single authorship. Handwriting analysts distinguish between two types of characteristic: individual characteristics and style characteristics. Style characteristics are those which are not unique to any given hand, and they can be divided into the following groups: system characteristics derive from the general style, such as the reversed e of Elizabethan secretary, copybook characteristics can be linked to the specific copybook from which the person learnt to write, while underground characteristics are shared with other writers but do not occur in any copybook, an example in modern handwriting being the formation of the dot on minuscule i as a circle.
The development of style characteristics in modern British handwriting is discussed in Tom Davis's article, Handwriting Acquisition in the UK.
The initial task in handwriting analysis is to distinguish the individual characteristics, which have evidential value, from the style characteristics, which do not.
The validity of a piece of handwriting comparison is dependent on individual characteristics being correctly distinguished from style characteristics, since a style characteristic is not unique to any particular writer so does not help to prove that documents share an author. The problem is that it is not always easy to know which characteristics are which. Different people with similar social status, form of employment, place and level of education, are particularly likely to have style characteristics in common.
The problem of correctly identifying individual characteristics is particularly acute with historical handwriting. When investigating modern hands there is a large amount of supporting evidence available; for example, which copybooks were in use where at what time. When looking further back in time, however, we know relatively little about how people were taught to write. Furthermore, professional hands (e.g. specialised legal hands such as 'Chancery') were much more widespread until the twentieth century, and such hands are carefully trained to lack individual characteristics. Did the parallel existence of two different writing systems (italic and secretary) in the sixteenth and seventeenth centuries have an effect on handwriting individuation?
The discrimination of evidentially important individual characteristics from stylistic features is something that is learnt almost entirely from experience. A corollary of this dependence on individual skill is that it is very difficult for a reader without considerable personal experience to judge a piece of handwriting analysis. In turn, this means that claims about handwriting are often judged either by the sheer quantity of evidence marshalled in support of a case, or by the prior authority of the person making the claim.
The Process of Handwriting Comparison
Handwriting analysis is about the comparison of samples. The samples should as similar as possible in origin: they should be from around the same date, and of around the same level of formality. The size of the samples is also important, for obviously the larger the sample the greater the likelihood of an accurate comparison. The fundamental problem with the identification of "Hand D" in Sir Thomas More as Shakespeare's hand is that the material for comparison is extremely limited.
The next steps are to analyse the samples for individual characteristics (i.e. characteristics that can reasonably be supposed to be unique), then to compare the samples for common individual characteristics. The technique involves finding correlations between non-adjacent portions of text - spotting features that recur through the samples. It is not enough to find a few striking features that are common to the samples: the comparison fails either if no common individual characteristics are found, or if there are individual characteristics unique to only one sample.
Professional handwriting analysts look first for evidence that the documents were written by different writers, whereas non-professionals tend to concentrate on similar features first. The problem with the latter is that if a similar feature is found, it is easy to assume that the hands must have been written by the same person, and not to look for differences with such care.
There will always be a certain amount of variation in anyone's hand: the question is whether differences between samples can be attributed to variation. An individual letter form should be expected to vary depending on its position within the word (initial, medial, terminal), or its neighbouring letters. There are also numerous contingent factors that can influence handwriting: when writing a particular document, the author could have been drunk, sick, hurried, or using someone's back as a writing surface.
Rather than focusing on a single instance of a letter-form, it is important to look at all the examples of that letter within each sample to get a sense of the underlying structure of the writing. Comparisons of handwriting ultimately depend upon the presence or absence of systemic similarities between the samples, rather than pictorial appearance alone.
Although letter forms are, on the whole the easiest feature on which to focus, the techniques of professional document examiners depend on the recognition that each penstroke is not just a pattern on paper, but is the trace of a physical movement: understanding the action that lies behind the letter form is crucial to being able to correctly identify handwriting. The question is not so much whether two samples look similar, as whether very similar motor actions lie behind their formation.
Characteristics, other than the appearance of letter forms, that are often suggested as being of evidential value include:
- The relative height of short and long letters.
- The relation between height and width of letters.
- The nature and placement of linking strokes.
- Slope of the hand.
- Relative placement habits such as use of indentations and treatment of margins.
- Formation of punctuation marks and other symbols.
- Spacing, including the spaces between words and between lines of writing (e.g. do projecting penstrokes intersect with strokes in conjugate lines?)
- The presence and regularity of pen-lifts, although the speed of writing can have a major effect on this.
- Line quality. A single writer's line quality will vary depending on (among other factors) the speed of writing; the slower the writing, the poorer the line quality. Tremor is a common feature of forgeries, since the writer will have to consider each stroke independently and will therefore write more slowly.
- Shading - differences in the darkness (and, with quills, thickness) of the stroke. This results from variation in writing pressure.
Discussion of handwriting analysis is inevitably deeply indebted to work in the forensic field. It is therefore worth considering forensic handwriting analysis in more detail, and the differences between forensic and scholarly handwriting analysis.