The similarity in handwriting would be due to the style characteristics that we were taught when we were learning handwriting in school out of a book. Thus, handwriting is as unique as a fingerprint.
Handwriting analysis is looking for small differences between the writing of a sample where the writer is known and a writing sample where the writer is unknown. Instead of beginning to look for similarities in the handwriting a process begins to search for differences since it’s the differences that determine if the document is a forgery. An expertise is looking at three things: letter form, line form and formatting.
This includes curves, slants, the proportional size of letters (relationship between size of short and tall letters and between the height and width of a single letter), the slope of writing and the use and appearance of connecting lines (links) between letters. A person may form a letter differently depending on where the letter falls in a word – beginning, middle or end. So an analyst will try to find examples of each letter in each placement.
This includes how smooth and dark the lines are, which indicates how much pressure the writer applies while writing and the speed of the writing.
This includes the spacing between letters, the spacing between words, the placement of words on a line and the margins a writer leaves empty on a page. It also considers spacing between lines — in other words, do strokes from words on one line intersect with strokes in words on the line below and above it?
Content, such as grammar, spelling, phrasing and punctuation should also be looked at.
A problem that arises during handwriting analysis is simulation, which is the attempt to disguise one’s handwriting or the attempt to copy another’s. Simulation is a huge problem because it can make it much harder to make a determination about a questioned document or it can make it impossible. It can be possible to determine simulation though.
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.
Analyzing handwriting is a long and careful process
It that takes a lot of time and, under ideal circumstances, a lot of comparison samples, or exemplars - documents that have a known author. It's not a matter of looking at two documents and saying "Hey, they both have a 'B' with a down-stroke extending below the line - same author!" In the case of the Lindbergh kidnapping in 1932, the police had a slew of questioned documents- in all, the kidnapper sent 14 notes to Lindbergh with ransom instructions. Handwriting analysts had no problem determining that all of the ransom notes were written by the same person. But pre-existing exemplars from the main suspect, Richard Bruno Hauptmann, were scarce, so the police had to get samples from Hauptmann in the police station by way of dictation. From those requested exemplars, handwriting analysts determined a match.
However, the police officers' methods of obtaining those samples has since been called into question - Hauptmann was forced to write for hours and hours until he nearly passed out from exhaustion. He was also told how to write, and he was shown a ransom note and told to copy the handwriting as best he could, to name just a few of the major no-nos. This of course means that the validity of the determined handwriting match is in question, and Hauptmann's execution makes a re-test impossible. There are now strict rules in place about how police should obtain a requested exemplar.